1058
ANALYTICAL C H E MISTRY
sarin procedure t o thorium nitrate solutions of various concentrations.
latter and for its determination a t concentrations of about to 10-6 mole per liter. I n contrast to the thorin procedures, light absorption due to the complex occurs in a spectral reZion where the reagent does not absorb, obviating correction for reagent absorption. The naphthazarin reaction is not specific for thorium, and the element cannot he determined by this means in admiuture with the rare earth elements. However, the success achieved with the mesityl oxide extraction separation of thorium ion from the rare earth metal ions ( 1 , 8, 4,5 , 9) in other cases suggests that thorium cnn be determined conveniently in monazite and other rare earth combinations by the naphthsznrin procedure after i iesityl ovide extraction.
EFFECTS OF OTHER SPECIES
Rare Earth Metal Ion-Naphthazarin Systems. The absorption spectra of rare earth metal salt solutions ( L a L + I , P r P + + , X d + + + , Sm+++,Gd+-+, E r + - + ,Y + + + )treated with enthanolic naphthaxarin in 2 to 1 mole ratios are all similar to that given for neodymium in Figure 3, the XTave length of maximum absorption varying from 6065 A . for lanthanum to G310 h. for erbium. T h a t such solutions absorb appreciably a t 6200 A. suggests difficulty in att'empting thorium determinations in the presence of rare eart,h species. Attempted Thorium Detsrminations in Presence of Rare Earth Metal Ions. Since the individual rare earth metal ions showed essentially identical behaviors with naphthazarin, mixtures amounting to a cerium group combination of average atomic weight about 141 and an yttrium group combination of average atomic weight about 125 were used. Both behaved in essentially t,he same fashion, with displacement of the thorium absorption peak by about 50 d.and considerable contribution of rare earth absorption t o this peak in equimolar thorium-rare earth metal ion systems and disappearance of the thorium peak with intensified rare earth ahsorption in an approximately 6 to 1 mole ratio Pystem. Detailed studies on systems containing varying ratios of thorium and rare earth metal ions a t 6200 ai. Bhowed marked variations between thorium fourid and thorium present, even after correction for absorptions produced by the rare earth systems when taken alone, as shown by the typical data in Table 111. Effects of Other Cationic Species. Cranyl, zirconyl, and titanyl ions all give bluish to purple colors with naphthazarin: with absorption spectra closely comparable witG that given hy thorium ion.
ACKNOWLEDGMENT
Appreciation is expressed to E. I. du Pont de Nemours and Co. for the grant-in-aid which rendered this investigation possible. LITERATURE CITED
Banks, C. V., and Byrd. C. H., A N ~ LCHEM., . 25, 416 (1953). and Byrd, C. H., Ibid.. 25, 992 Banks, C. V., Klingman, D. W., (1953). Formbnek, J., Z . anal. Chem., 39, 673 (1900). Ingles, J. C., Can. Chem. Process Inds., 35, 397 (1951). Ingles, J. C., Can. Dept. Mines and Tech. Surveys, Mines Branch, 3Iemorandum Series, No. 110 (1951). Job, P., Ann. chiin., 9 [ l o ] , 113 (1928). Kronstadt, R., and Eberle, A. R., U. S. Atomic Energy Commission, Rept. RMO-838 (1952). Kuanetsov, V. I., J . Gen. Chem. (U.S.S.R.), 14, 914 (1944). Levine, H., and Grimaldi, F. S.,V. S.Atomic Energy Commission, Rept. AECD-3186 (1950). E., and Calvin, AI., "Chemistry of the Metal Chelate Compounds," pp. 98-9, Prentice-Hall, New York, 1952. Moeller, T., Schweitaer, G. K., and Starr, D. D., Chem. Rws., 42, 63 (1948). Thomason, P. F., Perry, 11. A , and Byerly, W. hI., ANAL. CHEY.,21, 1239 (1949). Torihara, T. Y . , and Underwood, A. L., Ibid., 21, 1352 (1949). Underwood, A. L., and Neumann, W.F., Ibid., 21, 1345 (1949). T'oshurg. IT. C., and Cooper, G. R., J . Am. ChPm. Soc., 63, 437 (1941).
co\CLusIoYs
The colored complex formed between naphthazarin and thorium ion provides a sensitive method for the detection of the
RECEIVED for reiiew October 27,
1954. Accepted March 21, 1955.
Determination of Thorium and of Rare Earth Elements in Ceriwm Earth Minerals and Ores M. K. CARRON,
U. S.
D. L. SKINNER',
and R.
