ever, by the formation of cyanogen which condensed out in the capillary U-tube cold trap, interfering with carbon dioxide measurement. The technique of spiking metal samples with oxygen b y adding weighed amounts of the oxides to metal capsules did not necessarily approximate the physical distribution nor the particle size of oxide impurities in the metals. It is possible that the degree of entrainment of the oxide particles in the metal vapor was not the same for the two types of oxide distribution. However, i t seems improbable that this difference was significant. I n either case the twophase system of liquid metal and solid oxide had opportunity to approach the same physical distribution a t the begin-
.
ning of the evaporation, regardless of the original solid-metal-solid-oxide distribution. This method may be applicable to other metals of relatively high vapor pressures, provided the conditions of the procedure can be made to comply with the respective melting and boiling points, and with the temperatures of thermal decomposition and carbon reduction of the oxides.
LITERATURE CITED
(1) AlLsopp, H. J., Analyst 81,469 (1956). (2) Berry, R., Walker, J. A. J., Johnson,
R. E., DEG Report I11 (c), United Kingdom Atomic Energy Authority, 1960. (3) Goldberg, G., Meyer, A. S., White, J. C., ANAL.CHEM.32, 314 (1960). (4) Hartmann, H., Hofmann, W., Strole, G., 2.Metallk. 49, 461 (1958). ( 5 ) Sheft, I., hfartin, A. F., Katz, J. J., J. Am. Chem. SOC.78, 1557 (1956). (6) Smiley, IT.G., ANAL.CHEM.27, 1098 (1955).
(7) Smiley, W. G., Nuclear Sn'. Abstr. 3, 391 (1949). RECEIVEDfor review October 30, 1961. Accepted January 10, 1962. Presented a t the XVIII International Congress of Pure and Applied Chemistry, Montreal, Canada, August 1961. Based on work performed under the auspices of the U. S. Atomic Energy Commission.
ACKNOWLEDGMENT
The authors acknowledge the helpful suggestions of R. J. Ackermann and E. G. Rauh regarding the design of the apparatus, and the laboratory assistance of A. Venters and C. E. Plucinski.
A mperometric Titration
Of
.
Thorium in Monazite Sands
J. J. BURASTERO and R. W. MARTRES Administracio'n Nacional de Combustibles, Alcohol y Portland, Divisi6n lnvestigaciones Cienti,ficas, Pando, Uruguay
b A practical method for the separation and amperometric determination of thorium in monazite sands is proposed. The attack is carried out with sulfuric acid on 1 0-gram samples; thorium and the rare earths are separated b y a single precipitation with oxalic acid, and the final amperometric titration is made with ammonium paramolybdate as titrant. The composition of the thorium molybdate precipitated, as a function of the initial concentration of thorium, is verified. Possible interferences of some impurities that frequently occur in these sands are determined. The results agree with those of the iodate chemical method and other more laborious amperometric techniques.
