Fluorometric Determination of Submicrogram Quantities of Thorium

Fluorometric Determination of Submicrogram Quantities of Thorium CLAUDE W. SILL and CONRAD Health and Safety Division,

P.

WILLIS

U. S. Atomic Energy Commission, Idaho Falls, Idaho

b Interest in thorium i s increasing in the atomic energy program because of its use as a breeder material for production of fissionable uranium-233. As a result, the mill program for procurement of thorium is being expanded and investigations on the toxicology of the element and its compounds are increasing. Because of the toxicity of the element and federal regulations that prohibit concentrations of thorium in air larger than 0.009 pg. per liter from being released to unrestricted environments, a more sensitive and reliable procedure is required for its determination in both environmental and biological samples than i s currently available. A fluorometric method using morin in an alkaline solution of diethylenetriaminepentaacetic acid has been developed that has a detection limit of 0.01 pg. of thorium and a precision to about 0.5% on 5 pg., both at the 95% confidence level. Reliability and precision are achieved through use of a buffer system, an internal acid-base indicator, a permanent glass standard of fluorescence, and high concentrations of several different complexing agents. The fluorescent species contains thorium and morin in a mole ratio of 1 to 1. Because of its high sensitivity and tolerance to most of the common elements, the procedure i s particularly applicable to the determination gf thorium in air and water at concentrations well below the maximum permissible levels and in thorium ores and rare earths without separations of any kind. Even with siliceous materials, a determination can be completed in less than 1 hour.

D

a n investigation into the use of morin (Zt,4‘,3,5,7-pentahydroxyflavone) for the fluorometric determination of submicrogram quantities of beryllium (la), thorium was observed to produce a very sensitive greenish yellow fluorescence with the reagent in an alkaline solution containing (ethylenedinitrilo) tetracetic acid (EDTA). Zirconium, yttrium, scandium, lanthanum, and lithium also produced fluorescent complexes but a t lower sensitivity. During a subsequent 954

URING

m

ANALYTICAL CHEMISTRY

investigation ( I S ) , diethylenetriaminepentaacetic acid (DTPA) was found to decrease the fluorescence of yttrium, scandium, lanthanum, and lithium markedly while scarcely affecting that produced with beryllium, zirconium, and thorium. Since the formation constant of the thorium complex with DTPA is known to be about l o 3 times that of the complex with EDTA, the extreme stability of the morin complex in alkaline solution is particularly surprising. I n contrast, the thorium-morin reaction a t p H 2 (3, 7 ) is prevented completely even by small quantities of sulfate. The most widely used reagents for the determination of small quantities of thorium are either o-hydroxy azo dyes with another hydroxyl ( 5 ) or an arson0 (14) group in an ortho position at the other end of the azo group, or derivatives of 3-hydroxyflavone such as morin (3, 7 ) or quercetin (6). The most sensitive chemical procedure currently available appears to be the spectrophotometric one using morin (3) a t p H 2 which is capable of determining about 0.2 pg. of thorium in a 50-ml. volume using 5 c m . cells. A procedure using a n azo dye was reported to be capable of estimating quantities as low as 0.04 pg. but was shown later to have a lower limit for practical use of only 10 pg. (a). A fluorometric adaptation of the morin procedure was reported (7) to be capable potentially of detecting 0.044 pg. in 50 ml. but, in the author’s own words, “it is impossible to take advantage of this sensitivity a t present because random errors from unknown sources often occur.” I n the procedure using quercetin ( 6 ) , i t was found that “when the final test solutions contain less than 10 pg. of thorium, the absorbances are erratic and not reproducible.” Many of the other procedures show anomalous curvature, erratic tendencies, or other perturbations at the lower end of their ranges that prevent full utilization of the sensitivity available. Erratic behavior was also observed during the early part of the present investigation and was shown to be caused by failure to maintain the thorium in the proper ionic species necessary for the reaction during adjustment of pH. Even when dilute

alkali was used, some precipitation of thorium occurred locally around the drops which did not redissolve on subsequent mixing. If thorium solutions are made strongly alkaline in the presence of DTPA, thorium hydroxide is precipitated and will not redissolve even though several more drops of 72% perchloric acid are added than are required to neutralize the alkali and DTP.1. Because of the high stability of the thorium-morin complex in alkaline solution, DTPA and triethanolamine (TE.4) can be added to the thorium solution while it is still acidic to prevent any hydrolysis during the subsequent p H adjustment. TEA is used to prevent precipitation of thorium in the more alkaline region where the DTPA complex is unstable, and, particularly, to prevent local precipitation around the drops of strong sodium hydroxide during neutralization. Thorium can then be transposed at p H 11 to the more stable m o r b complex by the addition of morin and the fluorescence measured. By keeping thorium tied up at all p H values as water-soluble complexes, hydrolytic reactions are prevented and all the random errors and erratic tendencies referred to above are eliminated in the present procedure. INSTRUMENTATION

The instrumentation used has been described ( 1 2 ) . Fluorescence cells of quartz are highly recommended because Corex cells are etched very rapidly by t h e alkaline DTPA solutions. A combination of Corning filters Nos. 3387(454) and 5543(489) in standard thickness was used for the primary and numbers 3486(517) and 9782(582) for the secondary with a tungsten source. Transmittance spectra of the filters actually used have been published (8) but the figures in parentheses are given to ehow the wave lengths a t which the transmittance of the individual filter is 10%. REAGENTS

$11 DTPA must be recrystallized from water ( I S ) to remove impurities unless reagent grade material is available. If the decolorizing carbon used is not of high quality, a strong yellow color may be present in the filtrate due to the iron-DTPA complex that can be removed by making one additional re-

crystallization from ivater without addition of carbon. Triethanolamine is decomposed slowly on exposure t o air and light t o produce a bron-n color and bright blue fluorescence visible in ultraviolet light. If present in appreciable quantity, the fluorescence interferes seriously with the use of quinine as indicator. Standard Thorium Solutions. Fuse 0.2845 pram of nure ignited thorium dioxide ith 3 gr$ms o f anhydrous SOdium sulfate and 1 ml. of concentrated sulfuric acid in a 250-ml. Erlenmeyer flask. Roll the liquid melt around the sides of the flask n i t h continued application of heat t o dissolve any thorium dioxide t h a t may have spattered onto the sides. Cool the melt, :Idd 10 ml. of concentrated sulfuric acid, 100 nil. of water, and dissolve the melt with cooling. Dilute to 1 liter. Dilute 20.00 nil. of the stock solution and 10 ml. of concentrated sulfuric acid t o 1 liter. The solutions contain 250 and 5 pg. of thorium per milliliter, respectively. Sodium Perchlorate-DTPA-TEA. Dissolve 300 grams of anhydrous SO(Sum perchlorate in 300 ml. of lvater. Add 5 drops of 25% sodium metabisulfite t o ensure reduction of any hypochlorite t h a t might be present. Add 10 drops of 0.01% quinine sulfate and 1N sodium hydroxide until the bright blue fluorescence visible in ultraviolet light disappears. Filter the solution through a double glass fiber filter paper in a small Buchner funnel. Dissolve 6.0 grams of recrystallized diethylenetriaminepentaacetic acid in 50 ml. of water and 25 ml. of colorless 50yo,triethanolamine. Combine the solutions to obtain 500 ml. of solution. Add 72% perchloric acid n hile stirring the solution near an ultraviolet lamp until the light blue fluorescence that n-ill generally be present from the T E A lias changed to a brilliant one nhose intensity does not increase further n i t h a few inore drops of acid. Store the solution in a borosilicate glass bottle with polyethylene-lined screw cap. If the quinine fluorescence gradually disappears, incomplete elirniiiatioii of hypochlorite is indicated. Pipcridine Buffer. Transfer 44.0 grams of recrystallized DTP.1, 50 grams of sodium citrate, Na3C61150; 3 H20. and 20 grams of anhydrous sodium sulfite to a SOO-ml. volumetric flask with about 300 ml. of nater. Add 100.0 ml. of redistilled piperidine, stopper the flask, and swirl under a stream of cold water until cool. Dilute t o 500 ml. and store in a tightlystoppered borosilicate glass bottle with a polyethylene-lined screw cap. Norin. Prepare according to procedure in (18) Sodium Sulfate. Dissolve 140 grams of anhydrous sodium sulfate in 475 ml. of water to obtain 500 ml. of solution. Filter if necessary and store in a borosilicate glass bottle n ith a polyethylenelined screw cap. Sodium Sulfate in Sulfuric Acid. Ilieeolve 10 grams of anhydrous sodium sulfate in 100 ml. of concentrated sulfuric acid with heating as required.

