New Titrimetric Methods for Thorium C H A R L E S V. B A N K S
AND
H A R V E Y DIEHL
Institute f o r Atomic Research, Iowa State College, Ames, Iowa
A new oxidimetric method for the titrimetric determination of thorium is based on the precipitation of thorium as the normal molybdate followed by the reduction and titration of the molybdenum equivalent to the thorium. This method has been applied to the separation of thorium from calcium and uranium and to the determination of molybdenum by reversal. A new electrometric method is described in which the thorium is titrated with ammonium paramolybdate in a 7% acetic acid solution at 50" to 55" C. with a 0.1 N calomelmolybdenum wire electrode system.
0
molybdate (as described below). The thorium molybdate was filtered, washed, and dissolved in hydrochloric acid, and the molybdenum was reduced and titrated with ceric sulfate. The data recorded in Table I show definitely that thorium can be determined quantitatively by this method. RECOMMENDED PROCEDURE. Samples containing 0.15 to 0.2 gram of thoria are weighed out and placed in 250-ml. beakers. After the samples have been dissolved any large excess of mineral acid is destroyed by evaporating the solutions nearly to dryness. The samples are then diluted to 150 ml. with water and made about 7% in acetic acid by addition of 11 ml. of glacial acetio acid. Fifteen milliliters of thick filter pulp and 1 ml. of a diphenylcarbazide solution (0.5 gram per 200 d.of 95% ethanol) are added. The ammonium paramolybdate solution (7.6 grams per liter) is added from a buret with stirring until the indicator imparts a deep pink color to the solution. After the recipitates have settled the supernatant liquid may be tested or complete precipitation. The contents of the beakers are heated to boiling and filtered while hot through 11-cm. Whatman No. 42 filters into 400-ml. beakers. The precipitates are washed 5 to 6 times with hot 1 to 100 acetic acid. The 250-ml. beakers need not be scrubbed out with a policeman but only carefully rinsed 2 to 3 times with wash solution. The washed precipitates and filters are transferred to the 250-ml. beakers in which the precipitations were carried out and 25 ml. of concentrated hydrochloric acid are added to each beaker. The contents are stirred until the filters disintegrate. Seventy-five milliliters of water are added, and the mixtures are heated to boiling (long boiling results in reduction of molybdenum and decomposition of the filter pulp) and filtered while hot through 11-cm. Whatman No. 42 filters into 400-ml. beakers. The filter pulp and filters are washed 5 to 6 times with hot 1 to 100 hydrochloric acid. The filtrates, after being cooled to room temperature, are passed through an amalgamated zinc Jones reductor into an excess (5 times the theoretical of 10%) of ferric alum to which 2 to 3 ml. of concentrated phosphoric acid have been added and titrated with 0.1 iV ceric sulfate, 2 drops of 0.025 M ferroin (1,lO-phenanthroline ferrous sulfate) being used as indicator. The end point is taken as that point when the pink color of the solution changes to colorless or light blue.
F T H E large number of methods for the determination of thorium which have appeared in the literature, practically all are based on precipitating the thorium in some form from an acid or neutral solution and completing the determination by a gravimetric method. The precipitate is either ignited to the dioxide directly, or dissolved and the thorium precipitated as the oxalate, after which it is converted to the dioxide by ignition. The existing gravimetric methods have been reviewed by Bonardi (19) and Jiiatel (18). The work being reported, in which successful titrimetric methods for thorium are described, was prompted by the need for a rapid and accurate method for determining thorium in various alloys and mixtures. The precipitation-titrimetric method of Metzger and Zons (18) is rapid but not particularly accurate (13); in agreement with others (19) the authors found it very difficult to distinguish the end point. The oxidimetric oxalate method of Gooch and Kobayashi ,(8) is accurate but necessitates the addition of the thorium solution to the oxalic acid solution. The possibility of precipitating thorium as the oxinate (16) and titrating the equivalent amount of oxine by the method of Berg (3) has been studied extensively (4, 7 , 9, 10, 11, 17) and found to be suitable for both the macro- and microdetermination of thorium, but it is not a separation from any of the numerous metals precipitated by oxine from a solution buffered with acetate. The iodate method of Chernikhov and Uspenskaya (6) appears very promising but a critical re-examination of the method was not made. A few electrometric methods based on the precipitation of thorium oxalate ( d ) , the titration of thorium picrolonate with a titanous solution ( 1 4 ) , and the precipitation of thorium ferrocyanide (1,bO) have appeared but none is satisfactory.