S
workers have used ammonium paramolybdate as a precipitating agent for the determination of thorium by volumetric and electrometric methods (1, 4,5,15,22, 25). Kevertheless, few of these methods are used for the determination of thorium in the presence of the rare earths, as under certain conditions thorium molybdate may be precipitated, leaving the rare earths in solution (5, 15, 23). Direct titration has been questioned because of the difficulty of establishing the end point ( I , 9). Titration of the molybdenum combined with the thorium in the precipitate has the disadvantage of being rather lengthy (I). As a result, electrometric methods for EVERAL
378
ANALYTICAL CHEMISTRY
the determination of the end point appear desirable (1,16). ilpplication of the procedure proposed by Gordon and Stine (5) to monazite sands includes a prior treatment of the sample and separation of the thorium and rare earths from other metals occurring in the ore. This is a useful method, but i t is very complicated in the early stages. The present work was carried out on monazite sand samples obtained from Uruguayan deposits, to find a method for amperometric determination, which would involve easier manipulation, less time, and easily obtainable and cheaper reagents. For this purpose, a method of sample processing is proposed which does not require mechanical stirring for the precipitation of the thorium oxalate, temperature control and adjustment of pH, nor reprecipitation under the same conditions. The number of washings and filtrations are reduced. The use of methyl oxalate as a precipitating agent is eliminated and also treatment with hydrogen peroxide prior to precipitation. Perchloric acid is not used, as the sample is decomposed b y the classical sulfuric acid method. Once the sulfuric acid solution has been obtained, the only steps required are a single precipitation with oxalic acid, decomposition with nitric acid of the oxalates obtained, and conversion to chlorides prior to the final ampero-
metric determination. These steps can be easily performed in a few hours. The reliability of the method is further confirmed b y its application to samples from other sources and of different compositions. EXPERIMENTAL
Apparatus. The molybdenum(V1) polarograms were obtained with a Sargent Polarograph, Model XXI. This polarograph was also used in t h e amperometric titrations, working a t constant voltage. An H-type polarographic cell, like that of Lingane and Laitinen (IS),with a working initial volume of 35 ml. of solution, was used. The conventional dropping mercury electrode was used as a cathode, with the capillary and mercury reservoir arrangement of Linnane and Laitinen -
US).
-
Beckman p H meter, Model N-1.
Monazite Sand. The previous tests were carried out on a monazite sand fraction separated b y the magnetic process from a black sand concentrate obtained from Balneario San Luis, Departamento de Canelones, Uruguay (sample I). The total oxide (thorium and rare earths) content of the monazite fraction was determined gravimetrically by the oxalate method (20). The result obtained was 41.4%. The high content of impurities tests the effectiveness of the method under unfavorable conditions. Commercial samples generally contain about 60% of total oxide.
Figure 1. Polarogram of ammonium paramolybdate Supporting electrolyte. 7% acetic acid and 2.5M sodium chloride. MOO^-^ concentration. 2 i o - 3 ~ pH. 2.0 Sensitivity. 0.100 p a . per mm.
x
0
0.2 0.4 0.6 0.8 APPLIED VOLTAGE, VOLTS VS. S.C.E.
The following samples were used to check the developed method:
1.0
tained 2.07 for pure solutions and 2.03 for impure solutions, volumetrically n i t h external indicator (15); 2.07 potentiometrically ( 3 ) ; 1.93 to 2.05 amperometrically (6); and 1.99 volumetrically n ith internal indicator (4). Interferences. RIonazite sand is a concentrate of monazite with varying proportions of other minerals (ilmenite, zircon, magnetite, rutile, hornblende, olivine, etc.). Because of this composition, when it is dissolved, solutions of thorium a n d t h e rare earths are obtained which contain i arious metallic impurities (principally iron and titanium and calcium, niagnesium, aluminum, zirconium, uranium, etc., in minor quantities). Although interferences of uranium and calcium (1) and rare earths (5, 16) in thorium determination with paramolybdate as a precipitating agent have been studied, the literature does not account for the incompatibility of other metallic ions that generally occur in the solution obtained from the monazite attack by conventional methods. As a preliminary study, the general behavior