Cool and store in a glass-stoppered borosilicate glass bottle. PROCEDURE

The procedure given below for preparation and measurement of the fluorescence is that used in the development of the procedure using pure thorium solutions. It is also t o be used when thorium has been separated and can be obtained in concentrated sulfuric acid free of interfering elements. Holvever, most of the applications given in the present article do not require eeparations eo that slightly modified directions will be given later for each application to produce the same final conditions. Place the thorium standard or other pure thorium solution into a 100-ml. beaker. Add 2 drops of 72% perchloric acid, 1 ml. of concentrated sulfuric acid-sodium sulfate solution, and evaporate the solution carefully t o dryness on an asbestos-covered hot plate. If separation is required, extract thorium into a chloroform solution of thenoyltrifluoroacetone Place the chloroform extract into a 100-ml. beaker, add 5 ml. of concentrated nitric acid, 1 ml. of the sulfuric acid-sodium sulfate solution, and evaporate to fumes of sulfuric acid. Add 72% perchloric acid as required t o destroy all organic matter and evaporate carefully to dryness on an asbestos-covered hot plate. With either type of sample, heat until all sulfuric acid condensed on the beaker walls has been volatilized and fuming has ceased but avoid heating longer or hotter than necessary. Cool the sodium acid sulfate residue, add 2 ml. of water, and 1 drop of 25% sodium metabisulfite. Cover the beaker with a watch glass and boil the solution until the volume has been reduced to about 0.5 ml. to dissolve anhydrous sulfates that might be present in trace quantities. Remove the cover glass and rinse 1%-itha felv drops of water. Add 3 drops of 0.01% quinine sulfate and 5.00 nil. each of the sodium perchlorate-DTPA-TEA and sodium sulfate solutions. If many samples are to be analyzed, add 10.00 ml. of a 1 to 1 mixture of the two solutions. Transfer the solution quantitatively to a 25-ml. volumetric flask being careful that the total volume does not exceed 17 nil. Quantitative transfer can be accomplished easily 1%-ithinthe specified volume by rinsing the beaker three consecutive times n ith single drops of water added systematically around the sides of the beaker. Add 1 additional drop of 2570 sodium metabieulfite. While swirling the flask near a long-wave ultraviolet lamp, add 111 sodium hydroxide dropwise until the bright blue fluorescence disappears. Add 5.00 ml. of the piperidine buffer, swirl, and rinse the sides of the flask ~ i t ah few drops of water. Add 100 ml. of morin, mix, and dilute to volume carefully t o minimize drainage from the neck of the flask. Mix thoroughly and place in a constant-temperature bath for 20 minutes. Measure the fluorescence using the technique described previously (12,

IS). Permanent glass standards ( I d ) can be used to reproduce the same instrumental sensitivity from day to day. The time of measurement after addition of morin is very important and should be kept Lvithin 1 or 2 minutes of the recommended value of 20 minutes for blanks, standards, and samples for highest precision. Since the thoriummorin complex has a temperature coefficient of about -3% per O C. the temperature of the water bath should also be within 1 or 2" C. of the prevailing room temperature to minimize temperature effects \\-hile handling the cell. Place 1 ml. of water for a blank and 1 ml. of the 5-pg. per ml. standard thorium solution in 100-ml. beakers, add 1 drop of 72% perchloric acid and 1 ml. of the sulfuric acid-sodium sulfate solution, Evaporate carefully on a n asbestos-covered hot plate until evolution of sulfuric acid fumes has ceased and treat as described above. Subtract the blank from the standard and express the sensitivity as microgram thorium per net scale division. Correct the samples for an appropriate blank carried through the entire procedure including separations, if any, and calculate their thorium content from the sensitivity value obtained from the standard. FILTERS AND LIGHT SOURCES

The effect of various filters and light sources can be interpreted b y comparison of the published absorption spectra of morin and its thorium complex (9) mith the transmittance spectra of the filters involved (8). The absorbance of morin and its thorium complex a t equal concentrations is the same a t 339 and 428 mp with morin being the more absorbant between the two wavelengths. Consequently, filters having their primary transmittance in this rcgion will excite fluorescence from the blank in greater proportion than from the thorium complex. -kt wavelengths longer than 428 nip, the ratio of thorium fluorescence to blank fluorescence inci eases R ith incrcasing n a d e n g t h . To obtain the high sensitivity available from a 5-pg. thorium standard, the fluorescence of the blank must be suppressed by using light a t as long a wavelength as possible for excitation. The loss in intensity of fluclrescence rcsulting from the longer \va\ elengths can be offset easily by a slight increase in the voltage applied to the multiplier phototube. The use of a mercury lamp is much less desirable than a tungsten lamp for the light source becmse the only usable emission lines a t 365, 405, and even 436 m p produce blanks so large as to preclude use of a 5-pg. standard. To obtain quantitative data for standard thickness of various Corning filters with both tungsten and mercury sources, the instrumental sensitivity IT-asadjusted so that the gross fluoresVOL 34, NO. 8, JULY 1962

955

cence of a 5-pg. thorium st'andard read 95.0 scale divisions wit'h each filter combination and the blank as then measured. With a tungsten source, the ultraviolet filters 5860 (325 to 385 nip) and 5970 (305 to 415 nip) gave blanks of 54.1 and 62.9 scale dirisions? respectively. The blue filters consisting of combinations of 3.391-5543 (395 to 515 mpc), 3389-5543 (410 to 515 mpc) and t.he recommended one of 3387-5543 (450 to 515 nip) gave blanks of 36.1, 29.2, and 17.9 scale divisions, respectively, showing clearly the :Idvantage of moving the transmittance of the primary filter ton-ard longer wavelengths. TT7ith a medium pressure mercury vapor lamp and filters chosen to isolate each individual mercury line, the blanks were 55.9, 64.8, and 45.2 scale divisions for filter conibinat'ions 7380-5840 (365 mp), 33915970(1'2) (405 mp) arid 3389-5543 (436 inp), respectively. The secondary filter used in all cases was the recommended combination of S o s . 3486 and 9782 (515 to 600 mp). S o mercury line gives as low blanks as are obtainable with a tungsten continuum and the proper filter. Many other advantages result from use of light at longer wavelengths for excitation. Production of fluorescence from other complexes, impurities in reagents, filters, cuvettes, etc., will be reduced or eliminated in many cases. Errors due to absorption of the irradiating light by impurities in reagents or b y other metallic constituent's of the sample will also be reduced considerably in general. As pointed out previously (sj,the primary filter affects many other characteristics such as linearity, intensit'y of fluorescence, and interferences. However, a t the sensitivit'y employed in the present case, blank suppression is the primary colisideration.

'"-1

M O R I N CONCENTRATION ( X

Figure 1. 1. 2. 3.

Effect of morin concentration

Blank

5-pg. Th standard 5-pg.Th standard corrected for blank

justed so that a 5-pg. thorium standard always reads 95.0 scale divisions, the blank will read 26, 17, and 14 scale divisions for morin concentrations of 9 X 3 X loF4,and 1.5 X lop4%, respectively. Since alkaline solutions of morin are oxidized b y air much more rapidly than neutral ones, care should be taken t o prevent accumulation of traces of alkali in the morin solution from the pipet used. If the morin solution is made even slightly alkaline, the thorium standards will show a progressive decrease in fluorescence from day to day because of the rather critical effect of morin concentration shown in Figure 1.

EFFECT O F M O R l N CONCENTRATION

EFFECT OF ALKALINITY

The effect of morin concentration on the fluorescence of bot'h the blank and thorium complex is shown in Figure 1. As shown by the arrows, the concentration recommended occurs a t a relatively steep part of the curve and was chosen rather arbitrarily t o keep the blank a t an acceptable level. However, with a little care, the concentration can be reproduced easily without affecting the masiinum precision obtainable. Lower concentrations result in slightly smaller blanks and higher sensitivity but the range of linearity becomes smaller arid adequate coritrol of t'he concentration becomes increasingly difficult. Higher concentrations give significantly larger blanks without decreasing the effect of concentration appreciably. For esample. if the imtrumental sensitivity is ad-

The effect of changes in alkalinity on the fluorescence of the thorium-morin complex is a t least as serious as was observed with the beryllium complex (12). Because of the small quantity of alkali present at the p H required for maximum fluorescence (ea. p H 11), very little acid is required in unbuffered solutions to destroy the fluorescence completely. Use of larger quantities of alkali to lessen the effect of small changes in its concentration depresses the fluorescence tenfold or more. The excellent efficiency and high capacity of the buffer system used in the present procedure are sho\rn in Figure 2 . The arroRs mark the points on the buffer curves that result under the recommended conditions. The buffer has been adjusted off center deliberatel). to provide much greater capacity to

956


%)

acid than t o alkali. Nearly all of the errors that might be incurred tend to decrease the pH in a solution as alkaline as p H 11. The most common errors result from absorption of carbon dioxide from the air, the presence of ammonium salts from the oxidation of organic matter with nitric acid, or the presence of metals that either consume hydroxyl ions directly or liberate hydrogen ions during the formation of complexes. Because of its low equivalent weight, aluminum is particularly effective in consuming hydroxyl ions. The buffer system employed will tolerate 1 meq. of acid or 0.25 meq. of alkali with less than 1% error and up to 2 meq. of acid or 1 meq. of alkali with less than 5y0 error. STABILITY O F THORIUM-MORIN COMPLEX T O W A R D OXYGEN A N D IRON

I n an alkaline solution containing EDTA, a few micrograms of thorium cause complete decolorization of the yellow morin solution. Since stannite prevents decolorization, oxidation of morin seems highly likely, particularly in vie\v of its known susceptibility to oxidation (fa). The mechanism by which thorium causes Oxidation of morin in the presence of E D T A but which can be suppressed almost completely by formation of a more stable complex with DTPA is not kno1v-n. The present procedure employs a very high concentration of DTPA to achiere adequate stability for precision work. Tolerance to iron was desired in the 1)resent procedure to permit application to many types of samples without

I

I

+ I 20

I 5

10

05

1 N HC104, m l

0

I

l

05

IO

l 15

I N NaOH, m l

Figure 2. Effect of alkalinity 1. 2.