P
OXIDIMETRIC MOLYBDATE METHOD
Theoretical. This method is based on the quantitative precipitation of thorium as the normal molybdate and the subsequent reduction and titration of the molybdenum that is combined with the thorium. Rletzger and Zons (18) and also Britton and German (6) give ample evidence that thorium is precipitated as the normal molybdate. Determination of Thorium. The thorium nitrate used in all the following experiments was purified. The purified thorium hydroxide was dissolved in a minimum amount of nitric acid and diluted with water. The solution was standardized gravimetrically by precipitation as the hydroxide and ignition to the dioxide. Weight burets were used throughout this study.
Separation of Thorium from Calcium. Calcium molybdate is not precipitated from 7% acetic acid solutions containing as much as 1.5 moles of calcium nitrate per liter. This may be due to the incomplete dissociation of calcium acetate (16).
Table I. Determination of Thorium in Thorium Nitrate Trial
EXPERIMENTAL WORK. Samples of the standard thorium nitrate solution were weighed into beakers, diluted to 150 ml. with water, and made 7% in acetic acid. Filter pulp and diphenylcarbazide indicator were added to each sample and the thorium was precipitated by the additian of ammonium para-
z.
222
N
-
ThOs Taken Gram 0.1693 0.1531 0.1516 0.1709 0,1453 0.1607 0.1714 0,1572 0,1008.
Ce(HSO4)da MI. 38.15 34,44 34.12 38.42 32.70 36.20 38:64 35.40
ThOz Found
Error
Gram
%
0,1693 0.1528 0.1514 0.1705 0.1451 0.1606 0,1715 0.1571
*o.oo
-0.20 -0.13 -0.23 -0.14 -0.06 +0.06
-0.06
V O L U M E 19, NO. 4, A P R I L 1 9 4 7
223
EXPERINEKTAL WORK. The separation of thorium from calcium with molybdate was tested by adding different amounts of calcium nitrate to known amounts of thorium nitrate and separating the thorium as the molybdate. The data obtained in this series of experiments (Table 11) show that thorium can be quantitatively separated from as much as 0.4 gram of calcium in a 770 acetic acid solution but that high results are obtained when extremely large amounts of calcium are present. RECOMMESDED PROCEDURE. The recommended procedure for separating thorium from calcium is the same as that given above for the determination of thorium.
Table 11. Trial 1 2 3 4 6 6 7 8 9 10 11 12 a
-
N
Separation of T h o r i u m f r o m Calcium
Ca Present
ThOz Taken
Gram
Gram
0.1 0.1 0.2 0.2 0.4 0.4 3.0 3.0 6.0 6.0 9.0 9.0 0.1007.
0.1716 0.1480 0.1575 0.1632 0.1663 0.1743 0.1598 0.1756 0.1563 0.1705 0,1402 0.1693
Ce(HSO4)an M1. 38,72 33.42 35.60 36.90 37.56 39.38 36.38 39.90 35.95 39.15 33.20 38.35
Tho2 Found
Error
Gram
%
0.1716 0.1481 0.1578 0.1635 0,1665 0.1746 0.1623 0.1770 0.1595 0.1737 0,1473 0,1702
*o.oo +0.07 +o. 19 + O . 18 +o. 12
*
+0.17 $1.56
a
N
-
Us08
Separation of T h o r i u m f r o m U r a n i u m ThOn Taken
Gram
Gram
0.2 0.2 0.2 0.2 0.ioo.i.
