(12) Milton, C., Eugster, H. P:, “Researches in Geochemistry,” Philip Abelson, ed., pp. 118-50, Wiley, New York, 1959. (13) Mott, R. A., Fuel 29,53 (1950). (14) Powell, A. R., Bur. Mines Tech. Paper 254,21 pp. (1921). (15) Powell, A. R., Ind. Eng. Chem. 12, 887(1920). (16) Powell, A. R., Parr, S. W., Bull. 111, Eng. Exp. Station, Univ. of Illinois, 62 pp. (1919).
(17) Radmacher, W., Mohrhauer, P., Glueckauj 89,503 (1953). (18) Scott, W. W.,“Standard Methods of Chemical Analysis,” 5th ed., N. H. Furman, ed., Vol. 1, pp. 911-12, Van Nostrand, New York, 1939. (19) Smith,. J. W., Bur. Mines Rept.
Invest. 5725, 16 pp. (1961). 120) Stanfield. K. E.. Frost. I. C.. Mc’ Auley, W.S., Smith, H. N.’, Bur. h n e s Rept. Invest. 4825, 27 pp. (1951).
(21) Van Hees, W., Early, E., Fuel 38, 425-8 (1959). RECEIVED for review September 23, 1963. Accepted November 18, 1963. Division of Petroleum Chemistry, 144th Meeting, ACS, Los Angeles, Calif., April 1963. This work was done under a cooperative agreement between the Bureau of Mines, United States Department of the Interior, and the University of Wyoming.
Precipitation of Submicrogram Quantities of Thorium by Barium Sulfate and Application to Fluorometric Determination of Thorium in Mineralogical and Biological Samples CLAUDE W. SILL and CONRAD P. WlLLlS Health and Safety Division, U . S. Atomic Energy Commission, ldaho Falls, ldaho
b Investigation of the toxicology and mineralogy of thorium requires its determination at extremely low concentrations in many kinds of biological and mineralogical materials. An efficient method i s required for separation of thorium from the relatively large samples employed. From 100 to pg. of thorium i s precipitated to better than 99.5% by 25 mg. of barium or other elements forming insoluble sulfates from 75-ml. volume. The separation takes place from strongly acidic solutions and i s not affected appreciably over a wide range of experimental conditions. The barium sulfate i s dissolved in alkaline DTPA and the thorium i s determined directly in the alkaline solution by a fluorometric procedure, which has been applied to the following types of samples with the detection limits indicated: rocks, 2 X bone ash, 5 X lod7%; feces, liver, and grain, 2 x IO-8%; urine, 10-11 gram per ml.; and blood, 2 X 10-lo gram per ml.
D
an investigation into the determination of radium-226 in liquid effluents from mills processing uranium ores ( I ) , results were frequently thousands of times too high when conventional procedures employing coprecipitation of radium sulfate with barium or lead sulfates were used. The extraneous activity was shown to be thorium-230. I n a subsequent investigation on the fluorometric determination of thorium using morin ( 8 ) , barium produced a serious decrease in 622
URING
ANALYTICAL CHEMISTRY
fluorescence of thorium standards due to actual physical loss of thorium in the barium sulfate precipitate. I n both procedures, precipitation of the minute quantities of thorium by barium sulfate was so efficient that the method appeared to have great potential as an analytical separation. Pyrosulfate fusion can be employed for dissolution of refractory materials and at the same time ensure complete dissolution of the thorium itself. The resulting solution containing high concentrations of sulfates is well suited for precipitation of barium sulfate, while keeping other elements dissolved in the strongly acidic solution. Accordingly, the effect of experimental conditions and other elements on the separation was investigated using thorium-234 tracer. Approximately 2 x IO5 c.p.m. and a 5-minute counting time were used on each test to permit all thorium to be accounted for with a standard deviation of 0.1%. Preparation of the tracer and its use by gamma counting in a 3-inch thallium-activated sodium iodide well crystal are described elsewhere ( 7 ) . To determine if the coprecipitation of thorium were peculiar to barium or if other elements forming insoluble sulfates would act similarly, the element being tested was fused with 3 grams each of anhydrous sodium and potassium sulfates and 2 ml. of concentrated sulfuric acid in the presence of the thorium tracer. After cooling, the cake was dissolved in 5 ml. of concentrated sulfuric acid and 70 ml. of hot water. The solution was allowed to stand for about 15 minutes and was then centrifuged at 2000 r.p.m. for 5 minutes. The aqueous supernate mas
diluted to 100 ml. and 50 ml. was counted under the conditions described. With 25 mg. of barium or a n equal number of moles of lead or lanthanum, only 0.2% of the thorium remained unprecipitated. With the same number of moles of strontium, cerium, or praseodymium, or only 5 mg. of barium, the loss was 0.7%. The efficiency is much lower with the more soluble sulfates such as calcium and the double rare earth sulfates above praseodymium. However, m-ith benzidine sulfate, 99.iyo of the thorium mas present in the filtrate, in sharp contrast to the results found with metallic sulfates. The effect of experimental conditions on the barium system was tested using the basic conditions described above, with 25 mg. of barium. The data showed that recovery of thorium is not affected significantly by type or concentration of acid or salts, temperature of precipitation, length of digestion, or order of addition of barium. Recovery of thorium is complete even after a few minutes’ digestion with barium added as preformed barium sulfate. However, a high concentration o f ’ sulfate is required. The separated barium sulfate dissolves easily in alkaline diethylenetriaminepentaacetic acid (DTP-A),from which it can be reprecipitated without coprecipitation of thorium by addition of acetic or even sulfuric acid, provided the acidity is not allowed to become higher than about pH 3. On addition of 10 ml. of 1 to 1 sulfuric acid, the thorium i. again precipitated quantitatively with the barium sulfate. Seither ortho- nor pyrophosphate has significant effect, even if present during
the pyrosulfate fusion if the solutions are precipitated hot tc hydrolyze more highly condensed phos ,hates. Quadraphos (Rumford Cheriical Works) is very effective in preventing precipitation of thorium, but its effect is completely eliminated by boiling the solution for a few minutes. As might be espectei, the concentration of thorium affects the recovery greatly. With 25 mg. of barium, less than 0.5% of the thorium remains unprecipitated when 100 pg. down to at least 10-j pg. are present. When a few milligrams of thorium are present, the distribution rapidly favors the aqueous phase, so that the pre,ent procedure is obviously neither suita ble nor intended for the separation of macro quantities. Since the fluorometric procedure to be used n i t h the present separation uses only 5 pg. of thorium as the standard, the range through which precipitation is complete is a t least 20 times more than is required. Because the mechianism by which thorium is precipitated by barium sulfate is not understood, the procedure described abore was repeated in the presence of most of thiz elements of the periodic table to dettmrmine if any of them might prevent the reaction either entirely or in part. Twenty-five milligrams of barium was w e d and 1 ml. of 307, hydrogen perositle was added to the water in which the cake was dissolved in anticipation of its use to prevent precipitation of niobium and tantalum. If the mised sodium-potassium sulfate flus is used and barium is present during the fuqion, l e v than 0.2% of the thorium remains unprecipitated in the presence of 1 gram of uranium, 500 mg. of iron, copper, or titanium oxide, 250 mg. of aluminum, 100 mg. of beryllium, cobalt, nickel, cadmium, chromium, manganese, c lithium, magneqium, z vanadium, gallium, inclium, or thallium, 50 mg. of arsenic, germanium, silver, selenium, tellurium, or boron, 25 mg. of calcium, tin, antimony, or tungsten, or 10 ma. of scandium or the platinum metal;. The error w a y only 1.2% with 500 mg. of niobium or tantalum pentoxides and 1.7y0 with 50 mg. of yttrium. However, even a few milligrams of bismuth 01' the light rare earths cause recovery of thorium to be seriously incomplete. With rare earths heavier than praseodl mium, the error decreases as the ionic radius decreases and larger quantities are required to interfere. 1 similar error iq produced by relatively larger quantities of hafnium and zirconium. Significantly, the foreign elements are actually present in the barium sulfate lattice and are carried with an efficiency roughly proportional to the elements' abilities to prevent carrying of thorium.