Blank 5-pg. Th standard

separations in which this common element would likely be the principal interference. Hon-ever, even though sufficient TE,1 \\-as present to form the colorless iron complex and prevent errors due to light absorption, the presence of 1 nig. of iron produced results that n-ere low initially and decreased rapidly with time of standing because of the increased susceptibility of the thorium-morin complex itself to oxidation. Because of the steep slope of the morin curves shox-n in Figure 1, oxidation of even a small part of the morin will produce a serious decrease in fluorescence of both blanks and thorium complex. The total quantity of inorin present is only 0.25 pniole so that very little oxidizing capacity is required to destroy the reagent completely. Adequate stabilitjt o 1 mg. of iron has been achieved by using a high concentration of TEA t o decrease the oxidation potential of the ferric-TE.1-ferrous system. Cnder the recommended conditions, the fluorescence of a 5-pg. thorium standard increases by about 10% between 3 and 15 minutes after addition of morin, by about 47, during the next 45 minutes, and by less than 1% during the next 2 hours. dfter several hours, the fluorescence begins t'o decrease but the solution becomes completely colorless only after several days. The rate of decolorization increases markedly with increasing thorium concentration. Blanks increase Iiy about 3 7 , per hour from 15 minutes t o a t least 3 hours. The increase in fluorescence of blanks is due t o protluction of a fluorescent compound whose identity or origin has not been established. On standing overnight, I3lanks will develop a lemon yellow fluorescenee that is significantly different than the brownish yellow of

morin or the greenish yellow of the known metal complexes. I n the presence of 1 mg. of iron, the change in intensity of fluorescence with time IS very similar but decreases much earlier due to oxidation of the thorium-morin complex by tlie iron-TEA complex. The fluorescence starts out a little higher, reaches a maximum a t about 45 minutes and then decreases steadily. The maximum in the presence of iron is slightly smaller than in its absence and the two curves cross a t about 20 minutes. Accordingly, a standing time of 20 minutes is recommended since it is sufficiently long that the fluorescence is changing relatively sloir ly and the effect of 1 mg. of iron is small both as to magnitude of the error introduced and the rate of change of fluorescence n i t h time. For the best precision, a blank and standard and the unknonn samples should be prepared consecutively at a definite constant time interval. The fluorescence should be measured a t the same interval and in the same order in which the samples were mixed so that the time of standing of each sample will not vary more than 1 or 2 minutes from the recommended time. DETECTION LIMIT AND PRECISION

The detection limit is defined a t the 95% confidence level as that quantity of thorium which is equal to twice the standard deviation of its determination. T o determine its value and the precision obtained with larger quantities of thorium. 10 blanks and 10 5-pg. thorium standards were analyzed under the recommended conditions, including the evaporation of thorium solutions to dryness in the presence of sulfuric

acid and the transfer from beaker to the volumetric flask. The results indicate a detection limit of 0.01 pg. and a precision to about 0.5Oj, with 5 pg., both a t the 95% confidence level. I t was assumed t h a t the presence of the minimum quantity of thorium detectable would not change the standard deviation from that obtained with the blank. Khether such sensitivity and precision can be maintained on actual samples depends on how well the sources of error are controlled, particularly when separations are not used as in analysis of air dusts and ores. Maximum precision will be obtained if the samples are always compared to a standard and a blank run at the same time and under the same conditions. This includes temperature and timing of addition of reagents and measurrment of fluorescence. Kormal varia. tions will affect the maximum precision only slightly. LINEARITY AND EXTENT OF COMPLEX FORMATION

The effect of thorium concentration on the fluorescence was investigated at a morin concentration of 2.48 X 10-7 mole per 25 ml. t o determine both the linearity and the extent of complex formation under the analytical conditions. I n Figure 3, curve 1 s h o w the excellent linearity (and, incidentally, the precision) obtained up t o 5 pg. of thorium. Sonlinearity is not detectable and is, therefore, not greater than the precision of the procedure, Le., about 0.5y0. At 25 pg., nonlinearity has increased to about 150/0. As the thorium concentration is increased further, deviation from linearity becomes very pronounced as shon-n in curve 3.

Figure 3. Effect of thorium concentration 1.

2.

3.

0 to 5 p g . Th 0 to 25 p g . Th 0 to 500 pg. Th

THORIUM CONCENTRATION, JJg / 2 5

m1

VOL. 34, NO. 8, JULY 1962

957

The fluorescence is still increasing a t 500 pg. of thorium, or a mole ratio of nearly 9 t o 1 of thorium to morin, indicating t h a t the thorium-morin complex is highly dissociated under the conditions used. The extent of formation of the complex could not be determined precisely because absorbance differences Fere too small to be useful and fluorescence measurements are affected strongly by changes in the intensity of exciting light due to absorption of morin and nonlinearity over the range covered in curve 3. Probably, not more than 25 t o 50% of the thorium present a t the 5-pg. level is associated with morin even though the mole rat,io of morin to thorium is about 11.5. The incompleteness of the reaction makes the high precision obtainable all the more remarkable. It also explains why temperature, time, ionic strength, pH, concentrations of reactants, and all other factors affecting the position of the final equilibrium must be controlled exactly if precise results are t o be obtained.

plex than there is between their absorbances, much larger values are obtained and the results are much more precise. Hom-ever, to be sure that the deliberate variations in morin concentration employed did not introduce perturbations by changing the intensity of the exciting light ( I S ) , the experiments n-ere repeated using fivefold smaller concentrations of both thorium and morin. As shown by curve 2, identical results m-ere obtained. Because the solutions were virtually colorless at the lower concentration, proof of a 1 to 1 complex is felt to be unequivocal. Thorium has been shown to produce 1 to 2 complexes with both morin (3) and its isomer quercetin (6) from 15-eak acid solutions. The formation of a 1 to 1 complex from alkaline solutions of D T P A permits some interesting speculation as to the fate of the other three valence bonds and six coordination positions of thorium. The present authors have always thought

MOLE FRACTION OF THORIUM

Figure 4. Composition of thoriummorin complex b y method of continuous variations 1. 2.

3.5 X 10-SM thorium and morin solutions 7.0 X 10-6M thorium and morin solutions

Table 1.

THORIUM-MORIN REACTION

T o obtain information concerning the reaction involved, the composition of the thorium-morin complex was determined by the method of continuous variations (16). X milliliters of a 3.5 X 10-5AVI solution of morin was used with (5 - X) ml. of a n equimolar solution of thorium under the recommended conditions and diluted t o a total volume of 25 ml. Both absorbance and fluorescence measurements were used to follow formation of the complex. However, even with a 5-em. cell and working a t 460 mp nhere the difference in absorbance is greatest between morin and the thorium complex, the maximum Y-value (15) was only 0.053 absorbance unit, and was always obtained as the difference of tn.0 relatively larger numbers. The solutions could not be prepared sufficiently precisely to permit determination of the composition of the complex from such small values although the data Fere not inconsistent with those described below, Absorbance measurements \T ere successful in determining the composition of the beryllium complex (IS) under similar conditions, but the thorium complex appears to be more highly dissociated. Fluorescence measurements were made on the same solutions described above and were corrected for the calculated contribution of the unreacted morin in each solution to it5 total fluorescence. When plotted against mole fraction of thorium as shown in curve 1 of Figure 4, the data clearly indicate formation of a 1 to 1 chelate. Because there is a much greater difference in fluorescence between morin and its thorium com958


Quantity, Mg.