0,1787 0.1776 0.1740 0,1443
Ce(HSO4)rn MZ. 40.30 40.10 39.50 32.72
ThOz Found
Uranium
%
%
Total
%
97.60 82.60 76.90
99.27 98.70 100.50
Separation of Thorium from Rare Earths. The separation of thorium from rare earths such as lanthanum, cerium, samarium, neodymium, and also yttrium was tried with various acetic acid concentrations and at various temperatures but quantitative separation was not obtained. Determination of Molybdenum by Reversal. The determination of molybdenum by precipitating it as thorium molybdate is very useful in that it affords a means of separating molybdenum from certain other elements from which it cannot be conveniently separated otherwise. Such an example is the separation of molybdenum from uranium.
Table V. Trial
Present
Thorium
1 1.67 2 16.10 3 23.60 Analyses made by J. H. Patterson.
Separation of 3loiybdenum f r o m U r a n i u m
UaOs Present
Gram
Table 111.
1 2 3 4
Sample
+0.80
+0.24 +1.88 +5.06 +O. 53
Separation of Thorium from Uranium. Uranyl molybdate is insoluble in aqueous solutions and solutions containing acetic acid. However, ammonium acetate prevents its precipitation for several hours even if the solutions are boiled. A larger excess (4 to 6 ml.) of precipitant is necessary when ammonium acetate is present and diphenylcarbazide is not satisfactory to indicate complete precipitation in the presence of uranyl salts. The proper amount of precipitant for complete precipitation can be determined by a preliminary determination or by testing the supernatant liquid. EXPERIMENTAL WORK. A series of thorium solutions, to which had been added known amounts of uranium and 5 grams of ammonium acetate, was analyzed by precipitating the thorium with 4- to 10-ml. excess of ammonium paramolybdate. Table I11 shows that thorium can be quantitatively separated from uranyl salts by the molybdate method.
Trial
Table IV. Analysis of T h o r i u m - U r a n i u m Alloysa
Error
Gram
%
0.1786 0.1778 0.1751 0.1450
-0.06 +0.11 +0.63 +0.49
RECOMMENDED PROCEDURE. Samples containing 0.15 to 0.2 gram of thoria and not more than 0.5 gram of uranium oxide are weighed into 250-ml. beakers. After the sample is dissolved, any large excess of mineral acid is removed by evaporating the Bolution nearly to dryness. Five grams of ammonium acetate and 11 ml. of acetic acid are added to each sample and they are diluted to 160 ml. and analyzed by the procedure given above for the determination of thorium. ANALYSIS OF THORIUM-URANIUM ALLOYS.The above method has been used to advantage in analyzing thorium-uranium alloys. These alloys are easily decomposed with hydrochloric acid and then brought into solution by fuming with perchloric acid. Typical results obtained by this method are shown in Table IV. The uranium was determined on separate samples by reducing in a Jones reductor, aerating, and titrating the uranous solution with 0.1 N ceric sulfate in the usual manner.
6
1 0.24 2 0.24 3 0.24 N = 0.0783.
M o o t Taken Gram 0.1381 0.1381 0.1381
-
Ce(HSOd),O M1.