of such possible interferences TI as studied (Table 11). For each trial, a 50-ml. sample of the same thorium nitrate tetrahydrate solution nas used. To each, a n equal quantity of cerium chloride was added, since previous experiments showed that pure thorium oxalate resists decomposition with nitric acid. On the other hand, the presence of cerium in this precipitate greatly increases the ease of the decomposition. Metallic ions in the quantities indicated in Table IL nere added to samples B and C. Those that generally occur in major proportion in the monazite fraction were added in greater quantities, Trial A, without addition of ions, n as taken as a blank. Thorium n-as determined aniperometrically. Preliminary separation, n i t h cprium, n-as effected by a single precipitation n i t h o d i c acid, as indicated in thc proposcd technique. -According t o the results obtained, these impurities do not interfere I\-ith the method. Furthermore, there \\ere no significant diff erences in the values obtained by carrying out the precipita-
pH and supporting electrolyte (7,10,11, ' 4 1 17). Figure 1 shons the wave obtained under the conditions used for the Sample 11, from Balneario Atlitntida, amperometric titration. I t s half-n ave Uruguay, separated like sample I. potential is -0.47 volt us. S.C.E.; from Sample 111, from Balneario Aguas it a working voltage of -0.90 volt us. Dulces, Uruguay, separated like sample I. S.C.E., in which the wave reaches its Sample IV, large single crystals from limiting current, was selected. Madagascar. Thorium Molybdate Composition. Sample V, from Brazilian deposits. Samples of thorium standard solution S a m d e VI. semirated from a nionwere diluted t o a final volume of 100 azite sand from Cleveland County, ml. with distilled water, acetic acid, S o r t h Carolina, supplied by Ward's a n d sodium chloride solution, t o give Natural Science Establishment, Inc., a final concentration of 770 in acetic Rochester, N. Y. Reagents. S T A N D A RTDH O R I U M acid and 2.5M in sodium chloride S O L U T I O N .A thorium chloride solu(pH 1.8 to 1.9) ( 5 ) . tion containing approximately 2 mg. A 35-ml. aliquot n-as placed in the per ml. of thorium oxide mas prepared cell and titrated with standardized from 8 grams of Merck thorium niammonium paraniolybdate solution as trate tetrahydrate t h a t was dissolved indicated in the proposed technique. in concentrated hydrochloric acid The results obtained (Table I) supa n d evaporated t o dryness. This port the precipitation of normal thotreatment mas repeated until nitrates rium molybdate, Th(hIo04)2, because were totally eliminated. Subsequently the residue was dissolved in distilled the average for the ratio of molybdenum water, acidulated with 2 ml. of conatoms to thorium atoms is 2.04--i.e., centrated hydrochloric acid, and diluted the theoretical value. t o 2 liters. The exact content of This average is taken as a value thorium oxide was gravimetrically deteruseful for the total range studied. The mined by precipitation with oxalic ma\imum deviation from this value acid and final weighing as thorium does not exceed 1.5%1,,thus justifying oxide (8). STAKDARDIZED Amiox;ruai PARA- such a procedure. Similarly, the a i erage 4.37 mg. per ml. nas taken as a IIOLYBDATE SOLUTION. A solution which contained approximately 6 grams value for the paramolybdate solution. per liter was prepared with the Merck For the ratio of molybdenum atoms tetrahydrated salt. The molybdenum to thorium atoms other n orkers obwas gravimetrically determined b y the 8-quinolinol method (21). M E T H Y OXALATE L mas prepared from methanol and oxalic acid as described Table I. Thorium Molybdate Composition previously ( 2 ) . KITROGEN.The solutions mere deaRatio Molybdate ThC14 Tho, ThOz erated by bubbling the commercial >k.ThOi Required, Sample, Taken into Concn. X gas, which was freed from oxygen by MI. Ml. M O Cell, Mg 10 - 3 x AIL passing it through an aqueous solution 15.0 8.74 4.31 0.95 2.03 of vanadous salt. 4.42 17.0 9.91 1.07 2.24 Polarography of Molybdenum. 20.0 11.66 4.32 1.26 2.70 Several workers agree t h a t molyb25.0 14.57 4.38 1.58 3.33 denum(V1) is reduced at t h e dropping 30.0 4.36 17.48 1.89 4.01 35.0 4.43 20.40 2.21 4.60 mercury electrode in acid medium, though the number of waves and its Av. 4.37 half-wave potentials change with the
Ratio &Oms
Th Atoms
VOL. 34, NO. 3, MARCH 1 9 6 2
2.07 2.02 2.06 2 04 2.04 2.01 2.04
e
379
Table
II.
Effectiveness of Oxalate Precipitation for Removing Thorium from Metallic Impurities Prior to Amperometric Titration
A -
Impurities Added Fe Ti Zr A1
C
...
go"
... ... ... ...