Error, Sc. Div.” Blanks 5 pg. T h

Element Be Be AAb Be AAb Be AAb

0.00005 0.00005

Be

0 0006

Zr Zr Zr Zr Hf Ti Ti sc AI -41 A1 I,a I’ Li

0 010 0 010 0 100

10 0

1 0 0 1

8 -18 G -3 0

u

0 1

-1 0

Cr Cr

1 0 1 0

+2T 9 -3 5

Cr

Cr

0 1 0 1

-1 0

Ba

1 0

+4 7

Ba Ba

0 1 0 01 1 0 1 0 1 0 0 1 1 0 1 0

+0 2 0 0 1 0 1

++ +

u u

Sr

Ca Ce Ce Sb

Sb

Kb Nb

Ta

0.0005 0.0005

0 100 0 005 0 1 0 1 0 1 5 0 5 0 0 1

1 0

1 0

1 0 0 1 1 0

Effects of

+13 3

+o

9 +9 6 $9 4

+o

2

i-0 3

+30 7 +,0 1 $158 G +A 1 $11 9 +9 9

-r3 6 +14 +1 1 +37 6

+o

7

T O

1

t 2 1 4-(J

-0 8

0 0 0 0

-0 1 T O 9 $1 0 120 0 +0 2 +6 0

Remarks Fluorescent; 0.0037 pg./sc. div. Fluorescent; 0.05 pg.,/sc. div. Fluorescent; 0.052 pg./sc. div. +7.8 1-7.0 Acetylacetone added after morin - l , O l * Single extraction with acetylacetone as de- 1 . l J scribed. Be present n-ould have produced [fluorescence equal to 158 sc. div. Fluorescent; 0.33 pg./sc. div. Tyithout citrate $31.8 0 . 0 In presence of citrate buffer t l 5 1 . 5 Without citrate + 8 . 5 I n presence of citrate buffer Fluorescent; 0.42 pg./sc. div. +12.4 Fluorescent; 10 pg./sc. div. -5.8 Turbid -1G.5 Fluorescent; 71 pg./sc. div. $3.0 Slightly turbid; no citrate present -4.4 Fluorescent; 133 pg./sc. div. in citrate buffer T34.9 4 0 . 4 Fluorescent; in citrate buffer Fluorescent; 480 pg.jsc. div. +5.0 t 1 . 6 Fluorescent; > l o 4 pg./sc. div. Fluorescent; >lo4 f i g . l s c . div. -5.0 Rro\mish yellow color v-ith morin -98.5 -12.4 KO detectable color of uranium. Xorin solution nearly decolorized v-ithin about 2 hours Solution neutralized with l;aiCOs. S o de-70.8 colorization on standing overnight -Very turbid due to anhydrous Cr,(SO4), -11.4 -84.2 CrC18 added after fuming. Grayish blue c d o r of chromium visible -12.2 Presence of chromium color questionable Added as KanCr20; after making solution -3.0 alkaline. S o oxidation of morin -52 5 Very turbid. BaS04 does not redissolve on boiling alkaline DTPA solution Slightly turbid by Tyndall effect -10.8 -1.2 Y o turbidity detectable Slight turbidity -7.0 $13.5 Sl.5

I

-2.4 -2.8

-1.7

-72

G

+5.0 +i.3 -1.9 +0.4

Morin solution nearly decolorized overnight hlorin solution nearly decolorized overnight Turbid. Antimonic ”acid precipitates ?Jot fumed. Turbidity of hydrolyzed SbC1, cleared on addition of DTPd-TE-4 solution Very turbid. Siobic acid precipitates Very slight turbidity by Tyndall effect Turbid. Tantalic acid precipitates

it surprising that morin could remove thorium from its complex with DTPA. Not only is the thorium-DTPA complex known to be extraordinarily stable, but the molar concentration of D T P A is over 5000 times that of morin under the conditions used. However, because the thorium-morin fluorescence is not affected significantly b y concentration of DTPA, such exceptional stability did in fact seem indicated; yet, the thorium-morin complex is known t o be formed very incompletely. The evidence of a 1 to 1 complex would now seem to indicate t h a t the morin has simply replaced the two most loosely held bonds of the D T P A complex to form a mixed complex containing thorium, DTPA, and morin in a 1 t o 1 to 1 ratio. Such 1 to 1 to 1 mixed complexes have been shown (4) t o be formed with zirconium from EDTAtype complexing agents and a second multidentate ligand to form more completely coordinated compounds

which are more stable than those formed with either ligand alone. -4cetylacetone and 8-hydroxyquinoline-5-sulfonate were reported t o form particularly simple and stable chelates as the second ligand. The hydroxy-carbonyl structure of morin through the hydroxyl groups in either the 3- or 5positions should also be very effective as the second ligand. Such a postulate would explain the relatively slight effect of D T P A and the formation of a l t o l thorium-morin complex without requiring a superstable complexing agent capable of replacing all the D T P A bonds a t a very unfavorable ratio of morin to DTPd. EFFECTS OF OTHER SUBSTANCES

I n anticipation of applying the present procedure t o a wide variety of sample types without separations, a detailed investigation was made of the effect of many other substances on both

Other Substances

Element Ta Si Si

Quantity, Mg.

Error, Sc. Div." Blanks 5 fig. Th

0.1 1.0 1.0

+0.3 +o. 1 0.0

0.0 -5.9 -18.8

Si Sn Sn

1.0 1.0 1.0

-0.1 +5.3 +3.7

-23.6 -2.8 +3.1

Te W

1.0 1.0

-1.3 -0.8

-0.5 +l.5

Ag

Mn

1.0 1.0 1.0

+0.6

HI3

-0.3

-7.3 +3.1 -1.9

Fe

1.0

-0.5

-0.5

+4.8

co

1 0

-1 0

-6.4

Ni

1.0

-0.2

-3.0

cu

1.0

-0.8

-3.0

Pb

1.0

+0.2

-1.9

;is+:\

1.0 1 .o 1 .o 1. o 1.0 1 .o 1.0 1.0 1.0

-0.1 -1.6 0.0 +0.8 -0.3 -0.4 -1.1 -0.5 0.0 0.0 0.0 +0.5 0.0 -0.1 -0.7 0.0

-4.0 -0.8 -4.5 -1.5 -0.1 -1.4 -0.6 -3.3 -9.2 -9.5 -13.5 -1.5 -0.6 -0.7 -44.5 -0.3

Au Pt

os

Ru F F P P P P

s

1.0

1.0 1 0

i.0 Oxalic 10-4 mole Tartaric mole Citric lo-' mole

Remarks Very slight turbidity by Tyndall effect Few flocs of dehydrated SiOz present NazSiO:, boiled in acid solution to produce few flocs of hydrated silicic acid NazSi03added after making solution alkaline Turbid Turbid. NazSn03added after making alkaline Slight turbidity; presumably tellurous acid Tungstic acid precipitates but redissolves after making solution alkaline Turbid; probably elemental silver Turbid; probably elemental mercury Morin solution nearly decolorized on standing over night Deep yellow color disappears on addition of buffer. Solution completely decolorized on standing overnight Light ,pink color visible before addition of morin Faint ,blue color visible before addition of morin Strong blue color visible even after addition of morin PbS04 dissolves on boiling alkaline DTPA solution Bright yellow color in concentrated H2S0, Boiled with H2SOa; light blue color Gold reduced to element by SO2 Very slight turbidity Oso4 probably volatilized during fuming RUO4 probably volatilized during fuming NaF added after fuming while still acid XaF added after making solution alkaline Added as orthophosphate before fuming Added as orthophosphate after fuming Added as pyrophosphate after fuming Added as quadraphosphate after fuming Added m nitrate after fuming -4dded as the acid after fuming Added as the acid after fuming Added as the acid after fuming

Blank, 18.6 sc. div.; 5 rg. T h standard, 100.9 sc. div.; sensitivity, 0.0607 pg./sc. div. Differences larger than f0.2 sc. div. on blanks or zt0.5 sc. div. on standards probably indicate significant effect of added substance. * 0.25 ml. of 1Oyoacetylacetone added. Blanks and standards decreased to 17.2 and 92.3 sc. div., respectively, amounting to 7.5 and 8.5y0decrease.