far superior for the determination of sulfate as far as coprecipitation of other elements is concerned and should be more ITidely used. If the barium is added after the pyrosulfate cake has been dissolved in water, loss of thorium is increased drastically, even though the estraneous elements are also carried less efficiently than with the other order of addition. Apparently, if barium is added after the sample has been dissolved, barium sulfate is precipitated completely at the point of entrance of barium into the high sulfate solution and becomes nearly saturated with bismuth or rare earths before being dispersed throughout the solution to precipitate all of the thorium. If barium is present during the fusion, a homogeneous solution of thorium, barium, sulfate, and bismuth or rare earth is obtained. On subsequent dissolution of the cake in dilute acid, barium sulfate is precipitated homogeneously and the thorium is able to compete very successfully with the interfering elements for the available barium sulfate. Potassium ion ($1, 1.33 h.)used in the mised flus increases the distribution of the interfering ions into the barium (f2, 1.43 A) sulfate lattice very markedly in comparison n i t h the sodium ion ($1, 0.98 A\.), but decreases the effect of those ions on the recovery of thorium. The data of Table I show
The coprecipitation can be correlated very well with the charge and ionic radius of the ion. Lanthanum (f3, 1.22 *\.), cerous (f3, 1.18 A), praseodymium (f3, 1.17 A), bismuth ( f 3 , 1.09 A,), uranous ($4, 1.05 A), and ceric ( f 4 , 1.02A.) ions cause precipitation of thorium (+4, 1.10 h.) to be seriously incomplete and are themselves precipitated nearly quantitatively u p to about 1 mg. Elements having either lower charge or smaller radius must be present in much larger quantities to affect precipitation of thorium seriously and are not themselves carried on the barium sulfate nearly so efficiently. However, ions such as zirconium and hafnium (f4, 0.87 -\.) or scandium ($3, 0.83 a.) cannot be removed completely by repeated precipitation from sodium sulfate-sulfuric acid solution when present in small quantity. The effect of ionic radius and charge is particularly noticeable on comparing the errors in Table I produced by quadrivalent uranium with those for the uranyl ion. Other large ions of high charge, such as some of the transuranium element;, will undoubtedly also be coprecipitated efficiently when present in small quantities. It is very evident that precipitation of barium sulfate is neither a precise nor a reliable method for determining either barium or sulfate on unknou-n solutions. Precipitation of benzidine sulfate should be
Table I.
Effect of Other Elements on Recovery of Thorium with Barium Sulfates
Element Zr Hf Bi
La
Quantitv. ., mg.
100 100 10 10
10 10 10 3 1 1 10 10
3
3 1 1
Ce(1V) Ce(II1)
Pr
0.1 3 3 3 3
Kd U(IV) U
No
hTO
Yes No Yes No
Yes Yes
KO
Yes NO
Yes NO
Yes No No
3.5
Yes No Yes
0.8 13.3 3.0 51.0
NO
1 1
Yes
3
Yes
3 3
Yes
1 1000
Yes
3
Thorium in supernate, yo Potassium Sodium present present 1.9 1.8 18.6 1.4 0.3 30.2 77.1 5.4 76.7 21.5 1.6 18.2 0.3 57.7 5.3 36.2 0.4 25.2 56.1
Barium fused? Yes
5.1 0.4 29.5 0.5 11.6 2.0 0.4
No
K O
NO No
VOL.
0.9
77.3 0.3 49.9 2.6 27.7
38.1 0.5
36, NO. 3, MARCH 1964
623
Table II.
Effect of Alkali Metal on Coprecipitation of Zirconium with Barium Sulfate Distribution of Zr, 70 Zr present, Barium Aqueous
Conditions Na2S04-H2S04
mg. 100 10 1 1
0.1
0.01 100
10
1 1 0.1 0.01 1 1
sulfate 2.0
5.1 9.1 3.4" 12.6 4.8 20.3 42.7
21.6" 65.9 82.8
31.1 H2S04-110 salts 3.7 a BaCL added after sulfuric acid-salt solution diluted with water. ( NHa)zSO4-HzS04
that the thorium remaining in the aqueous supernate is much larger when the barium is not present during the fusion or in the absence of potassium. n'hen both conditions prevail simultaneously, the errors are particularly large. Accordingly, whenever possible, potassium should be present and the barium should be added before the pyrosulfate fusion to minimize loss of thorium caused by other elements. If neither can be done, the errors are maximized but are still not very large except for bismuth and the light rare earths. Since other elements besides thorium are coprecipitated by barium sulfate to some extent, some of them might be carried in quantities large enough to interfere with the subsequent fluorometric determination even if they do not affect recovery of the thorium itself. Zirconium was expected to be a particularly serious interference because of its tendency to coprecipitate and because of the sensitivity of its reaction with morin to produce a highly fluorescent complex similar to that produced with thorium. Recauqe potassium had been found to increase the distribution of many elements into barium wlfate, the effect of type of salt and concentration of zirconium on the distribution of zirconium between barium sulfate and the aqueous solution was investigated quantitatively using zirconium-95 tracer separated from its niobium-95 daughter
(4) '
The zirconium carrier and tracer were heated to strong fumes with 7 ml. of concentrated sulfuric acid and 6 grams of sodium sulfate or ammonium sulfate or 3 grams each of the mixed sodium and potassium sulfates in the presence of 1.0 ml. of 6.6% barium chloride solution. After the concentrated acid had been diluted to 75 ml. with water, the barium sulfate was removed by centrifuging, dissolved in alkaline DTP.1624
ANALYTICAL CHEMISTRY
supernate 97.8 94.3 90.8 96.7" 87.1 87.5 95.1 78.8
57.4 78.6" 33.6 16.7 68.4 95.0
TEA solution, and counted along with the supernate. The distribution data are given in Table 11. The percentage of the total zirconium distributing into the barium sulfate increases as the quantity of zirconium present decreases, particularly in the presence of potassium. Consequently, the last of the zirconium becomes increasingly difficult to eliminate. For the minimal quantities of zirconium detectable by the fluorometric procedure, each reprecipitation of the barium sulfate can be expected to reduce the zirconium content by a factor of about 10 in the presence of sodium ion but by less than a factor of 2 in the presence of potassium ion. The ammonium ion is very similar in its effect to pot:issium, while the hydrogen ion is even more effective than sodium in eliminating zirconium from the barium sulfate-Le., maximum decontamination is obtained in the absence of salts. .Addition of barium chloride after dilution of the sample with water further reduces coprecipitation of zirconium by another factor of 2 to 3 in the presence of either sodium or potassium. T o determine the effect of other elements on the fluorometric determination of thorium after the barium sulfate separation, most of the elements of the periodic table were carried through the procedures described below on both blank. and 5-pg. thorium standards arid the fluorescence was measured (Table 111). Results not having a footnote letter in the second column were by a procedure similar to that recommended for rocks and soilLe., a fusion in the presence of both barium and potassium-but the barium sulfate was reprecipitated from sodium sulfate-sulfuric acid in the absence of potassium. The other results were obtained under conditions used with biological materials-Le., a single precipitation of barium sulfate in the