E. STEVENS'
Geological Survey, Washington 25, D. C.
The conventional oxalate method for precipitating thorium and the rare earth elements in acid solution exhibits definite solubilities of these elements. The present work was undertalcen to establish conditions overcoming these solubilities and to find optimum conditions for precipitating thorium and the rare earth elements as hydroxides and sebacates. The investigations resulted in a reliable procedure applicable to samples in which the cerium group elements predominate. The oxalate precipitations are made from homogeneous solution at pH 2 by adding a prepared solution of anhydrous oxalic acid in methanol instead of the more expensive crystalline methjl oxalate. Calcium is added as a carrier. Quantitative precipitation of thorium and the rare earth elements is ascertained by further small additions of calcium to the supernatant liquid, until the added calcium precipitates as oxalate within 2 minutes. Calcium is remo\ed
by precipitating the hydroxides of thorium and rare earths at room temperature by adding ammonium hydroxide to pH>10. Thorium is separated as the sebacate at pH 2.5, and the rare earths are precipitated with ammonium sebacate at pH 9. Maximum errors for combined weights of thorium and rare earth oxides on synthetic mixtures are f 0 . 6 mg. Maximum error for separated thoria is i O . 5 mg.
R
ESETVED interest b y the U. S. Geological Survey in the cerium earth minerals, with the recent discoveries of bastnasite, monazite, and associated minerals in California and Mcntana led to studies of chernical and spectrochemical methods (3, 9) for determining the rare earth elements and thorium. The procedure developed is applicable to the analysis of ores and 1 \
Present address, U. P. Geological Surrey, Denver Federal Center, Den-
er, C o l a
V O L U M E 27, NO. 7, J U L Y 1 9 5 5 minerals in ivhich t h e rare earth elements are predominant'ly of the cerium group. With minerals in Lvhich yttrium is the major const,ituent, the procedure does not give quantitative recoveries. Although yttrium is not one of the rare earth elements, it must be considered with them because of its similar chemical behavior and. its occurrence in all cerium earth minerals. I n the procedure presented, four major features are described : Double precipitation of thorium and the rare earth oxalates a t pH 2, calcium being int,roduced as a carrier before and after each precipitation. T h e additions of calcium t o t h e supernatant liquid after t h e first precipitation indicate complete removal of thorium and the rare earth elements bj- t,he appearance of precipitated calcium oxalate within 2 minutes, longer periods of time indicating incomplete removal. Removal of most of the calcium b y precipitating thorium and the rare earth elements a t room temperature lvith ammonium hydroxide a t pH > 10. Separation of thorium from the rare eart,h elements by a single precipit,ation of thorium sebacate a t about p H 2.5. Precipitation of the rare earth sebacates in the filtrate with ammonium hydroxide a t p H 9. REAGENTS
Alcoholic Methyl Oxalate Solution. Heat reagent, grade oxalic acid dihydrate in a n oven a t 100" C. for 3 hours. Break up the crust and heat a t 100" C. for an additional hour. When cool, dissolve 40 grams of the anhydrous oxalic acid in 100 ml. of reagent grade methanol. A4110wthe solution t o stand a t least 3 days. Filter before using: 15 ml. of this solution provide approximately 4 grams of oxalate ion. Calcium Nitrate Solution. Dissolve 22 grams of C.P. calcium nitrate in 500 ml. of distilled water. Adjust to the green color of bromophenol blue indicator with very dilute nitric acid; 5 ml. are equivalent to 0.05 gram of calcium oxide. Sodium Hydroxide Solution. Dissolve 200 grams of reagent, grade sodium hydroxide in 250 ml. of n-ater. Let etand overnight and decant. Dilute t o 400 ml. Ammonium Hydroxide. Specific gravity, 0.90. Prepare fresh from tank ammonia. Bromophenol Blue Indicator. Dissolve 0.1 grain of watersoluble bromophenol blue in 100 ml. of water. Thymol Blue Indicator. Dissolve 0.04 gram of thj-ixo! blue in 100 ml. of 60% ethyl alcohol. Ammonia-Ammonium Nitrate Wash Solution. Dissolve 5 grams of reagent grade ammonium nitrate in 4 i 5 ml. of distilled water, and add 25 ml. of ammonium hydroxide, specific gravity of 0.90. Oxalic Acid Wash Solution. Dissolve 5 grams of reagent grade oxalic acid dihydrate in 500 ml. of distilled water. Hydroxylamine Hydrochloride. Reagent grade. Sebacic Acid, C.P. Ammonium Sebacate Solution. Dissolve 0 . i 5 gram of sebacic acid, c.P., in 18 ml. of distilled water and 1.5 ml. of ammonium hydroxide, specific gravity of 0.90. Hydrogen Peroxide. Reagent grade, 307& All other materials used were of C.P. or reagent grade. PROCEDURE