blanks and 5-pg. thorium standards. Unless otherwise indicated, the element or compound was added before fuming with sulfuric and perchloric acids t o determine its effect under the recommended conditions. In other tests, the element was added at later stages in the procedure t o determine the effect of a specific oxidation level or other factors such as hydrolysis, dehydration, or precipitation. After most of the present work had been completed, i t was found that sodium citrate would suppress markedly the fluorescence produced b y zirconium without affecting either blanks or 5-pg. thorium standards significantly. Accordingly, sodium citrate has been added t o the buffer t o provide increased tolerance t o zirconium. I n general, sodium citrate is not likely to change the results significantly because of the generally lower stability of its complexes in comparison t o those formed with DTPA. However, the buffer curve of Figure 2 and most of the interference tests of Table I using 6-pg. thorium standards were checked with the citrate buffer. Where the results are different than those shown, specific comments will be made. No error could be detected on blanks and less than 1% error on 5-pg. thorium standards in the presence of 1 mg. of potassium, rubidium, cesium, magnesium, zinc, cadmium, bismuth, gallium, indium, thallium, aluminum in the absence of citrate, boron, germanium, selenium, arsenic, molybdenum, or any of the rare earths. Errors produced b y other substances or under other conditions are shown in Table I. Beryllium. Beryllium produces b y far t h e most sensitive interference of a n y substance tested with quantities larger t h a n about 0.7 millimicrogram being detectable. On a weight basis, sensitivity of t h e morin reaction with beryllium is about 16 times t h a t with thorium under t h e conditions used i n t h e present procedure. T h e only complexing agent found t h a t would suppress the beryllium fluorescence significantly without producing a prohibitive depression of the thorium reaction was acetylacetone. The sensitivity of the beryllium reaction in the presence of acetylacetone is about the same as for the thorium reaction and the maximum quantity of beryllium t h a t can be present without producing detectable error is about 0.01 pg. The same suppression of the beryllium fluorescence can be achieved even if the acetylacetone is not added until after the fluorescence has been developed showing t h a t the fluorescent reaction is rapidly and quantitatively reversible. Thus, a suspected fluorescence can be checked very conveniently b y adding 1 drop (0.026 ml.) of pure VOL. 34, NO. 8, JULY 1962

959

acetylacetone after the measurement has been made. The fluorescence will change very little if due to thorium but will decrease by 14 times if due to less than 0.5 pg. of beryllium. If acetylacetone is used t o increase the tolerance to beryllium, both the blank and 5-pg. thorium standard should be treated similarly and comparison made t o them rather than to the usual ones without acetylacetone. Also, the 10% acetylacetone solution should be freshly prepared to prevent hydrolysis. T o determine if acetylacetone might affect either the linearity or precision of the procedure, a linearity curve was run from 0 to 5 pg. of thorium using the same concentration of acetylacetone used in the tests. The results n-ere identical with curve 1 of Figure 3. A simple procedure is given for separation of beryllium when it might be present in interfering quantities. Zirconium and Hafnium. Zirconium is t h e next most sensitive interference with quantities greater t h a n about 0.07 pg. producing detectable fluorescence in the absence of citrate. Hafnium acts similarly. The fluorescence produced by zirconium is visually identical with that from thorium, so that, unlike beryllium, no indication of interference is available from visual inspection in ultraviolet light. I n the presence of the high concentration of sodium citrate used in the present procedure, 10 pg. of zirconium produces no detectable error on either blanks or standards which amounts to a decrease in sensitivity of over 100-fold. Furthermore, 100 pg. of zirconium gives fluorescence equivalent t o that produced by only 0.25 pg. of thorium. I t is also interesting to note that the accelerated oxidation of morin that .;till occurs in the presence of zirconium despite the high concentration of D T P A is eliminated b y addition of citrate. It seems likely that the model proposed for the thorium-morin complex is equally applicable to zirconium, Le., a 1 to 1 t o 1 zirconium-DTPAmorin complex as the fluorescent species. At high molar ratios of citrate t o zirconium complex, the morin is displaced by the citrate to form a nonfluorescent zirconium-DTPA-citrate complex. Titanium. Titanium produces a dull green fluorescence with morin of sensitivity such t h a t not more than about 2 pg. of titanium can be present 1%ithout producing detectable error. If very much titanium is present, other difficulties n-ill be encountered due t o its extreme hydrolytic tendencies. A solution containing 1 mg. of titanium will become turbid within 2 minutes after addition of the buffer, 0.5 mg. within about 20 minutes, and 0.1 mg. after a few hours. 960

0


Uranium. Uranium forms a coniplex with morin even in t h e presence of D T P A and interferes seriously b y consuming t h e reagent necessary for production of fluorescence with thorium a n d whose concentration is very critical as shown in Figure 1. T h e maximum quantity of uranium t h a t can be present without causing error on either blanks or standards is about 5 pg. Addition of sodium carbonate to complex uranium makes the interference much worse. A brownish yellow color of the morin complex 11-ill be visible with about 1 mg. of uranium. Solutions containing 0.1 mg. of uranium will be nearly decolorized within about 2 hours due to the accelerating effect of uranium on the air ovidation of morin. Aluminum. I n t h e absence of citrate, 1 mg. of aluminum produces no interference. Only a small error is produced a t the 5-mg. level due t o turbidity of anhydrous aluminum sulfate. However, when citrate is used to suppress zirconium fluorescence, a bright greenish yellow fluorescence similar to t h a t produced with beryllium becomes detectable in the presence of about 50 pg. of aluminum. If the sample size is kept small enough to eliminate the more serious interferences, aluminum will present no problem even in the presence of citrate. I n special cases in which aluminum interference is more serious than that of zirconium, the original buffer with which most of the present work was done should be used. This buffer contained 42.0 grams of D T P A and no citrate, the other components remaining as described under Reagents. The formation of mixed complexes when several complexing agents are present appears t o be a more common process than has been previously supposed and towhich moreattention should be given in the development of analytical procedures. I n addition to the complexes proposed for thoriuniDTPA-morin, probably a similar one for zirconium-DTPA-morin, and the zirconium-DTPA-citrate, the evidence is quite conclusive that the fluorescent species produced by aluminum in alkaline solution is another three-n-ay complex composed of aluminum, morin, and citrate. The fluorescence will develop on addition of any one of the three components to a nonfluorescent solution of the other two. The same results are obtained in a system without DTPA. Rare Earths. During t h e initial stages of the present investigation, significant fluorescence was observed with several rare earths and was shown subsequently t o be caused b y traces of thorium even though only 1 mg. of rare earth was used in t h e interference tests and oxides of purity

greater than 99.9% were used as the starting material. After further purification to remove t'races of thorium, each rare earth except promethium was retested individually. KO interference could be detected from 1 mg. of any of the rare earths except cerium on either blanks or 5-pg. thorium standards. The serious error produced by rare earths in the presence of E D T d (12, 1 3 ) is due to consumption of t'he morin reagent which is prevented by formation of more stable complexes n i t h DTPA. Lanthanum and the light rare earths form relatively insoluble double sulfates with the sodium sulfate present, which, however, will dissolve on warming an alkaline solution containing DTPA. Cerium. I n the quadrivalent state, cerium interferes seriously b y oxidizing t h e morin reagent'. Less t h a n 100 pg. of ceric ion is required to oxidize the 0.25 pmole of morin present quantitatively in a few seconcia. Even if present initially in the trivalent state, cerium is oxidized alo~vly by air in alkaline solution even in the presence of DTPA. The resulting quadrivalent cerium will then oxidize niorin and the thorium fluorescence mill decrease rapidly with time. This catalytic action \Till continue until all niorin has been oxidized and the solution becomes completely colorless. If all cerium is reduced to begin \vit,h, addition of sulfite vi11 slow the air oxidation sufficiently so that the error is virt'ually eliminated if the measurement of fluorescence is made \Tithin the 20 minutes specified. However, sulfite will not reduce quadrivalent cerium in alkaline solution. The ceric-DTPA complex is reduced relatively slowly even in acid solution. Therefore, after boiling an alkaline DTPA solution to dissolve rare earth double sulfates, the solution must be reacidified and n-armed in the presence of sulfite to reduce the ceric-DTPA complex quantitatively. Other Fluorescent Complexes. As shown in Table I, ytt'rium, lanthanum, and particularly scandium shorn some interference a t the I-mg. level due t o incomplete suppression by D T P A of their normal fluorescence n-ith morin. P a r t of the residual fluorescence observed n-ith scandium could be due t o traces of thorium since the oxide used t o prepare the scandium solution was not purified specifically to remove traces of thorium. Chromium. Trivalent chromium is a particularly serious interference probably because of its strong absorption of both the emitted thoriuninlorin fluorescence and t h e exciting radiation and because of the insolubility of anhydrous chromic sulfate. The presence of chromium can be detected quite sensitively by evap-

orating t h e solution t o fumes of sulfuric acid in t h e presence of perchloric acid. il bright yellow color will develop with less than 0.1 mg. of chromium but will disappear quite rapidly because of thermal decomposition of the dichromate in the fuming sulfuric acid. Similar yellon to red colors are produced by less than 0.1 mg. of cerium and vanadium which are more resistant to thermal decomposition. Chroniiuni can be removed easily from sulfuric-perchloric acids b y volatilization n ith hydrogen chloride gas as chroniyl chloride. Chromic acid cleaning solution should be avoided if possible. Niobium and Tantalum. T h e error produced b y niobium a n d tantalum on t h e fluorescence measurement is simply a turbidity effect caused b y precipitation of t h e earth acids a n d is relatively unimportant with less t h a n 0.1 mg. of either element. However, because of their frequent occurrence in thorium ores, their effect under conditions encountered in t h e treatment of ores was investigated. When 50 mg. of each element was separated from 250 pg. of thorium by hydrolysis from dilute acid, loss of thorium in the precipitate amounted to 5.7 and 26.87, for niobium and tantalum, respectively. When the solutions were made alkaline and boiled before removal of the precipitate, as is necessary in the presence of rare earth double sulfates, the losses were reduced to 0.1 and 5.7%, respectively. The errors 4 o u l d be considerably smaller in actual practice when only 50 mg. of total sample is taken for analysis. However, if larger samples are taken or the highest accuracy is desired, any significant precipitate should be filtered off and reprecipitated to recover the thorium it contains. Barium. Barium produces one of the most serious negative interferences encountered because of exten*ive coprecipitation of thorium n i t h t h e barium sulfate formed in t h e present sulfate system. S o t more than about 10 pg. can be present nithout producing significant error. Strontium and calcium act in a similar manner hut are much less serious. Lead or rare earths produce little error in the tlevelopment of the fluorescence bevause their sulfates dissolve in the dkaline D T P A solution used. I n contrast, the alkaline earth sulfates will not dissolve completely in alkaline D T P A when excess sodium sulfate is present. However, any insoluble sulfates including the double rare earth sulfates that are filtered off during treatment of a n actual sample will contain the major part of the thorium. This process is so efficient that i t is currently being developed into a method

for the quantitative separation of submicrogram quantities of thorium.