30 30 30 30 8 8
*.. *..
Mg
Ca Mn Pb Cerium added, mg. Oxalate pptn. pH Tho2 found, mg.
Approx. metal-Th ratio 5 3
90
...
300 0.3 34.1
Approx. metal-Th ratio 5 3
WIg 150 90 30 30 30
1 1 1 1 /4
300 0.3 34.0
1 1 1
30
1
8 8
1/4 1/4
300 0.1 34.3
Table 111. Comparative Thorium Estimation in Monazite Samples Method A B C D Gravim. Attack HClO, H2SOd &SO4 HzSOI &SO4 Thorium sepa- Double pptn. Double pptn. Double pptn. Single pptn., Pptn. as ration methyl methyl oxalic oxalic acid iodate oxalate oxalate acid 0.8-0.9 0.8-0.9 0.8-0.9 0.8-0.9 0.3-0.4 Final pH ThOpfound, % I 2.72 2.71 2.73 2.69 2.74 2.71
I1 I11 IV V
VI
...
... ... ... ...
... ... ... ... ...
tion at a different pH. Sample C, analogous to B, was determined in a similar way, but the oxalates were precipitated in a more acid medium. These trials point out the possibility of utilizjng a routine method for thorium determination, obviating the necessity of precipitating thorium and rare earth oxalates under conditions of strict purity by several precipitations from homogeneous solutions. COMPARATIVE STUDY AND SlMPLlCATlON OF METHODS
Decomposition of Monazite. T h e procedure of Gordon (5, 6, $4) for decomposition of monazite sands uses perchloric acid on 1-gram samples. The attack with perchloric acid is rapid and the precipitate obtained is easily filtered, but an expensive reagent needs to be utilized. In the present work, perchloric acid is replaced b y sulfuric acid, a cheaper product, which is more convenient for the attack of larger monazite samples. Furthermore, in the case of monazite sands, large samples are necessary to ensure uniform and representative sampling. Precipitation of Thorium and Rare Earths and Determination of Thorium. h I E T H o D A. T h e procedure of Gordon and Stine (5) was followed. It consists of three steps: decomposition of monazite sand with perchloric acid, separa380
ANALYTICAL CHEMISTRY
...
...
...
... ...
2.72
...
... ... ... ...
3.56 4.69 14.11 5.49 6.18
2.76 3.51 4.66 14.22 5.43 6.26
tion of thorium and the rare earths by the hydrolysis of methyl oxalate in acid solution, and amperometric titration of thorium with ammonium paramolybdate as titrant. METHODB. On a n aliquot of the sulfate solution obtained as indicated below, a double precipitation by the hydrolysis of methyl oxalate, as for Method A, was made. METHOD C. Thorium was determined in a similar aliquot, and a prior separation, together with rare earths, carried out by a double precipitation with oxalic acid at pH 0.8 to 0.9. METHODD. From the same sulfate solution, thorium and rare earths were separated as oxalates by a single precipitation with oxalic acid. I n first trials, the pH of the solution was adjusted to 0.8 to 0.9 after addition of oxalic acid. Subsequently, a similar single precipitation was made, but the p H was not adjusted. Because an excess of oxalic acid may precipitate the thorium eyen in very acid solutions ( I @ , it was unnecessary in this last case to control the pH during the precipitation of oxalates. The excess of oxalic acid added was the same as in earlier procedures, and the p H resulting from the addition to the solution, after neutralization. of 5 ml. of concentrated hydrochloric acid for each 100 ml. of solution, was not adjusted. Thus, the final pH is about 0.3 to 0.4. Furthermore, precipitation in stronger acid solutions is
advisable , because heavier and more easily filterable precipitates are obtained. I n all the procedures pointed out, a residue of thorium and rare earths as chlorides was finally obtained, in which the thorium was amperometrically determined as indicated below. Thc thorium oxide content obtained by these several trials is showed in Table 111 (sample I). The data obtained with the classical method of precipitation as thorium iodate, transformation to oxalate, and final weighing as oside (19) are also given. The results obtained amperometrically, through the consecutive simplifications of the technique for the previous separation of thorium as oxalate, agree with each other and \Tith those obtained by the amperometric method proposed by Gordon and by the iodate chemical method (19). Five other samples of different origin and very different thorium content were analyzed by the simplified proposed method (Table 111). These samples had been previously analyzed by the iodate method.