Decomposition of Sample. Ignite 0.5 to 2.0 grams of sample in a porcelain crucible a t dull red heat, to expel moisture. Add 6 t o 8 grams of sodium peroxide and fuse a t dull red heat for 1 minute. Cool. Transfer the crucible t o a 400-ml. beaker and leach the fusion product in 200 ml. of water. Digest the solution on the steam bath for 0.5 hour and allow it t o stand overnight, a t room temperature. Filter the solution through a double filter paper, one 12.5-em. Whatman No. 42 above one 12.5-em. Whatman KO. 41H filter paper (previously rinsed with 1% sodium carbonate wash solution). Wash 8 to 10 times with 1 % sodium carbonate solution. Discard the filtrate (silicon, aluminum, phosphorus, and sodium salts). Wash the precipitate from the papers with a stream of Tvater into the original beaker. Pass through the papers, in small portions a t a time, 50 ml. of hot 10% nitric acid containing 5 ml. of 30y0 hydrcgen peroxide. Rinse the papers with water. Heat until dissolution of the precipitate is complete. Remove and thoroughly police and rinse t h e crucible. Ignite the papers in the crucible, and add the ash to the solution. Removal of Silica. Evaporate the solution to dryness. Remove the silica in 570 nitric acid containing 1 gram of hydroxyl-
1059 amine hydrochloride. Volatilize silica with a few milliliters of hydrofluoric acid, and fuse the residue with 1 gram of sodium bisulfate. Dissdve the melt in 50 nil. of water. Bring the solution to boiling. and add 50% sodium hydroxide sufficient to provide 5% excess. Digest on the steam bath for 20 minutes If no precipitate appears, discard the solution. If a precipitate forms, filter the solution through a small K h a t m a n S o . 40 paper, and wash the precipitate 10 times with hot 1 % sodium carbonate solution. Transfer the precipitate and paper to the main solution, and adjust the volume to 125 ml. Precipitations of Thorium and Rare Earth Oxalates with Alcoholic Methyl Oxalate Solution. To the solution add 10 ml. of calcium nitrate solution (equivalent to 0.1 gram of calcium oxide), unless the sample is known to contain calcium as a major constituent. .4dd 5 t o 10 ml. of 307, hydrogen peroxide. Heat the solution on the steam bath, add 4 drops of 0.1% bromophenol Idue, then add 50% sodium hydroxide solution dropwise and with vigorous stirring to attain the blue-green color of the indicator. If iron precipitate masks the indicator, adjust t o p H 3.8 using suitable p H indicator paper. d d d 15 ml. of prepared alcoholic methyl oxalate solution slowly and with stirring. With the beaker uncovered digest the solution for 0.5 hour, maintaining the volume a t I25 nil. by replacing water lost b y evaporation. Remove the beaker from the steam bath, and adjust to p H 2 with sodium hydroxide solution using suitable p H indicator paper. Allow the qolution to stand 1 hour. Pour the supernatant liquid into another beaker, or if the liqiiid is not clear. filter about 75 ml. of it through a 9-em. Whatman S o . 42 paper. Reserve the paper for subseqient filtering Add 5 ml. of calcium nitrate solution Stir and allow the solution to stand until a precipitate of calcium oxalate forms. Precipitation of the added calcium nithin 2 minutes indicates that precipitation of rare earth elements and thorium has been essential117 completed. Return the solution and calcium oxalate to the beaker to &nd 1 hour longer. If the added calcium had precipitated in the supernatant liquid within 2 minutes, add 5 ml. more of calcium nitrate solution directly to the beaker and stir. (Otherwise repeat the transfer of the supernatant liquid or t h e filtration and t h e treatment n i t h 5 ml. of calcium nitrate solution until precipitation of calcium oxalate