presence of potassium salts, the barium being added after the pyrosulfate cake was dissolved in dilute sulfuric acid. Results obtained in the absence of potassium are also given for a few elements for comparison. The third column gives the approximate quantity that can be present without producing a n error greater than 0.2 scale division on blanks, which is the detection limit for thorium with the instrumentation used, or about 1 scale division with 5 pg. of thorium, corresponding to an error of about 1%. (The errors given in scale divisions can be interpreted using the information given in footnote a,) Whether or not others will be able to detect interference a t the same levels will depend on the sensitivity and precision of the instrumentation employed. The error shown for beryllium, zirconium, aluminum, scandium, and yttrium is due to fluorescence of the residual element not eliminated in the separation and is larger in all cases than the corresponding error due to loss of thorium. The errors with bismuth and cerium result from loss of thorium and are the only ones (including the other light rare earths) in which physical loss of thorium is the limiting factor. The limiting concentration of lanthanum is about the same for either error. The errors due to chromium and uranium result from their serious effect on the thorium fluorescence due to light absorption and complexation of morin ( 8 ) . The fluorescence produced by scandium, yttrium, and lanthanum after the separation is nearly identical to that produced by the same quantity added directly to the fluorescent solution (8), indicating little loss of these elements during the separation. rilthough scandium and yttrium produce little loss of thorium, they act like lanthanum in being carried nearly quantitatively by barium sulfate when present in microgram quantities. The error with the platinum metal group is due entirely to reduction of ruthenium and iridium to black metal which absorbs most of the irradiating light used in the fluorometric measurement. Quantities of strontium, barium, or lead larger than those shown interfere with complete dissolution of the main barium sulfate precipitate in the quantity of alkali and DTP.1 for which the procedure is balanced. The error found with zirconium in the presence of potassium i j much less than was expected from the tracer data of Table 11. *Apparently, chemical discrimination is preventing the zirconium shown by the tracer studies to be present from reacting with morin to produce the fluorescent comples. The most likely explanation is the precipitation of zirconium hydroxide by the alkaline DTPA solution which is not
redissolved by the DYPA. As little as 1 x lo-5yo thorium t a n be detected in 100 mg. of zirconium metal b y precipitating the barium wlfate once in the presence of potassium and twice in the presence of sodium, loss of thorium being only about 2%. Because the barium sulfate separation does not produce significant elimination of lead, strontium, barium, lanthanum, and the light rare earths, or bismuth, scandium, yttrium, and the heavy rare earths when present in small quantity, the quantity permitted in the total sample cannot be larger than the procedure will tolerate mithout separation. Fortunately, in the t5,pe.s of samples for which the present pro3edure is intended, these elements shoLld not occur in quantities larger than the permissible levels shonn in Table 111. The most likely source of error on biological samples is bismuth compounds used for medicinal purposes, where high concentrations may be encountered occasionally, particulai*ly in urine and feces. The rare earth, are the only other elements producing serious losses of thorium during thcl barium sulfate separation and shculd not be encountered frequently. Comparison of the data of Tables I itnd I11 shows that production of fluorescence with scandium and lanth2 num becomes the limiting error rather than loss of thorium caused either by lanthanum or the cerium occurring with it. There is conclusive evidence that the lanthanides with odd atomic numbers are nearly always less abundant than those with even numbers in both cosmic and terrestrial surroundings. I n 55 samples of nine different minxals in which rare earths are known to concentrate, Lyakhovich and Barinski: (3') show only one sample that contained more lanthanum than cerium. Accordingly, simple tests for b?th bismuth and cerium have been mz,de an integral part of the procedures to prove their absence as well as that of lanthanum and most other rare earths and thus restore complete confidence in the results. Many additional data and comments supporting the forc>going conclusions have been published (9), as well as the effect of both experimental conditions and other elements cn the precipitation of thorium b y lead sulfate.
No. 5543, CS 5-60; KO.3486, CS 3-69; b a t h until t h e solid has dissolved. Add 17.0 ml. of 50% triethanolamine and No. 9782, CS 4-96. Reagents. D T P A - T E A SOLUTION. to obtain 100 ml. of solution. Store Dissolve 10.0 grams of dry sodium in a polyethylene bottle and protect from light to prevent accumulation hydroxide in 68 ml. of water and cool thoroughly. Add 20.0 grams of reof color due to decomposition of t h e crystallized diethylenetriamine pentatriethanolamine. SODIUM PERCHLORATE, 831. Dissolve acetic acid and swirl in a cold water
Table 111.
Effect o f Other Elements on Fluorometric Measurement of Thorium after Separation by Barium Sulfate
Quantity, mg. Used Permissible 10 0.3 0.01b 0.01 0.01c 0.01 10 10 1* 0.5
Element Be Zr
1C
500d
Ti02
Ti
1 b
1" 250
A1
5b
5c
sc
10
0.1 0.5b 0.5c 50
Y
1 1*
La
100
1 b
U
1000b
Fe
500 50 500
1
lo*
Co, Ni, Mn, Cd Co, Ni, Mn, Cd Cs, Rb, Li Cs, Rb, Li Mg, Zn, Mo, V Mg, Zn, Mo, V As, Ge, Ag As, Ge, Ag In, T1 Ga In, T1, Ga Sn, Sb, W Se, Te, B, Hg Se, Te, B, Hg Au, Pt, Pd, Ir Os, Rh, Ru NbzOb
1 )'
Taz05 Ba
25 25 * 100 ea. 10 eaeb 100 ea. 10 ea.b 100 ea. 10 ea.* 50 ea.
'3ifi
25 ea. 5 ea.* 50 ea. 5 ea.* 10 ea. 500d 5* 500d
5b 5
5 b
EXPERIMENTAL
Sr
5 5b
All instrumentation, including the primary and secondary color filters used in the measurement of fluorescence, is exactly as described previously (8). However, the glass numbers formerly used to identify polished Corning filters in standard thickn3ss should be replaced with color specification numbers as follows: KO. 3387, CS 3-72; Instrumentation.
Pb
25
$0.3
+0.5
+0.2
-0.1
-0.3
+0.5 0.0
-0.6
f1.4 +0.3
$0.9
$61.1
+61.0 $0.6
0.0
0.03 0.03 0.03
+0.9
+3.0
+3.2 +184.
...
$0.1 +0.3
1 1 1
+0.3 +0.3
+o. 1
...
0.1
1"
Ca
1 1 35 5 5
1
1 1
cu
250
IC
l b
Ce Bi Cr
1
Error, sc. div.a Blanks 5 pg. Th +7.5 +5.8
0.0
+2.4
0.1 0.15 0.25
+o. 1 +0.7
+2.0
25
+1.3 -0.4 0.0 +0.6
-4.4 -3.1 -2.4
200
-0.4
-5.3
$0.6
-0.4 -2.0
0.25 1
1
100 25 500 10 15
10
+0.8
0.0
+0.4
+O.l
-0.8
-0.8 -0.3 -0.1 $0.3 -0.9 -0.8
100 ea. 10 ea. 100 ea. 10 ea. 100 ea. 10 ea. 50 ea. 10 ea. 100 ea. 1 50 I 10 ea. 10 ea. 5 ea. 50 ea. 5 ea. 0 . 3 ea.