PROCEDURE
Carry out the attack on 10 grams of finely ground monazite sand (-200 mesh) with 30 ml. of concentrated sulfuric acid. Heat the mixture for about 5 hours at 200" to 210" C. u i t h frequent stirring. Allow it to cool, dilute m-ith crushed ice or ice-cold n-ater to about 300 ml., then heat to about 40' C. Add 1 ml. of 1% gelatin solution, drop by drop, with stirring. With the addition of gelatin a clear filtrate is obtained. After standing overnight, filter the solution into a 500-ml. volumetric flask and wash the residue with water acidified with sulfuric acid. Make the solution up to the mark with water. Measure out 50 ml. of this solution into a 600-ml. beaker. If the thorium oxide content of the sample is out of the range 2.5 to 6%) take an aliquot in a higher or lower proportion. Neutralize with 1 to 1 ammonium hydroxide drop by drop with stirring, until the appearance of a permanent turbidity which does not disappear with vigorous stirring. Add 10 ml. of concentrated hydrochloric acid and dilute to 200 ml. with water. lF7arm to boiling and add, slowly and with stirring, a warm solution of 14 grams of oxalic acid in 200 ml. of water. Let stand for 1 hour in a hot place. Filter through a sintered-glass funnel (Jena 3G4), so that the bulk of the precipitate remains in the beaker. Wash five times with a cool 2% oxalic acid solution adjusted to p H 1.0 with hydrochloric acid. Transfer the precipitate into the beaker containing the oxalates, with hot water. Pass through the glass filter
20 ml. of hot concentrated nitric acid and add this liquid to the beaker. Decompose the precipitate with nitric acid, and evaporate t o dryness on a hot plate. Carry out a t least three evaporations with 5 ml. of concentrated hydrochloric acid each time, to ensure complete removal of nitric acid. Dissolve the residue in 50 ml. of a 5M sodium chloride solution and add 7 ml. of glacial acetic acid. Transfer the solution to a 100-ml. volumetric flask by a water wash and dilute to volume (the resulting DH is 1.8 to 1.9). Transfer a 35-ml. aliquot into the cell. Before reading and adding the titrating solution, deaerate the solution by passing nitrogen through it for 15 minutes. Continue this bubbling for 3 minutes after each addition of the paramolybdate solution, prior to turning on the current. “
240
i
I
i
f
/
2 160
w LL Lz
3 0
80
I
The readings were carried a t each 0.1 ml. a t the beginning of the titration, then for each 1 ml. around the equivalence point, and next around each 0.2 ml. until various points in line were obtained. After the dilution correction is made ( l a ) , the equivalence point is graphically determined in the usual way. Figure 2 shows a typical curve. LITERATURE CITED
(1) Ranks, C. V., Iliehl, H., ANAL. CHEM.19,222 (1947). (2) Blatt, A. H., ed., “Organic Syntheses,” Coil. Vol. 11, p. 414, Wiley, New York, 1944.
0.8
0
Figure
2.
1.6
2.4 3.2 4.0 MOLYBDATE SOLUTION, ML.
4.8
5.6
6.4
Amperometric determination of thorium in a monazite sample
(3) Britton, H. T. S., German, W. L., J. Chem. SOC.1931, 1429. (4) Deshmukh, G. S., Bokil, I., Bull. Chem. SOC.Japan 29,449 (1956). (5) Gordon, L., Stine, C. R., ANAL. CHEM.25,192 (1953). (6) Gordon, L., Vanselow, C. H., Willard, H. H., Zbzd., 21, 1323 (1949). (7) GuibB, L., Souchay, P., J . chim. phys. 54,684 (1957).