SAMPLE PREPARATION

Because of the exceptionally high sensitivity and precision of the present procedure, the thorium content of many types of samples of practical importance can be determined precisely with very small samples. Because of the excellent tolerance to many other elements, no separations of any kind are required in most cases and no loss of reliability results if the sample size is kept so small that potentially interfering elements cannot acquire the concentrations necessary to interfere. I n fact, the size of sample taken for analysis should never be larger than necessary to obtain the desired sensitivity when i t is known. -4s discussed in the preceding section, a t least 1 mg. of many of the commonly occurring elements, including iron and the rare earths, can be present without causing significant error. illthough not as sensitive in their reactions, zirconium, titanium, and uranium are perhaps more serious interferences than is beryllium. They are more likely to accompany thorium in significant concentrations, they give much less sensitive warning of their presence, and their separation is considerably more difficult and time consuming than is true with beryllium. Honever, they can still be accommodated without loss of reliability by proper choice of sample size. For example, if the total sample taken for analysis is limited to 1 mg., thorium can be determined in concentrations greater than 1 X 10-37, a i t h o u t detectable interference from as much as 17, zirconium, 0.57, uranium, or 0.27, titanium. If the sample size is reduced to 50 pg.) as much as 2070 zirconium, 10% uranium, or 47G titanium can be present and still permit detection of as little as 0.02% thorium. On the other hand, with certain types of samples knonn to be free of interferences, such as purified rare earth compounds, as much as 10 mg. of sample can be used safely extending the detection limit don.n to 10-470. These concentrations nould seem to include most samples to be encountered so that many valuable practical applications can be made without separations and with a high degree of confidence in the results. The other interferences listed in Table I are not likely to be encountered in concentrations high enough to interfere when less than 1 mg. of total sample is used. A single determination can be completed in about 40 minutes if the sample can be dissolved directly in pyrosulfate fusion. If prior fusion with potassium fluoride is neces-

sary, the time required will be about 1 hour. Determination of thorium in bone, urine, rocks, or other samples in which very low concentrations must be measured will necessarily require separations because of the large samples taken for analysis. Because of the refractory nature of the compounds of thorium and associated elements and the inability to dissolve them even in boiling concentrated acids, pyrosulfate fusion is always employed t o ensure their complete dissolution and conversion to a soluble ionic form. illso, solutions containing thorium should not be evaporated to dryness e w e p t under conditions known to prevent hydrolysis or pyrolysis because of the danger of converting the thorium to a refractory oxide that will not dissolve completely in the limited quantity of acid t h a t may be permitted. Sulfate f o r m a relatively stable complex with thorium in acid solution that virtually eliminates hydrolysis or pyrolysis during evaporations. It is also of considerable assistance in preventing formation of insoluble anhydrous sulfates during fuming with sulfuric acid. Air Dusts. Title 10, part 20 of t h e Code of Federal Regulations (10 C F R 20) ( 1 ) requires that the concentration of either soluble or insoluble natural thorium compounds in air be less than 3 x 10-11 pc. per ml. in restricted areas and less than 1 X lopL2pc. per ml. in unrestricted areas. For application to these regulations, the curie was redefined for natural thorium to include 3.7 x 1010 disintegrations per second of the thorium-228 daughter of thorium232. Using the resulting value of 9.0 X 106 pg. per pc. for the specific activity of natural thorium, the corresponding concentration limits in terms of mass are, therefore, approximately 0.27 pg. per liter and 0.009 pg. per liter, respectively. To determine compliance with the regulations by those engaged in processing thorium ores, analytical procedures capable of determining a t least one tenth of these levels are required. Because of the high sensitivity of the procedure, the maximum permissible concentration (MPC) of thorium in restricted areas can be determined with a precision of about 39; with a sample volume of only 2 liters of air. Furthermore, concentrations as low as one fiftieth of M P C can be detected and as high as ten times MPC can be measured without having to change the size of the aliquot. Concentrations ne11 below the maximum permitted in unrestricted areas can also be determined without separations if 60-liter samples are used in the analysis but more caution should be observed in its application. Because beryllium occurs widely in clay dusts u p to about VOL. 34, NO. 8, JULY 1962

961

lows%,the simple and rapid procedure given below for separation of beryllium should be used routinely unless the dust concentration is below about 100 pg. per 60 liters. Dust concentrations u p to about 1 mg. per 60 liters can be tolerated if acetylacetone is used to suppress the beryllium fluorescence. For fairly clean samples, LTet-ash the filter paper with nitric and sulfuric acids in a 125-ml. Erlenmeyer flask and evaporate to fumes. .4dd 2 grams of anhydrous sodium sulfate and heat to a pyrosulfate fusion. If thorium might be associated with siliceous materials, e.g., thorite, or if abnormally dusty samples are obtained, muffle or wet-ash the paper in a 5O-ml. platinum dish. Fuse the residue with anhjdrous potassium fluoride, add sulfuric acid, and transpose t o a pyrosulfate fusion ~ i adescribed previously (10). Boiling with hydrochloric acid to prevent interference of condensed phosphates with extraction of beryllium (12) is unnecessary with the small samples used in analysis of air dusts. Cool the flask Bad add 5 ml. of 10% disodium DTPA, 20 ml. of water, and a drop of 25% sodium metabisulfite to reduce any quadrivalent cerium or other strong oxidants that might be present. Add 1 ml. of 50% TEA, 5 drops of 0.01% quinine sulfate, and 8M sodium hydroxide dropwise with continuous swirling until the bright blue fluorescence visible in ultraviolet light disappears. Warm slightly as necessary to dissolve the cake and any double alkali-rare earth sulfates that might be present. &id additional 8M sodium hydroxide as required to keep the fluorescence extinguished. When dissolution is complete, add 10N sulfuric acid dropwise until the full quinine fluorescence just reappears. Add 1 drop of 25% sodium metabisulfite and allow the solution to atand fsr a few minutes while still warm to ensure reduction of cerium. Cool, transfer the solution quantitatively to a 50-ml. volumetric flask, and dilute to the mark with water. If large quantities of dust were collected, increase the quantity of 10% disodium DTPA sufficiently to complex all metals present, Place an aliquot of the prepared solution corresponding to 2 liters of air into a 25-ml. volumetric flask. Add 5 ml. each of the sodium perchlorateDTPA-TEA, sodium sulfate, and buffer solutions. If many samples are t o be analyzed, add 20 ml. of a mixture of equal volumes of each of the three solutions and one of water. Add 1.00 ml. of morin, mix, dilute exactly to volume, and mix thoroughly. Place the flask in the constant temperature water bath for 20 minutes and measure the fluorescence. If the fluorescence is too high for measurement, smaller aliquots can be taken and the sample i s never lost from this cause. With the restricted range of 5 pg. of thorium resulting from the high sensitivity of the present procedure, the capability of handling a wide range of air concen-

962


trations without loss of sample or duplication of work is very important. Treat 1 ml. of the 250-pg. per ml. standard thorium solution in exactly the same manner as the samples. Measure the fluorescence of a 5-pg. thorium standard obtained from a 1-ml. aliquot of the prepared solution. Carry a blank through a similar procedure. Air samples from 50 liters to 10 cubic meters can be accommodated conveniently by using aliquots of the prepared solution of the air dust of from 2 ml. to 10 pl., respectively. From a 50-ml. volume of solution, the aliquot size in milliliters equivalent to 2 liters of air is simply the reciprocal of the number of hundreds of liters of air collected in the total sample, e.g., 0.2 nil. for 500 liters, etc. EXTRACTION OF BERYLLIUM. If there is any question of the presence of beryllium in concentrations large enough to interfere, pour enough of the prepsred solution into a 25-ml, vial with a Bakelite screw cap with a Poly-Seal liner to fill the vial about one third full. Add an equal volume of chloroform, 1 drop of 8M sodium hydroxide, 4 drops of acetylacetone, and shake the vial vigorously for 2 minutes. An attachment for a commercial shaking machine consisting of an aluminum frame with spring clips has been described for making 40 such extractions simultaneously (11). Allow the phases to separate, add a few drops of chloroform t o remove the drop of beryllium-containing extract remaining at the top of the aqueous layer, and allow the vials to stand uncovered for a few minutes to allow the drop of chloroform t o evaporate. Develop the fluorescence on an aliquot of the aqueous layer as before. CALCULATION OF RESULTS.T o minimize the human error in calculating results, the data are placed on a calculator decimally correct and the answer is taken off the calculator also decimally correct without personal contact with the exponential. Consider the calculated quantity of natural thorium in microcurie per milliliter corresponding t o a net fluorescence of 1 scale division: div'

5.00 pg. X 100.9 - 18.6 sc. div.