MOOSFound
Gram
Error %
36.55 36.70 36.65
0.1373 0.1379 0.1377
-0.58 -0.14 -0.29
EXPERIMENTAL WORK. The separation of molybdenum from, uranium was studied by adding known amounts of molybdate to uranyl nitrate solutions and separating the molybdenum thorium molybdate. The data in Table V show that this separation is satisfactory. RECOMMENDED PEOCEDURE FOR ANALYSIS OF URANIUMMOLYBDENUM ALLOYS. Samples containing 0.1 to 0.16 gram of molybdenum trioxide are weighed into 400-ml. beakers and dissolved in as small a volume as possible of 1to 1hydrochloric acid. The samples may be warmed to aid solution. Hydrogen peroxide is added to dissolve the black hydrated uranium dioxide and to oxidize both elements to the hexavalent state. The solutions are boiled about 10 minutes to destroy the excess hydrogen peroxide and diluted to 200 ml. Sixteen milliliters of glacial acetic acid are added to each sample and enough ammonium acetate, usually 1 gram, to react with the mineral acid present. About 15 ml. of thick filter pulp are added to each beaker and the molybdate is precipitated as thorium molybdate by adding slowly with stirring a 25% excess of thorium perchlorate solution, prepared by fuming purified thorium nitrate with perchloric acid to near dryness and diluting until 1 ml. is equivalent to about 5 mg. of molybdenum. From this point on the molybdenum analysis is the same aa that described above in the procedure for determining thorium. In case the 1 to 1 hydrochloric acid does not dissolve the sample, a little concentrated nitric acid is added, and the solution is boiled. It is best to avoid the use of nitric acid if possible, for it must be removed prior to reducing the uranium in the filtrate by the Jones reductor. The use of nitric acid is, however, necessary for the higher (20y0 or more) molybdenum alloys. The nitric acid, when used, cannot be fumed off immediately after dissolving the sample because the use of sulfuric acid precipitates thorium sulfate later, and fuming with perchloric acid causes molybdic oxide to precipitate. The filtrates from above are concentrated to a convenient volume, or fumed with perchloric acid if nitric acid was used in dissolving the samples, and analyzed for uranium in the usual manner. Larger samples may be weighed out, dissolved, and diluted to a known volume, and aliquots used for analysis if the sample form demands it. Typical analyses of uranium-molybdenum alloys obtained by the above method are shown in Table VI.
ANALYTICAL CHEMISTRY
224
Tahle VI.
Analysis of Uranium-1Iolybdenum Alloys
Sample
u
%
%
0 4 7 15 20
la
2 3 4 5 a
3t 0 47 00
99 99 91 83 78
24
92 66
Total
%
30 06 91 25 16
99 100 99 99 98
77 06 15 17 82
Sample analyzed by R W Holniberg
Table VII. Potentiometric Standardization of Ammonium Paramoly hdate Trial
Q
ThOl Taken Giam 0 1061 0 1048 0 09624 0 1095 0 09852 0 1088 0 1811
Molybdate
Titer
ErrorU
.Ill 18 00 17 80 16 35 18 60 16 80 18 55 30 78
Gram ThOdml
%
Calculated as deviation from the average titer of 0 005881
Tahle VIII. Determination of Thorium in the Presence of Calcium Trial 1 2
Ca Present Gram 0 2 0 4
T h o ? Taken Gram 0 1619 0 1538
llolybdate .Ill
27 50 26 23
..
ThOz Found Gram
Error
0 1617 0 1542
- 0 12 + O 26
7c
ELECTROMETRIC METHODS
, Determination of Thorium in the Presence of Calcium. Even
,
though thorium can be determined very accurately by the osidimetric molybdate method discussed above, .the method has the disadvantage of being rather lengthy. The possibility of detecting the end point electrometrically when thorium is titrated with ammonium paramolybdate was investigated and found very promising. Of the various conditions tried for this titration the best was a 7% acetic acid solution at 50" to 55" C. with a 0.1 .L' calomel reference electrode and a molybdenum wire indicator electrode. EXPERIIIEKTAL WORK. Since it was found that the thoria titer of the ammonium paramolybdate solution w-as about 0.6% higher when determined potentiometrically than when determined oxidimetrically, it is necessary to standardize the ammonium paramolybdate solution potentiometrically. The results obtained in this manner are reproducible, as shown by the data in Table 1711. In each case the data were plotted and the end point was obtained from the graph. -4typical curve is shown in Figure 1. The slight perturbation just preceding the end point was invariably present, even though equilibrium was readily attained in all cases. The pH of the solution remained sensibly constant throughout the titration, changing only from 2.33 to 2.61. The determination of thorium in the presence of calcium was studied by adding known amounts of calcium nitrate to k n o m amounts of thorium nitrate, making the solutions 7% in acetic acid, warming to 50" to 5 5 O , and titrating with amnionium paramolybdate. Table \?I1 shows that thorium can be determined quantitatively in the presence of calcium. ACKNOWLEDGMENT
The authors wish to acknowledge the helpful suggestions of J. C. Warf from time to time during the preparation of this paper. hlost of this work was done under Contract W-7105eng-82 n-ith the Manhattan Project at the Department of Cheniistry, IOWA State College.