(8) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., p. 542, TViley, New York, 1953. (9) Kaufman, L. E., Trav. inst. &ut radium (U.S.S.R.) 4 , 313 (1938). (10) Kawahata, M., Mochizuki, H., Kajiyama, R., Bunseki Kagaku 8 , 2 5 (1959). (11) Kolthoff, I. M., Lingane, J. J., “Polarography,” 2nd ed., Val. 11, p. 457, Interscience, New York, 1952. (12) Zbzd., p. 890. (13) Lingane, J. J., Laitinen, H. A., IND. ENG.CHEM.,ANAL. ED. 1 1 , 504 (1939). (14) Manning, D. L., Ball, R. G., Menis,
O., AXAL.CHEM.32,1247 (1960). (15) Metzger, F. J., Zons, F. W., J. Znd. Eng. Chem. 4,493 (1912). (16) Moeller, T., Schweitzer, G. K., Starr, D. D., Chem. Reus. 42, 63 (1948). (17) Pecsok, R. L., Parkhurst, R. M., ANAL.CHEM.27,1920 (1955). (18) Rodden, C. J., ed., “Analytical Chemistry of the Manhattan Project,” p 169, McGraw-Hill, New York, 1950. (19) Zbid., 171. (20) SchoeEer, W. R., Powell, A. R., “Analysis of Minyals and Ores of the Rarer Elements, 3rd ed., p. 108, Griffin, London, 1955. (21) Zbid., p. 262. (22) Smales, A. A,, Airey, L., At. Energy
Research Establishment, Harwell, England, C/M 131. (23) Tung, S. C., Kang, E. K., Hua
Hsueh Hsueh Pao 25, 33 (1959) (24) Willard, H. H., Gordon, L , ANAL. CHEY.20,165 (1948).
RECEIVED for review February 10, 1961. Accepted Kovember 28, 1961.
Differentiation of Vitamins D2 and DB by Infrared Spectrophotometry W. W. MORRIS, Jr., J. B. WILKIE, S. W. JONES, and LEO FRIEDMAN Food and Drug Administration, U. S. Department o f Health, Education, and Welfare, Washington 25, D . C.
b Infrared spectrophotometry can b e used to determine the form of vitamin D present, either b y visual examination o f the spectrum between 10 and 1 1 microns or b y spectrophotometric neutralization techniques when nonuniform background i s present. The amounts of vitamin Dz or D1 can also b e estimated b y spectrophotometric neutralization. A technique i s described that i s potentially useful for the determination o f the proportion of each form of vitamin D present in mixtures of th= two by means of a reference curve relating the ratio of a b sorbance differences (A10.3p - Ala.5 p ) /
A10.4 p - A10.5 p) to the per cent composition of the mixture. The amount of each form could then b e calculated from the total vitamin D content of the sample. The accuracy o f these procedures i s within f 15%.
U
there has been no means of differentiating between vitamin D2and vitamin DB,except by bioassay procedures. Both forms are equally potent as measured by the rat bioassay ( I S ) , while vitamin D2 has only 1/100th the potency of vitamin D3 as measured by the rhick bioassay (I). Horn-erer. bioassay procedures are P TO THE PRESENT TIME
costly and too time-consuming for routine control work, and the need exists for a more rapid procedure for the differentiation of the two forms of vitamin D. Infrared spectrophotometry appeared to offer a rapid and sensitive means of differentiating between vitamins D2 and DB. Jones (7) found that the infrared spectrum of vitamin Dz exhibits a strong band a t 10.3 microns (970 cni.-l) due to the carbon-hydrogen bending vibration of the -C-H group in the unsaturated side chain; the saturated side chain of vitamin D3 does not exhibit this strong band. VOL. 34, NO. 3, M A R C H 1962
381