+o. 1
+2.0
-36.7
500 5 500 5 5 5 5
+0.3
+1.6
-0.1
-0.9 +0.7
5 25
0.0
-0.6 -0.3
+0.5
+0.5 -0.4 0.0
0.0 $0.4
+0.2 +0.4
$0.7 +O. 1
+0.5
-1.2
+0.3 +o. 1 +2.1
-0.3 -0.5 -1.2
$0.1
+0.6 -0.6 -0.6 -0.3 -0.3
-0.1 5b 5 Blank 14.5 sc. - v.; 5 rg. Th standard, 92.- sc. div.; sensitivity, 0.0644 pg./sc.,div. Differences larger than f 0 . 2 sc. div. on blanks or 1 sc. div. on standards probably indicate significant interference of separated substance. b Single precipitation of barium Fulfate in presence of potassium with barium added t o aqueous solution.
c Single precipitation of barium sulfate in absence of potassium with barium added to aqueous solution. 5 M1. of 30% H202 added.
VOL. 36, NO. 3, MARCH 1964
625
500 grams of anhydrous sodium perchlorate in 300 ml. of water to obtain 500 ml. of solution. Add 5 drops of 25% sodium metabisulfite to ensure reduction of any hypochlorite that may be present. -Idd 10 drops of 0.01% quinine sulfate and then IN sodium hydroxide with continuous stirring near a 100-watt ultraviolet lamp until the bright blue fluorescence disappears. Filter the solution through a double glass-fiber filter paper in a small Buchner funnel. Store the solution in a borosilicate glass bottle with a polyethylene-lined screw cap. PIPERIDINE BUFFER. Prepare exactly as described previously (@, but use 87.0 ml. of redistilled piperidine instead of 100.0 ml. BARIUMCHLORIDE,6.6%. Dissolve 33.3 grams of barium chloride dihydrate in water and dilute to 500 ml. One milliliter contains 37.5 mg. of barium. Prepare standard thorium and morin solutions as described previously ( 8 ) . Sample Preparation. T h e present procedure is particularly suited for biological samples and for certain mineralogical types such as rocks, meteorites, etc., in which the thorium concentration is sufficiently small t o require separation but the concentration of interfering elements is also expected to be small. T h e barium sulfate separation is inadequate for application to rare earth minerals or a n y unknown material in which the quantity of interfering element might exceed the permissible levels derived from Tables I and 111, whichever is limiting. With the exception of bone, all organic samples are wet-ashed in borosilicate glass vessels to avoid contamination from both spattering and corrosion of the container that occurs when several samples are muffled simultaneously, particularly if conventional porcelain evaporating dishes are used. Bone is dry-ashed in porcelain dishes to avoid the corrosive action on expensive platinum containers. However, that part of the bone coming in contact with the dish must be cut out and discarded, since it could contain thorium from corrosion of the refractory dish.
As shown in previous work ( 8 ) , 1 mg. of silicon produces about 30% error in the determination of 5 pg. of thorium if present as silicate. If the silicate is dehydrated to silica but allowed to remain in the solution after addition of alkali, the error is still about 7% and will undoubtedly be larger in the present procedure because the dehydrated silica will be dissolved more completely at the higher temperature and alkalinity employed. Since silicate does not precipitate thorium in alkaline DTP.2 solution, the decreased fluorescence obtained in the presence of thorium but not on blanks is probably due to competition of the silicate for the bonds occupied by morin in the 1 to 626
ANALYTICAL CHEMISTRY
1 to 1 thorium-DTPX-morin complex (8). Use of platinum containers for pyrosulfate fusions requires certain precautions. If the temperature is too high or the duration of the fusion is too long, a few milligrams of platinum are dissolved that can interfere seriously in several ways. If unoxidized gases or carbon particles from the burner are allowed to enter the melt during fusion, the platinum is reduced to the metal, which is collected quantitatively in the barium sulfate. Since it is not redissolved in the alkaline DTP*I with the barium sulfate, the black elemental platinum interferes seriously in the subsequent fluorometric measurement by absorbing both the exciting and emitted radiation. If present in the quadrivalent state, the platinum is carried on the barium sulfate, producing a light buff to yellow color that is not eliminated efficiently by reprecipitation. The occluded platinum causes the barium sulfate to form a colloidal suspension during the washing with 0.1Ji sulfuric acid that is difficult to centrifuge clear. It also prevents complete dissolution of the barium sulfate in the alkaline D T P h and errors due to both turbidity and incomplete dissolution of thorium result.
ROCKSAND SOIL. Place 0.5 gram of finely powdered siliceous material in a 50-ml. platinum dish and moisten the sample with 1.0 ml. of 6.6% barium chloride solution. Immediately swirl the solution gently until a free-flowing suspension is obtained. (If the sample is allowed to cake, complete dissolution becomes very dificult or impossible in some cases.) Add 1 ml. of 48% hydrofluoric acid dropwise around the sides, washing down any sample swirled onto the vertical part of the dish, evaporate gently to dryness, and fuse with 3 grams of anhydrous potassium fluoride. Add 5 ml. of concentrated sulfuric acid and 3 grams of anhydrous sodium sulfate and transpose to a mixed alkali pyrosulfate fusion as described previously (6). Be particularly careful to heat the solution just hot enough and long enough to transpose the potassium fluoride cake completely to minimize dissolution of platinum. Also, use a maximum of air in the blast lamp even with very small flames to minimize reducing gases. Clean the outside of the dish if necessary and place it upright in a 400-ml. beaker. Add 5 ml. of concentrated sulfuric acid and 30 ml. of water containing 1 ml. of 30% hydrogen peroxide to the dish, swirl some of the acid into the beaker, and heat the solution to boiling. Swirl as required until the main cake becomes detached from the dish and all small isolated pieces of the cake around the sides have been dissolved. Remove the dish and rinse it with 25 ml. of water. Boil the solution for a few minutes to dissolve the cake and any anhydrous sulfates of iron or aluminum that might
be present, leaving only a fine turbidity of the barium sulfate. If silica is known to be absent, omit the potassium fluoride fusion and fuse the sample in a 250-ml. Erlenmeyer flask in the presence of 1.0 ml. of barium chloride solution with 3 grams each of anhydrous sodium and potassium sulfates and 2 ml. of concentrated sulfuric acid until a clear melt is obtained. Add 5 ml. of Concentrated sulfuric acid and 55 ml. of water containing 1 ml. of 30% hydrogen peroxide and boil. With solutions, evaporate a n appropriate sample to dryness in a 250-ml. Erlenmeyer flask and fuse as described. If large quantities of niobium or tantalum are present, as in oxides or high-grade minerals, redissolve the pyrosulfate cake in 10 ml. of hot concentrated sulfuric acid and cool to about 30" C. =idd 25 ml. of water containing 5 ml. of 30% hydrogen peroxide, followed by 25 ml. of m r m water not hotter than about 50" C. Allow the solution to stand with occasional swirling until all soluble salts have dissolved. (Tantalum is hydrolyzed rapidly a t the boiling point, the quantity increasing with time of boiling and decreasing acidity. Titanium and niobium are not hydrolyzed in strongly acid solutions of hydrogen peroxide even on prolonged boiling.) Transfer the hot solution to a 90-ml. round-bottomed borosilicate glass centrifuge tube with a pouring spout, and rinse the container with enough water to give a total volume of about 7 5 ml. Mix thoroughly by swirling the solution vigorously until the agitation reaches the bottom of the tube. Allow the solutions to stand without further heating for 10 minutes, then centrifuge at 2000 r.p.m. for 5 minutes. Decant the aqueous solution carefully, SO that the barium sulfate cake is not disturbed, and allow the tube to drain thoroughly. K a t c h the cake closely as the last of the solution drains off, to make sure that none of the barium sulfate is poured off with the supernate. Discard the aqueous solution. Add 7 ml. of concentrated sulfuric acid and 3 grams each of anhydrous sodium and potassium sulfates to the centrifuge tube containing the barium sulfate. Heat the tube gently over a blast burner until the sulfuric acid boils and a clear solution is obtained. Use a face shield and swirl the solution continuously with the tube pointed away from personnel to prevent bumping and possible injury. Remove the tube from the flame and allow to cool for about 30 seconds. h d d 5 drops of 72% perchloric acid down the sides and cool to room temperature. Examine the solution for a yellow color that might indicate the presence of cerium.