The second term is the fluorescence sensitivity in microgram per net scale division, the third term is the volume ratio of total solution to size of aliquot taken for analysis, the fourth term is the specific activity of natural thorium, and the fifth term is the reciprocal of the total volume of air collected. The product of the first four terms gives 0.338 A-, ppc. of thorium in the total sample for each net scale division of fluorescence. If this value is divided

by the number of hundreds of liters of air collected, Le., B in the fifth term, the answer will always be in units of 10-11 pc. per ml. and all exponentials are eliminated from the calculations. sc. div. Thus, 0.338 X Afluorescence, ml. X B answer ( x pc. per ml. As mentioned above, A should be chosen to be the reciprocal of B as nearly as standard pipet sizes will allow. However, the relationship \\-ill seldom be exact and a small correction will need to be applied. The final answer in units of pc. per nil. Kill always be sufficiently close to one third of the net fluorescence reading in scale divisions to make administrative checking very simple and convenient. Water. The concentrations of thorium in n a t e r permitted b y 10 C F R 20 are lo6 times the respective ones for air and correspond in terms of mass to 270 mg. per liter and 9 mg. per liter for restricted and unrestricted areas, respectively. Such high concentrations are determined easily by the present procedure without separations and with virtually no possibility of interference. The same multiples of MPC quoted for air dusts can be obtained on -water even in unrestricted areas with only 50 p1. of sample. The analytical procedure given below ensures that all thorium is solubilized and that the aliquot taken for analysis is truly representative of the sample received. However, the small sample required for analysis in no way lessens the necessity of using good sampling techniques to ensure adequate representation of the original population. Place 50 ml. of water and 3 ml. of concentrated sulfuric acid in a 125-ml. Erlenmeyer and evaporate to fumes. Add 2 grams of anhydrous sodium sulfate, heat t o a pyrosulfate fusion, and finish as described for Air Dusts. Use a 50-pl. aliquot of the prepared solution for development of the fluorescence. Observe the same precautions concerning siliceous material and beryllium in air dusts.

Thorium O r e s a n d Rare Earths. T h e determination of thorium i n the presence of rare earths presents special problems. If fusion with potassium fluoride and/or sodium pyrosulfate is employed t o ensure dissolution of refractory compounds, the lighter and more plentiful rare earths form very insoluble double sulfates with t h e alkali metals that will not dissolve even in strong acids. Fortunately, they can be dissolved by boiling in alkaline DTPA solution. I n the analysis of monazite sand or other phosphates, the high temperature and dehydrating conditions present during fusion convert the orthophosphates to more highly condensed phosphates that interfere with extraction of beryllium by acetylacetone. Boiling with strong acid is

necessary to hydrolyze the sequestrant (1%'). Both orthophosphate and pyrophosphate interfere in the determination of thorium. Homver, no interference should arise in the present applications from either source. The high concentration of phosphat'es present in nionazite sands will be offset b y the limited sample permitted by the thorium prcsent. Other samples n-ill not be affected by their phosphate content with the small sample sizes required to prevent the more serious interferences of zirconium, et?.

THORICMORES. Fuse 50 mg. of sample \\-it11 2 granis of anhydrous potassium fluoride in a 50-nil. platinum dish, add 3 ml. of concentrated sulfuric acid, and 1 gram of anh>-drous sodium sulfate, and transpose to a mixed alkali pyrosulfat,e fusion as described previously ( I O ) , If silica is known to be absent, fuse the sample with pyrosulfate in an Erlenmeyer flask as described for Air Dusts. Clean the outside of the platinum dish if necessary and place it upright in a 400-ml. beaker. Add 10 ml. of 0.2531 disodium DTPA, 1 ml. of 507, TEA, and 20 ml. of water to the platinum dish, and swirl some of the solution out of the dish into the beaker. Heat the solution gently ivith frequent sxirling to facilitate separation of the cake from the sides of the dish and t o complete the dissolution of small pieces of melt still adhering to the dish. Remove the dish and rinse it with about 30 nil. of water. Add 10 drops of 0.01% quinine sulfate to the solution. Khile sn-irling the solution continuously over a long-ware ultraviolet lamp, add 831 sodium hydroxide dropwise unt'il the bright blue fluorescence is extinguished. If much iron is present, the fluorescence will be visible only a t the surface where the ultraviolet light enters the solution. Heat the solution to boiling to complete the dissolution of the cake and add more 8.11 sodium hydroxide until the fluorescence is again extinguished and then 2 drops in excess. Boil the solution gently for 5 niinutes to dissolve an>- double rare earth sulfates that might be present. Add 1 0 s sulfuric acid until the fluorescence is restored and 2 drops in escess. Add 5 drops of 22% sodium metabisulfite and :illon- the solution to stand hot for a t least 5 iniiiutes bo ensure reduction of the ceric-DTPd complex. Cool, transfer to a 100-nil. volumetric flask, and dilute to the mark. If a precipitate of niobic or tantalic acids is present, transfer some of the solution to a dry centrifuge t'ube and centrifuge a t 2000 r.p.m. for 5 minutes before taking an aliquot' for analysis. If more than about I mg. of barium is present, centrifuge the solution before diluting to volume and dissolve the barium sulfate in 10 mi. of tetrasodium DTPA solution (08.3 grams of DTPA and 45 grams of sodium hydroxide per liter) by n-arming in a boiling ivater bath for 5 minut'es. Dilute the alkaline solution to 100 ml. and measure t,he fluorescence on an appropriate aliquot. Transfer an appropriate aliquot of

the prepared solution to a 25-ml. volumetric flask. Add 1 drop of 25% sodium metabisulfite, 5 ml. each of the sodium sulfate and sodium perchlorateDTPB-TEA solutions, neutralize to quinine end point with 1 S sodium hydroxide, and finish as described under Procedure. A 100-,~~1. aliquot will permit precise determination of thorium in concentrations from 10% down to about 0.2% with anything over 0.02% being detectable on only 50 fig. of total sample. Thus, most thorium ores of value can be accommodated without loss of reliability. A 1-ml, aliquot (0.5-mg. snmple) will give maximum precision at the lY0 level and extend the detection limit to 0.00276 n-it'liout significant loss of reliability in most, cases. If sufficient information about the composition of the sample is available, even larger samples can be used. Carry a reagent blank and 2 ml. of the 250-pg. per ml. thorium standard through the same procedure including boiling in alkaline solution. Develop and measure fluorescence on 1 ml. (5 pg. of Th) of the standard but use the same volume of the blank as is taken for the sample. PCRIFIED RARE EARTHS.Place u p to 10 mg. of rare earth oxide containing less than 0.1% scandium, 1% lanthanum, or lOy0 yttrium into a SO-ml. round-bottomed centrifuge tube 1 inch in diameter. Add 1 gram of anhydrous sodium sulfate, 0.5 ml. of concentrated sulfuric acid, and heat t o a pyrosulfate fusion. Cool and add 5 ml. of T d e r and 1 drop of 25% sodium metabisulfite t o reduce quadrivalent cerium or other strong oxidants. Add 5 ml. of the sodium perchlorate-DTPA-TEA solution and 3 drops of 0.017, quinine sulfate. Keutralize iTith 851 sodium hydroxide, dissolve rare earth double sulfates, and reacidify 11-ith 10s sulfuric acid as described for ores. Add only 1 drop of excess sulfuric acid and 1 drop of 25'35 sodium metabisulfite. Let the solution stand hot for a few minutes, cool to room temperature, and transfer t o a 25-ml. volumetric flask. Add 1 drop of 25%, sodium metabisulfite and 1 s sodium hydroxide dropn.ise until the quinine fluorescence disappears. Add 5.00 ml. of the buffer, 1.00 ml. of morin, dilute to the mark, niis, and place in the ivater bath for 20 minutes before measuring the fluorescence. Run a blank and a 5-pg. thorium standard through the same procedure including boiling in alkaline solution. For some unknon-n reason, boiling in alkaline solution causes an