occurs within 2 minutes.) - 1 1 1 0 ~the solution to stand 0.5 hour, and filter it through a %em. Whatman KO. 42 paper or the reserved paper. Wash the precipitate 8 to 10 times Rith lye oxalic acid Transfer the paper and precipitate to t h e original beaker, add 20 ml. of concentrated nitric acid, cover, and destroy the paper and oxalates b j gent11 boiling until a few milliliters remain. Evaporate to dryness on the steam bath. Dissolve the salts in 50 ml. of 5% nitric acid containing 5 ml of 30% hydrogen perouide, by digesting the solution on the steam bath until clear. Adjust the volume to I25 ml., heat on the steam bath, and repeat the entire procedure for precipitating the rare earth and thorium oxalates, omitting further addition of calcium nitrate solution except for tests on the supernatant liquid. Destroy the paper and oxalates in the original beaker b r boiling a i t h 20 ml. of concentrated nitric acid until a few milliliters remain. Evaporate t o dryness on the steam bath. rilloiv the salts to bake a t steam-bath temperature for 0 5 hour. Separation of Calcium from Thorium and Rare Earth Elements a t pH > 10. T o the dry nitrates add 1 ml. of 1 to 1 nitric acid, 25 nil. of water, and 0.5 gram of hydroxylamine hvdrochloride. Digest on the steam bath until completely dissolved. Cool the solution to room temperature and dilute to 100 to 125 ml. Add concentrated ammonium hydroxide v, ith constant stirring to p H > 10, indicated by the blue color produced by a small drop of solution applied to pHydrion (Micro Lab , Brooklyn, S.Y.) indicator paper. Add 5 ml. of ammonium hydroxide in excess and some paper pulp, and allow the precipitate to stand 1 hour covered a t room temperature; stir occaqionally. Filter t h e s o h tion through K h a t m a n S o . 40 filter paper, previously rinsed with ammonia-ammonium nitrate wash solution, and n ash 6 times n i t h the same solution. Drain the excess ammonia from t h e precipitate by applying gentle suction. If thorium and the rare earth oxides are t o be determined together, omit the separation of thorium from rare earth elements and proceed with the precipitation of rare earth elements plus thorium Separation of Thorium from Rare Earth Elements. Transfer the precipitate and paper to the original beaker, add 20 ml. of concentrated nitric acid, and destroy the paper by gentle boiling. Evaporate to dryness. Dissolve the salts in 25 ml. of ivater containing 1 ml. of concentrated nitric acid. Dilute to 75 ml , and add 2 grams of hydroxylamine hydrochloride and 1 ml. of thvmol blue indicator solution. Heat the solution to boiling and add 10% ammonia qlowly t o obtain the bright orange color of the indicator, and the complete disappearance of all pink in the solution. Sloxly and with constant stirring add 50 ml. of a boiling solution containing 0.5 gram of sebacic acid and 2 drops of 5% nitric acid. 4 d d a small amount of paper pulp, and digest the
ANALYTICAL CHEMISTRY
1060 precipitate on the steam bath for 10 minutes. Filter the s o h tion through TVhatman KO.42 paper, and wash the precipitate 15 times with nearly boiling water acidulated with 5% nitric acid to the bright orange of thymol blue. Ignite the precipitate to constant weight a t 1000° C. Report as thorium dioxide. Precipitation of Rare Earth Elements plus Thorium. Transfer the precipitate and paper as described for separation of calcium to the original beaker, and pulp the paper with 6 ml. of 1 to 1 nitric acid. Heat on the steam bath and dilute to 150 ml. Add 1 gram of hydroxylamine hydrochloride and cool to room temperature. If thorium has been previously separated, adjust the volume of the filtrate t o 150 ml. and cool to room temperature. Excess sebacic acid crystallizes on cooling. Add concentrated ammonium hydroxide dropwise t o pH 9, indicated by pHydrion paper becoming dark green when a small drop of solution i p applied. Add 20 ml. of ammonium sebacate solution and, if not already present, a small quantity of paper pulp. Allow the solut,ion to stand 0.5 hour with occasional stirring. Filter the solution through Whatman KO. 40 paper, previously rinsed with ammonia-ammonium nitrate solution, and wash the precipitate 8 t o 10 times with the same wash solution. Drain excess ammonia by applying gentle suction. Ignite under oxidizing conditions a t 1000" C. to constant weight. Report as CeO?, (RE)203(+Tho%). EXPERIMENTAL DATA