ri yellow color is generally produced by perchloric acid in hot sulfuric acid solution due to gaseous decomposition products, but disappears on standing for a fern minutes or on vigorous swirling or cooling. I n the presence of a few milligrams of iron or titanium, the solution is bright yellow while hot but
becomes virtually colorless at room temperature. The color of cerium does not disappear on cooling and 0.1 mg. will give a detectable light yellow color. If the solution is color less, as is generally the case, absence of :ignificant cerium, and consequently lanthanum and other light rare earths, is amured. Heat the sulfuric w i d solution back to boiling to destroy residual perchloric acid and add about 25 mg. of hydrazine sulfate to reduce any small permissible quantities of cerium that would otherwise interfere by oxidizing the morin reagent. Cool the :,ohtion and add 20 ml. of water containing 5 drops of 3Oy0 hydrogen peroxide, rinsing the walls of the tube as the solution is added. (There is 110 spattering or boiling, if the sulfuric acid is cooled to a t least 40" C. before the water is added.) Fill the tube to 75 ml. with water, mix thorougl-ly, and allow to stand for 10 minuteiq. Centrifuge the solution at 2000 r.p.m. for 5 minutes, decant carefully, and discard the supernate. If the barium sulfate contains more than a slight yellow color of peroxytitanic acid, rvprecipitate i t one additional time. Rinse the walls of the tube with 25 ml. of 13.1.11 sulfuric acid and add 2 drops of 25%, sodium metabisulfite. Swirl the tube vigorously to suspend the precipitate and centrifuge for 5 minutes. Discard the aqueous wash solution. Add 1.5 ml. of the DTPA-TEA solution to the washed precipitate and disperse the barium sulfate in the solution. Since swirling is usually unsuccessful, use a glass stirring rod to smear the tightly packed lumps on the walls of the tube. (If the barium sulfate is not dispersed completely, dissolution will be incomplete or greatly retarded.) Add 8 ml. of water, rinsing the stirring rod, and place the t L bes in a beaker of boiling water for 5 minutes with occasional swirling. (The barium sulfate will dissolve completely within 2 minutes if the precipitate is dispersed completely.) Roll the l o t solution carefully around the sides of the tube to dissolve any barium sulfate that might have deposited theime, cool to room temperature, and transfer to a 25-ml. volumetric flask. Rinse the walls of the centrifuge tube UTt h two consecutive portions of water, not exceeding 2 ml. each or the final volume will be too large. Add the wajhes to the volumetric flask. If the solution was not completely clear, centrifuge the main solution and each oj the 2-ml. washes before decanting into the volumetric flask. (If not removed, the turbidity will cause serious errors due to scatter or absorption of light, particularly if some platinum mere dissolved inadvertently during the pyrosulfate fusion and later reduced to the hlack metallic state during the treatment with hydrazine.) Add 5 ml. of 8.11 sodium perchlorate, 5 ml. of the buffer solution, and 1 ml. of morin, swirling the flask after each addition. Dilute tc volume, stopper the flask, and mix 1,horoughly. Place the flask in a constant-temperature water bath for 20 minutes and measure
the fluorescence as described (8). Check the solution for presence of bismuth by adding 3 ml. of a freshly prepared 5T0 solution of stannous chloride in 15% sodium hydroxide to the remaining solution from the fluorometric measurement. -4 distinct black turbidity will develop within 1 or 2 minutes if as much as 0.1 mg. of bismuth is present. Carry a blank and a 5 - ~ g .thorium standard through the entire procedure, using the same batch of reagents. Subtract the instrument reading obtained on the blank from that obtained on both standard and samples and calculate the thorium content of the samples from the sensitivity value obtained from the standard. BONE. Add 5 ml. of 72% perchloric acid and 5 ml. of a ferric perchlorate solution containing 1 mg. per ml. of iron to 2 grams or less of bone ash in a 250-ml. beaker. Cover the beaker with a watch glass and boil the solution until the ash has dissolved completely except for a small quantity of silica and all organic matter has been oxidized. Remove the cover glass and allow the excess perchloric acid to evaporate until the total volume has been reduced to about 2 ml. but do not allow t o dry. Cool the solution, add 60 ml. of water and a boiling chip, and heat t o boiling. Add 4 drops of 0.1% tetrabromophenolsulfonephthalein followed by a saturated solution of sodium acetate dropwise with continuous swirling until the indicator changes to the first distinct blue color in the greenish solution to precipitate ferric and thorium phosphates. (The color will appear reddish if the solution is viewed in tungsten light. If the p H is permitted to go much higher, too much calcium phosphate will be precipitated.) Boil the solution vigorously for 2 minutes and transfer to a 90-ml. roundbottomed centrifuge tube. Rinse the beaker with about 15 ml. of water and add the rinse to the tube. Mix the solutions by swirling the tube rapidly and centrifuge a t 2000 r.p.m. for 2 minutes. Decant the supernate carefully and discard. Add 1 ml. of concentrated hydrochloric acid and 5 ml. of water to the beaker in which the precipitation was made and swirl the solution around the sides until all the phosphate precipitate has been redissolved. Pour the hydrochloric acid solution into the centrifuge tube and swirl until the main precipitate has been dissolved. Rinse the beaker with 50 ml. of water and add the rinses to the tube. Do not transfer the boiling chip. .Idd 3 drops of 85% phosphoric acid and place the tube in a beaker of boiling water. When the solution is nearly boiling, add 4 drops of 0.1% tetrabromophenolsulfonephthalein indicator and repeat the precipitation with sodium acetate while swirling the tube rapidly. Place the tube back in the boiling water bath for 10 minutes and centrifuge again a t 2000 r.p.m. for 2 minutes. Discard the supernate. Add 2 ml. of concentrated sulfuric acid and 3 grams each of anhydrous sodium and potassium sulfates to the centrifuge tube and heat gently over a
blast burner until the water has been expelled and the sulfuric acid begins to fume. Increase the heat and swirl continuously until excess sulfuric acid has been expelled and a clear yellow pyrosulfate fusion is obtained, to ensure complete dissolution of all refractory thorium compounds. Cool the tube, add 7 ml. of concentrated sulfuric acid, and heat gently to boiling over the blast lamp to redissolve the pyrosulfate cake in the concentrated acid. ridd 5 drops of 72% perchloric acid, cool the solution to room temperature, and examine i t for the yellow color of quadrivalent cerium. ;Idd 30 ml. of water containing 5 drops of 30% hydrogen peroxide, mix thoroughly, and heat the solution in a boiling water bath for 5 minutes to hydrolyze condensed phosphates and dissolve all soluble salts. If more than about 0.5 mg. of silica appears to be present, filter the hot solution through a double ll,'s-inch Whatman S o . 41 filter paper in a S o . 000.1 Hirsch funnel containing a small quantity of paper pulp. Use only slightly reduced pressure until the solution has cooled sufficiently so that the solution does not boil and spatter. Wash the filter with about 20 ml. of water. If much insoluble material remains or if there is any question about its possible thorium content, treat the paper as described below for feces and add the resulting solution to the main filtrate. Heat the solution back to about 50" C., and transfer it to a clean 90-ml. centrifuge tube to which has been added 1.0 ml. of 6.6% barium chloride solution. Rinse the flask with sufficient water to bring the volume to 75 ml., mix thoroughly by srrirling vigorously, and allow to stand for 10 minutes. Centrifuge the solution a t 2000 r.p.m. for 5 minutes, decant carefully, and discard the supernate. Complete the determination as for rocks and soil, beginning with the washing of the barium sulfate with 0.1Jf sulfuric acid. A single precipitation of barium sulfate is generally adequate for bone, urine, tissue, blood and grain but should be repeated with feces. SEPARATIOX O F BISMUTH. If the stannite test indicates the presence of bismuth in any of the biological samples, repeat the analysis and add about 100 mg. of thioacetamide to the boiling solution just before filtration of the silica. Boil the solution for 2 or 3 minutes, filter, add 1.0 ml. of barium chloride solution, and evaporate to fumes of sulfuric acid until all barium sulfate has dissolved. =idd a drop of 727, perchloric acid to oxidize all organic matter. Cool, add 20 ml. of water containing 5 drops of hydrogen peroxide, and finish as described for rocks and soil from the second precipitation of barium sulfate.