VOLUME of
MOLYBDATE (rnl)
Figure 1. Standardization of Ammonium Paramolybdate Solution
LITERATURE CITED (1) Atanasiu, I. A , , Bul. chim. SOC. romdna stzinte, 30, 51-9 (1928). (2) A t a n a s h , I. A , , 2. a n d . Chem., 112,19-22 (1938); 113, 276-9 (1938). (3) Berg, R . , P h a r m . Ztg., 71, 1542-3 ( 1 9 2 6 ) ; J . prakt. Chem., [2] 115,178-85 (1927). (4) Berg, R . , a n d Becker, E., 2 . anal Chem., 119,1-4 (1940). (5) Britton, H . T . S., and German, IT. L., J . Chem. SOC.,1931,142935. (6) Chernikhov, Yu. A,, and Uspenskaya, T. A , , Zavodskaya Lab., 9, 276-83 ( 1 9 4 0 ) . (7) Frere, F. J., J . Am. Chem. SOC.,55, 4362-5 (1933). Gooch, F. A., a n d Kobayashi, -M., Am. J . Sci., [4] 45, 227-30 (8) (1918). (9) Gotb, H . , J. Chem. SOC.J a p a n , 54, 725-40 (1933); Sci. Rept. TBhoku I m p . Univ., [ l ] 26,391-413 (1937). (10) Hecht, F., and E h r m a n n , W., 2. anal. Chem., 100,98-103 (1935). (11) Hecht, F., and Reich-Rohrwig, IT,, Monztsh., 53-54, 596-606 (1929). (12) Justel, B., Die Chemie., 56, 157-8 (1943). (13) Kaufrnan, L . E., J . Applied Chem. (U.S.S.R.), 10, 1693-5 (1937); Traw. inst. &at radium (U.S.S.R.), 4, 313-17 (1938). (14) K i b a , T . , J . Chem. SOC.J a p a n , 58, 1283-7 (1937). (15) Kolthoff, I. R.I., Chem. Weekblad, 24, 606-10 (1927). (16) MacDougall, F. H., and Larson, W. D . , J . Phys. Chem., 41, 417-29 (1937). (17) Mannelli, G . , A t t i Xo congr. intern. chim., 2,718-25 (1938). (18) Meteger, F. J., and Zons, F. IT., I n d . Eng. Chem., 4 , 493-5 (1912). (19) Moore. R. B., Lind, S. C., Marden, J. W., Bonardl, J. P., Davis. C . W,, and Conley, J. E , U. S. Bur. Mines, Bull. 212, 51-70 (1923). (20) Shemyakin, F. M., and Volkoi-a, T'. A,, J . Gen. Chem. ( U S . S.R.), 7, 1328-32 (1937). ABSTRACTED from a thesis presented by Charles V. Banks to the Graduate School of Iowa State College in partial fulfillment of the requirements for the degree of doctor of philosophy i n chemistry. The complete thesis will be on file at the Iowa State College Library. Contribution No. 2 from the Institute for .\tomic Research, Iowa State College, Ames, Iowa.