Table II.

increase of about 1 scale division in the blank and a similar decrease in the standard. However, boiling must be kept to a miniinuin because of danger of leaching elements producing fluorescence such as beryllium, zirconium, or thorium from the gl3ss. These errors are relatively unimportant on air dusts and ores when sinall aliquots are taken for analysis but become important when the entire sample is taken for fluorometric measurement. PRACTICAL APPLICATIONS

T o test the applicability of the present procedure to the determinat'ion of thorium in ores, standard thorium samples rvere obtained from the New Brunswick Laboratory of the U. S. Atomic Energy Commission. One sample was a nionazite sand; the others were prepared by dilution of a similar monazite ivith an exact quantity of dunite (essentially a magnesium silicate containing large quantities of iron). The samples mere dissolved completely by fusion n-it'h potassium fluoride and sodium pyrosulfate and appropriate aliquots of the solution analyzed as described using the citrate buffer. As shown b y the results in Table 11, agreement with the certificate values is excellent. KO separations were employed and each analysis required about 1 hour. Since both the monazite and dunite u-ere knorrn to be free of interferences in the sample sizes used, as much as 5 mg. of the most dilute samples was used successfully. Xornially, i t is not advisable to use such large samples. Also, there is no intent to compare either sensitivity or precision of the tn-o lonest concentrations with results obtainable after adequate chemical separations using large samples. Because of the small sample size taken for analysis, the results obtained rrith ores should be coml)letely representative of the precision to be expected with air dusts and water. Yo attempt \vas made to compare results obt,ained by the present procedure a t very Ion levels nit11 those obtained by other procedures. Existing procedures are believed to lack both the sensitivity ssary to test the ies of the present procedure. The internal evidence pre-

Determination of Thorium in Standard Monazite Samples

Sample Taken, llg.

T h Found*

________ d e Div. C' ,Inalyzed Samples 7-A Monazite sand 8 52 o n.mn T,:i 8 F( 41 _ . -i 00 79 lIonazite/dunite i 01 =t0 01 0 500 I 0 9 0 101 i 0 003 15 1 0 100 80 llonazite/dunite 1 00 0 0101 0 0102 + 0 0002 5 00 83 XIonazite/dunite 7 7 0 0013 0 0011 i. 0 0002 5 00 84 Monazite/dunite 1 0 a Values are given at confidence level. b Blank, 18.1 sc. div.; 5-pg. T h standard, 94.0 sc. div.; sensitivity, 0.0659 p g . / s c . div. NBL

Certificate Valuen Th, %

-

I

VOL. 34, NO. 8, JULY 1962

963

sented is thought to be a more unequivocal proof of the sensitivity. precision, and reliability of the method. Since no separations are involved and many elements likely to be encountered can be tolerated in quantities greater than the total sample taken for analysis, precision and accuracy of the samples ought not to be significantly less than that of the pure thorium standards to which they are compared. ACKNOWLEDGMENT

The authors acknowledge the assistance of their associates during many helpful discussions. Special thanks

are extended to C. J. Rodden for the generous gift of the standard thorium samples. LITERATURE CITED

(1) Code of Federal Regulations, Title

10, Part 20, Federal Register, Septem-

her 7. 1960.

( 2 ) Cooper, J. A, Vernon, 11.J., .~nal. Chzm. Acta 23, 351 (1960)

(3) Fletcher, h1. H., Milkey, R. G., ANAL. CHEM.28, 1402 (1956). (4) Intorre, B. J., Martell, A. E., J . Am. Chem. SOC.83,3618 (1961). (5) Lott, P. F., Cheng, K. L., Kwan, B. C. H., ANAL.CHEM.32, 1702 (1960). (6) hknis, O., Manning, D. L., Goldstein, G., Zbid., 29, 1426 (1957).

( 7 ) Milkey, IC. G., Fletcher, AI. H., J . A m . Chem. Soc. 79, 5425 (1957). (8) Sill, C. IT., -4s.i~.CHEW 33, 1584

(1961). (9) IIbzd., b i d . , p. 1579. (10) Zbid., Zbzd., p. 1684. (11) Sill, C. IT., LaFleur, LaFleur,, P. D., G. S. Atomic Energy Commission Document, I D 0 12017, 1961. 112) Sill. C. W..Willis. C. P.. AXAL. .~ .._ \ - -

- - >

(13) Sill, C.’ W.,Willis, C. P., Flygare, J. K., Jr., Ibid., 33, 1671 (1961). (14) Thomason, P. F., Perry, XI. A,, Byerly, F.&I.,Ibid., 21, 1239 (1949). (15) Vosbureh. W. C.. Cooaer. G. R.. J . Am. Ch&. Soc. 63; 437 (1941). RECEIVED for review January 15, 1962. Accepted April 16, 1962.

Separation of Ruthenium from Base Metals by Cation Exchange H. ZACHARIASEN and F. E. BEAMISH Department o f Chemistry, University o f Toronto, Canada

b A method for the separation of ruthenium from large amounts of associated base metals in dilute hydrochloric acid solution b y cation exchange resin is described. The method has also been applied for the determination of ruthenium in a copper-nickel-iron assay button.

T

HE low content of the platinum metals in ores and concentrates necessitates some method of concentration prior to the quantitative determination of these metals. The classical method is the collection of the platinum metals in molten lead. A recent method proposed by Beamish and coFvorkers involves the collection of the platinum metals in a copper-nickel-iron button. The button is dissolved in hydrochloric acid plus a small amount of nitric acid, and the resulting solution is passed through a cation exchange resin under specified conditions, thus separating the platinum metals from the base metals. The platinum metals solution is then concentrated further by evaporation of the effluent to a small volume and is determined either gravimetrically or colorimetrically. This method has proved t o be successful for platinum and palladium ( 6 ) , rhodium (9), and iridium ( I O ) . However, no data for the quantitative separation of osmium and ruthenium from base metals by cation exchange have been reported. The usual method of parting the assay button would finally give a hydrochloric acid solution of the platinum metals

964


with large proportions of iron, copper, and nickel. Methods of separation by precipitation or solvent extraction are inapplicable under these conditions. Initial attempts to separate osmium and ruthenium by ion exchange resulted invariably in some retention of these metals by the exchanger. Thus, the only applicable procedure involved the dissolution of the button by perchloric acid and a subsequent selective distillation of the octavalent oxides ( 5 ) . This method gives good recoveries both for ruthenium and osmium. However, the method introduces difficulties in the subsequent operations if the other platinum metals are to be determined. Among the difficulties are the separation of the base metals from the platinum metals in perchloric acid solution, and the hazard in removal of large amounts of perchloric acid when a perchloric acid fume hood is not available. This paper deals with a quantitative method for separation of ruthenium from base metals in dilute hydrochloric acid solution by cation exchange resin. EXPERIMENTAL

Apparatus and Reagents. FisherLindberg gas-air furnace. Pyro-optical Pyrometer made by Pyrometer Instrument Co. Unicam Spectrophotometer Sp. 500 was used for the spectrophotometric determination of ruthenium. Beckman RIodel A2 p H meter. Ruthenium distillation apparatus : as used by Kestland and Beamish ( I d ) . Dowex 50 X 8, Na-form cation mchange resin, 20- to 50-mesh.

lon exchange column containing 500 grams of wet resin. Synthetic base metal solution was made b y dissolving 800 grams of FeC13.6Hz0, 250 grams of XiCl2.6H20, and 130 grams of CuC12.2Hz0 in 50 ml. of concentrated hydrochloric acid and some water. The solution was filtered and diluted with water to 2 liters. Aliquots of 150 ml. containing approximately 20 grams of base metals were taken. Standard solution of ruthenium was made by dissolving 1.7 grams of amnionium chloro ruthenate, (KH4)2RuC16 (obtained from Johnson, Matthey Co. Ltd.), in about 200 ml. of distilled water and 20 ml. of concentrated hydrochloric acid. The solution was filtered and diluted with distilled water to 1 liter. Upon standardization with thionalide as precipitating reagent the solution contained 0.510 mg. of Ru per ml. Initial experiments carried out in which a chlororuthenate solution was passed through the cation exchange resin Dowex 50 a t p H 1.5 showed that only 80 to 90% of ruthenium was recovered in the effluent. The rest Iva5 retained by the resin. When such a solution was treated with various amounts of sodium chloride the recovery of ruthenium in the effluent increased. However, quantitative recovery of ruthenium in the effluent vias not obtained before the ruthenium solution was evaporated to near dryness in the presence of hydrochloric acid and sodium chloride, then diluted with water and the acidity adjusted to pH 1.0 before the solution was passed through the column. Quantitative recoveries of ruthenium were also obtained when synthetic base

Fluorometric Determination of Submicrogram Quantities of Thorium

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