FECES.The most important problem in the analysis of feces results from the presence of phosphate. If the sample is dry-ashed or is allowed to dry and bake during wet ashing, the phosphoric acid is conrerted to a white solid of unknown composition that is extremely insoluble VOL. 36, NO. 3, MARCH 1964
627
and contains most of the components of the sample. For example, when carrierfree thorium-234 tracer was evaporated to dryness with only 3 drops of 85y0 phosphoric acid, over 95% of the thorium was present in the insoluble material formed even after extensive boiling with concentrated hydrochloric acid. The material can be dissolved and the thorium recovered by fusing with pyrosulfate. However, corrosion of the glass container is extensive and the consequent contamination of the sample prohibits baking of phosphatecontaining samples in glass containers prior to trace analysis. Because of the large quantities of calcium and potassium normally present, sulfates should not be added until after removal of calcium and the concentration of perchloric acid should be kept as small as possible. Also, aluminum has been encountered in unexpectedly high concentrations (probably from medicinal use of antacids) and is precipitated quantitatively along with the iron and thorium phosphates. If not removed prior to the pyrosulfate fusion, anhydrous aluminum sulfate is produced t h a t is difficult to redissolve completely when present in large quantity. Add 50 ml. of concentrated nitric acid to 50 grams of feces in a 1-liter Erlenmeyer flask and heat gently with occasional swirling until frothing subsides. Evaporate the solution nearly to dryness, add 50 ml. of nitric acid, and repeat the evaporation. Add 50 ml. of nitric acid and 25 ml. of 72% perchloric acid and evaporate again to fumes of perchloric acid. (Shortly after fuming begins, the solution will usually char and turn black and should be cleared immediately by the addition of a few milliliters of concentrated nitric acid. If much of the organic matter is allowed to be oxidized by the perchloric acid rather than nitric, other organic compounds are produced that are much more resistant to further oxidation and the time required will be significantly increased.) Continue boiling the solution until essentially all of the organic matter has been destroyed and the solution is nearly colorless. Evaporate as much of the excess perchloric acid as possible by alternately heating and swirling the flask, but do not permit the solution to dry and bake even in local spots a t any time during the entire decomposition. Add 25 ml. of water, 5 ml. of ferric perchlorate solution containing 1 mg. per ml. of iron, and 3 drops of 85% phosphoric acid and boil the solution gently until the cake has disintegrated. Transfer the solution and insoluble material into a 250-ml. beaker with 30 ml. of water, heating the wash water as necessary to dissolve the potassium perchlorate and calcium sulfate. Add a boiling chip, cover the beaker with a watch glass, and boil for a few minutes to dissolve all soluble salts. Complete the determination as described for bone, beginning with the precipitation of
628
ANAWTICAL CHEMISTRY
ferric phosphate from the hot solution and including a reprecipitation of the barium sulfate as described for rocks and soil. Also, if much aluminum might be present, boil the second phosphate precipitate for 2 or 3 minutes with 3 ml. of 8.U sodium hydroxide and 25 ml. of water, centrifuge, and discard the supernate before proceeding with the pyrosulfate fusion. If sufficient insoluble material is present to require additional treatment, place the filter paper containing the insoluble material filtered off after the pyrosulfate fusion as described under bone in a 25-ml. platinum crucible. Heat the crucible gently over a blast lamp until the paper has dried and ignited, then increase the heat until all carbon has been burned off. Cool the crucible, add 0.5 gram of anhydrous potassium fluoride (6), and heat over the blast lamp until the salt has fused and all insoluble material has dissolved. If large quantities of silica are present, evaporate to dryness with 48% hydrofluoric acid before making the potassium fluoride fusion. Cool the crucible, add 1 ml. of concentrated sulfuric acid, and heat gently over the blast lamp until the excess sulfuric acid has been volatilized and a clear pyrosulfate fusion is obtained. Cool the crucible, add 5 ml. of water, and warm on a hot plate until the cake dissolves completely. Combine the solution with the main filtrate just before precipitation of barium sulfate. URINE. To eliminate the use of strong acids at the collection station, send samples to the laboratory without acidification. Transfer the entire solution collected to a beaker of appropriate size, rinse the original container with 5 ml. of concentrated hydrochloric acid, and add the rinse to the main solution to recover any thorium that might have precipitated on the walls of the container during the intervening period. For 1000-ml. samples, place the samule in a 2-liter beaker and add 5 ml. bf a ferric perchlorate solution containing 1 mg. per ml. of iron and a boiling chip. Cover the beaker with a watch glass and heat the solution to boiling. -4dd 8M sodium hydroxide with continuous stirring until copious precipitation of calcium phosphate occurs and add 10 ml. in excess. Leave the cover glass slightly askew to prevent frothing and boil the solution vigorously for 10 minutes. (The solution must boil actively or precipitation of thorium will be incomplete by a t least 2 or 3%. The solution can be filtered immediately if desired, but filtration of the entire solution is tedious and timeconsuming when large volumes are taken for analysis. If the precipitate is allowed to settle for 30 minutes, most of the supernate can be decanted and discarded without losing a significant quantity of the precipitate. Cooling the sides of the beaker to eliminate thermal currents will greatly facilitate settling of the precipitate. Loss of thorium is directly proportional to the loss of total precipitate and need never exceed I%.) Filter the remaining solu-
tion through a 12.5-cm. paper in a 60' funnel containing a small quantity of Paper Pulp. Rinse the 2-liter beaker in which the ureciuitation was made with 5 ml. of concentrated nitric acid and three 5-ml. portions of 72% perchloric acid to recover the calcium phosphate adhering to the walls. Cse a stirring rod to work the acid around the sides of the beaker. Transfer each wash consecutively to a 250-ml. beaker in which the filter paper containing the main precipitate has beenplaced. Do not transfer the boiling chip. Cover the beaker with a watch glass and heat slowly to fumes of perchloric acid. (The solution may bump if heated too rapidly before the acids have become sufficiently concentrated to dissolve the paper, particularly if the sample contained large quantities of calcium phosphate. When most of the nitric acid has been expelled, the solution will frequently turn black and should be cleared immediately by the addition of several drops of concentrated nitric acid and repeated if necessary. Sometimes the organic matter does not char but is oxidized vigorously with copious evolution of fumes of perchloric acid. Although the reaction is not violent, organic compounds resistant to further oxidation are sometimes produced when perchloric acid is used alone.) Add nitric acid as required to moderate the reaction and to assist in the oxidation of the last traces of the organic matter. Remove the cover glass and allow the perchloric acid to evaporate to a volume of about 2 ml. Add 30 ml. of water and 3 drops of 85% phosphoric acid and heat to boiling to dissolve all soluble salts. Transfer the solution to a 90-ml. centrifuge tube with 20 ml. of water, and place tube in a boiling water bath until the solution is nearly boiling. Finish the determination as described for bone, beginning with the reprecipitation of ferric phosphate by addition of sodium acetate. -1single precipitation of ferric phosphate is adequate for separation of calcium from urine samples. TISSUE. Place 50 grams of muscle, liver, or other tissue in a 1-liter Erlenmeyer flask and add 15 ml. of concentrated sulfuric acid. Place the flask on a hot plate and evaporate carefully to fumes of sulfuric acid to remove water and to char the organic matter, but do not permit volatilization of much sulfuric acid. Swirl the solution or adjust the temperature of the hot plate as required to minimize bumping and spattering. Remove the flask from the hot plate and allow the solution to cool for about 5 minutes. Aidd50 ml. of concentrated nitric acid and evaporate the solution until dense fumes of sulfuric acid just appear and the organic matter has charred again to a thick black solution. Repeat the addition of 50-ml. of nitric acid and evaporation to dense fumes of sulfuric acid two additional times, allowing the organic matter to char extensively each time. Finally, add 25 ml. each of concentrated nitric and 727, perchloric acids and evaporate until the perchloric acid has been removed.
When the nitric acid has been driven off, the remaining organic matter is oxidized rather vigorously by the perchloric acid, but per:)onal attention is generally not required. However, a considerable amount of boiling is required to complete the oxidation. When osidation is complete, the solution should have a pale to deep greenish yellow color, depending on the iron content. If red or orange tints are present, organic m a t k r is probably still present. If the solution is red, add a 1 to 1 mixture of nitric and perchloric acids until the color l i g h t m to yellow or light orange. Add 3 grams each of anhydrous sodium and potassium sulfates and complete t h e determination as described for bone, beginning with the pyrosulfate fusion. ( A small quantity of organic matter can be osidized during the pyrosulfate fusion, but even moderate quantities will t a m e severe frothing and excessive time for complete osidation in the pyrosulfate fusion. When considerable charring is evident in the fusion, cool the flask, add 5 ml. of concentrated sulfuric acid and 2 or 3 ml. of 1 to 1 mixiure of nitric and perchloric acids, and re-evaporate to a fusion .) If large samples are not available or if the greater sensitivity obtainable f r m i their use is ~niiecessary,the decomposition can be simplified considerably. Place 10 grams or less of tissue in a 500-ml. Erlenmeyer flask. Treat as described, using one evaporation to fumes with 10 ml. of concentrated sulfuric acid, one evaporation with 25 ml. of ccncentrated nitric acid, and one evaporation with 25 ml. of nitric and 15 ml. of 72% perchloric acids. BLOOD. Treat 50 ml. of blood by the procedure described for tissue, using only 10 ml. of concentrated sulfuric acid for the initial evaporation, one evaporation to dense fumes with 50 ml. of nitric acid, and one evaporation with 50 ml. of nitric and 10 ml. of 72% perchloric acids. After decomposition of the organic matter, the solution will have a deep yellow cclor due to the iron present. Ten milliliters or less of blood can be oxidized in thv same way except that only one evaporation with 10 ml. each of Concentrated nitric and 72% perchloric acids and 5 ml. of concentrated sulfuric acid is required. GRAIN. Treat 50 grams of grain by the procedure described for tissue, but omit the initie 1 evaporation to fumes with sulfuric acid and make three evaporations to dense fumes each with 100 ml. of concentrated nitric acid in the presence of 15 ml. of concentrated sulfuric acid, and one evaporation with 50 ml. of nitric and 25 ml. of 72% perchloric acids. Grain reacts much more readily than tissue. After the first evaporation to fumes, the reaction with the charred grain is self-sustaining and the flask should not be placed on the hot plate
until the copious evolution of dark reddish brown fumes has abated and most of the carbonized solids have redissolved. Otherwise, the reaction will proceed too vigorously and severe frothing and loss of sample will result. Ten grams of grain can be handled in a 500ml. Erlenmeyer flask with one evaporation with 50 ml. of nitric acid and 5 ml. of concentrated sulfuric acid to strong fumes and charring followed by one with 25 ml. of nitric and 10 ml. of 72% perchloric acids. RARE EARTHS.Since even small quantities of rare earths interfere seriously with recovery of thorium by barium sulfate, a special procedure must be used for their removal before the barium sulfate separation can be applied. I n the following procedure, pyrosulfate fusion is used t o ensure dissolution of all refractory compounds of thorium as well as the difficultly soluble osides of lanthanum and cerium. After separation of the alkali sulfates, the thorium and rare earth hydroxides are dissolved in perchloric acid and the thorium is estracted into thenoyltrifluoroacetone (TTA). The organic extract iq decomposed and the barium sulfate separation is applied. The procedure should be generally applicable to all kinds of samples containing rare earths as well as to the pure osides. Scandium interferes. Fuse 0.5 gram of rare earth oxide with 3 grams each of anhydrous sodium and potassium sulfates and 2 ml. of concentrated sulfuric acid in a 250-ml. Erlenmeyer flask. (The maximum temperature obtainable from a blast lamp will generally be required.) Fuse siliceous materials with potassium fluoride as described for rocks and soils. Add 50 ml. of water to the cool pyrosulfate cake and heat gently to boiling to avoid bumping caused by the dense crystalline precipitate of double alkalirare earth sulfates. When the cake has disintegrated completely, add 4 drops of 0.1% o-cresolsulfonephthalein indicator followed by ammonium hydroxide until the purple alkaline color is obtained and about 0.5-ml. excess has been added. Digest hot for a few minutes and transfer to a 90-ml. roundbottomed centrifuge tube. Centrifuge a t 2000 r.p.m. for 2 minutes and discard the supernate. Transfer the precipitate back to the Erlenmeyer flask with 50 ml. of water and add 5 ml. of 72y0 perchloric acid. Boil the solution until clear, adding a few milligrams of hydrazine sulfate as necessary to reduce cerium, and repeat the precipitation of thorium and rare earths with ammonium hydroxide to remove the last of the alkali sulfates. Centrifuge and discard the supernate. Add 2 ml. of 72% perchloric acid and 5 ml. of water to the Erlenmeyer flask in which precipitation was made, adding the solution around the sides of the flask t o dissolve the small quantity of hydroxides remaining there. Transfer the acid to the centrifuge tube
and rinse the flask with 10 ml. of water. Heat the solution gently until the precipitate has dissolved completely. Cool and transfer the solution to a 125-ml. separatory funnel, rinsing the tube with 20 ml. of water. If the precipitate does not dissolve completely, as happens frequently with 0.5-gram samples of ceric oxide, centrifuge the solution and decant the supernate into the separatory funnel. Add 1 ml. of concentrated nitric acid to the residue and evaporate to near dryness. h d d 2 ml. of water and a few milligrams of hydrazine sulfate and heat for a few seconds. Cool, transfer to the separatory funnel and rinse the tube with the 20 ml. of water. (The solution must be clear, or an emulsion will result during the subsequent extraction.) If iron is present, add about 100 mg. of solid ascorbic acid to the cold solution to reduce the iron. Add 4 drops of 0.1% o-cresolsulfonephthalein indicator followed by a saturated solution of ammonium acetate dropwise until the last trace of the orange acid color of the indicator changes to its yellow form (pH ca. 2.0). Extract the solution with two consecutive 5-ml. portions of 0.5.11 T T X in chloroform, shaking vigorously on a mechanical shaker for 5 minutes each time. .ifter each extraction, add 5 ml. of straight chloroform, draw off about 2 ml. to rinse the concentrated extract from the funnel stem, then shake for 5 seconds and add the chloroform to the extracts. Shake the combined extracts for 1 minute with 10 ml. of a wash solution prepared by adjusting 1% nitric acid to pH 2.0 with ammonium acetate and indicator as was done with samples. Draiv off the organic extract into a 250ml. Erlenmeyer flask and extract the wash solution for 2 minutes with 1 ml. of 0.531 TT*i and 5 ml. of chloroform. Xdd the extract to the flask. Xdd 2 ml. of concentrated sulfuric acid, 5 ml. of concentrated nitric acid, and 1 ml. of 7270 perchloric acid to the flask. Evaporate carefully to fumes to remove chloroform and to destroy organic matter. Add a few additional drops of nitric and /or perchloric acid as required to oxidize the last of the organic matter. Add 3 grams each of anhydrous sodium and potassium sulfates, 7 ml. of concentrated sulfuric acid, and 1.0 ml. of 6.6% barium chloride dihydrate solution, and heat gently until the sulfuric acid boils and all barium sulfate has dissolved. Cool and finish as described for rocks and soils, beginning with the second precipitation of barium sulfate. With samples other than fairly pure rare earth oxides, reprecipitate the barium sulfate. PRACTICAL APPLICATIONS
TKe physical distribution of thorium in the various fractions resulting from each of the procedures described was determined using approximately 1.5 X 106 c.p.m. of thorium-234 tracer (9). At least 98y0 of the thorium was recovered in the barium sulfate in every VOL. 36, NO. 3, MARCH 1964
629
Table IV.
Fluorometric Determination of Thorium in Standard Samples
Sample NBL 79
Recommended value Th, % 1.01 f 0.01
NBL 80
0.101 f 0.003
NBL 83
0.0102 f 0.0002
NBL 84
0.0011 f 0.0002
G-1
0.0052 f e
UT-1
0.00024 f
a
div.
Th foundo Sample taken, mg. Net sc. div. 70 0.512 70.1 0.96 70.1 0.96 69.5 0.95 0.537 72.9 0.95 72.8 0.95 72.9 0.95 0.522 71.8 0.96 71.7 0.96 69.2b 0.96b 7O.lb 0.97b 5.12 69.8 0,095 0.095 69.8 0.095 69.7 5.33 71.6 0.094 0.094 71.5 71.8 0.094 54.3 0.0097 75.3 74.1 53.5 0.0097 70.4 50.2 0.0098 503.4 66.0 0.00092 513.0 68.1 0.00093 O.OO096 501.4 68.4 105.9 85.6 0.0056 0.0051 110.3 79.5 73.0 0.0051 101.1 503.7 0.00022 15.5 514.8 15.3 0.00021 500.7 0.00022 16.0
Blank, 15.7 sc. div.; 5.11-pg.Th standard, 88.7 sc. div.; sensitivity, 0.0700 pg./sc.
Direct measurement without separations (8). Blank, 22.0 sc. div.; 5.11-pg.Th standard, 92.9 sc. div.; sensitivity, 0.0721 pg./sc. div. c Average of 40 determinations, values ranging from 0.0045 to 0.0061% (10). Average of 25 determinations, values ranging from 0.00019 to 0.00036% (10).
Because of the excellent material balances obtained with the tracer and comparison of the samples to thorium standards put through the same procedure, it is difficult to see how the present results could be low. It is felt that the values recommended by the originators of the samples are probably a little too high and that the samples are not entirely homogeneous. The two USGS samples have been analyzed very extensively for thorium but the results show a considerable spread of values ( 2 , I O ) . I n addition to 96 chemical determinations on G-1 and 38 on W-1, a determination by gammaray spectroscopy by Adams [quoted in ( g ) ] gave 51 pap.m. for G-1 and 2.3 p.p.m. for M7-l. More recently, Morgan and Lovering obtained 42 and 2.2 p.p.m., respectively, by neutron activation analysis ( 5 ) . It is rather surprising that the average of such a wide spread of values agrees so well with the present high-precision determinations. As with NBL 84, the high value on G-1 is undoubtedly real. This sample has been reported on several occasions to be somewhat inhomogeneous. ACKNOWLEDGMENT
The authors acknowledge the assistance of their associates during many helpful discussions, particularly D. B. Hawkins. Special thanks are extended to C. J. Rodden for the generous gift of the standard thorium samples and to J. J. Tregoning for the samples of G-1 and W-1. LITERATURE CITED
(1) Ebersole, E. R., Harbertson, A. R.,
case. I n general, about 0.3% loss of thorium can be expected for each barium sulfate or ferric phosphate precipitation, and about 1% loss from the original calcium phosphate precipitation on 1 liter of urine. Each procedure was also checked using fluorometric measurement on samples with and without addition of 5 pg. of thorium to prove the absence of any interference with the final chemical determination. The recovery of the added thorium was in excellent agreement with that found with the tracer studies. In addition to the recovery tests, standard thorium samples obtained from the Kew Brunswick Laboratory of the U. S. Atomic Energy Commission and from the U. S. Geological Survey were also analyzed, with the results shown in Table IV. All analyses were traced with about 1.5 x 105 c.p.m. of
630
ANALYTICAL CHEMISTRY
thorium-234 tracer and complete material balances were obtained to ensure accountability of all thorium. Because of the small quantity of sample required on NBL 79 and 80, 50 to 500 mg. of sample was dissolved, the solutions were diluted to volume, and aliquots were taken with siliconized and calibrated pipets. The results demonstrate the precision of which the procedure is capable, which is particularly remarkable in view of the very small concentrations of thorium present. The precision is slightly better between aliquots of the same solution than between different solutions of the same sample and probably indicates a detectable inhomogeneity in the sample. The highest value on NBL 84 is clearly outside the precision of the method and represents a real difference in sample composition.
Flygare, J. K., Jr., Sill, C. W., U. S. At. Energy Comm., Doc. I D 0 12023 (1959,revised 1960). 12) ~. Fleischer, hl.. Stevens, R. E., Geochim. et Cosmoch$m.Acta 26, ’525 (1962). (3)Lyakhovich, V. V.,BarinskiI, R. L., Geochemistry (Engl. trans.), No. 6, 495 (1961). (4) Moore, F. L., ANAL.CHEM.28, 997 (1956). (5) Morgan, J. W.,Lovering, J. F., Anal. Chim. Acta 28, 405 (1963). (6) Sill, C. W.,ASAL. CHEV. 33, 1684 (1961). (7)Ibid., 36, 679 (1964). (8) Sill, C. W.,Willis, C. P., Ibid., 34, 954 (1962). (9) Sill, C. W., Willis, C. P., U. S. At. Energy Comm., Doc. I D 0 12034 (1963). (IO) Stevens, R. E., “Second Report on a Cooperative Investigation of the Composition of Two Silicate Rocks,” U. S. Geological Survey, Bull. 1113 (1960). ~,
RECEIVEDfor review August 5 , 1963. Accepted December 16, 1963.