Thorium, cerium, and lanthanum nitrates, C.P. grade, n-ere purified b y precipitation as oxalates, converted to nitrates, and dissolved in 5% nitric acid. Yttrium nitrate solutions were prepared from spectrographically pure yttrium oxide and froin yttrium group osides furnished by T h e George Washington University, Washington, D. C. The solutions were standardized 1)y evaporation and ignition to the oxides. The results given in Table I illustrate the efficiency of removal of calcium from cerium and lanthanum. Alternatively, a second precipitation with ammonium hydroxide alone a t p H > 10 can replace the ammonium sebacate precipitation, starting with a 0.5% nitric acid solution in order t o minimize the solubility of lanthanum oxide in excess ammonium nitrate ( 2 ) . Second precipitation as the sebacates is preferred because of the more easily filterable flocculent precipitate formed and because of its apparent insolubility in the presence of considerable ammonium nitrate salts a t a lox-er pH. Results in Table I1 show separations of cerium from varying quantities of thorium b y a single precipitation with sebacic acid a t pH 2.5. T h e p H adjustment is critical, requiring careful addition of ammonia. Precipitation made from solut,ions with the indicator color even slight,ly pink gave low results. Solutions approaching t,he yellow color of t,hymol blue gave positive errors of as much as 2 mg. T h e results in Table I1 show a maximum error of 0.5 mg. where proper p H adjustment was made. Spectrographic esamination of t h e ignit,ed thoria (thorium dioside) precipihtes showed contamination b y cerium to be 0.1 % or lem T h e separation is rapid, requiring less than 0.5 hour, and is dependable if the proper conditions are maintained. I n Table I11 results are given for rare earth plus thorium oxides in standard solutions starting with the oxalate precipitat,ions of the procedure. I n experiments 1 to 4,0.2 gram of calcium oxide added only prior t o the oxalate precipitation failed to effectively act as a carrier except when thorium and the rare earth elements were present in minor quantities. Recovery of milligram quantities (esperiment 3 ) was complete; with larger amount,s, lospea were 2 to 3 mg. Experiment 5 ) showing complete loss of small amounts of rare earth elements in the absence of any calcium, illustrates the need for using a scavenger. The results of experiments 6 to 8 indicate t h a t a t least a 1 t o 1 ratio of cerium plus lanthanum to yttrium plus thorium-i.e., esperiment 12-mu~t exist for complete recovery wit,h the procedure, cerium and lanbhanum perhaps acting as carriers. T h e losses were probably due to the high solubi1it.j- of yttrium oxalate. Experiments 9 to 12 show complete recoveries of large quantities of rare earth elements and thorium by repeated additions of calcium ion. T h e data given in Table TV were obtained with the over-all procedure on simulated rare earth and thorium silicates and phos-
Table I. Separation of Calcium from Cerium and Lanthanum by .4mmonium Hydroxide and Ammonium Sebacate Precipitations CaO = 0.4 gram Ct.02, LatOa. Gram
Taken
Found
0 2190 0 2196 0 0026
0,2197 0.2197 0,0027
Table 11.
Error +0 ,0001 +o ,0001 +0.0001
Separation of Thorium from Cerium with Sebacic Acid at pH 2.5
Taken
CeOz = 0.2 gram TliOt, Grani Found
0.0016 0.0312 0.0624 0.0937 0.1248 0 . 1.761
0.0018 0.0313 0.0622 0.0940 0,1283 0.1561
Error
+o. 0002 +o.
0001 - 0.0002 +0.0003 +o, 0005
Xone
Table 111. Determination of Rare Earth plus Thorium Oxides in Solution (Effect of repeated additions of calcium ion on collection of thorium and rare earth oxalates) Common elements taken (gram). CaO, 0.1-0.2: FetOa, 0.1; A1t08, 0.1; BlgO, 0.1; N n O , 0.05; PtOs, 0.002; TiOz, 0.005 CeOz, Lap03
ThOz
0.2970 0 0821 0.0017 0. 2927a 0. 0013a 0.0045 0.0045
0.0096 0.0096 0.0010 0.0049 0.0010 0.0096 0,0096 0.0096
0.0366 0.0366 0.0008 0.0183 0,0008 0.0330 0.0330 0.0330
0.2927 0.4098 0.1849 0.0471
0,0048 0.0096 0,0096
0.0183
'
9 10 11 12 a
Taken, Gram
Yzos
No.
None
0.0096
CaO Additions None Sone None h-one None
0.0330 0.0330 0.0330
RE, T h Oxides Found, Gram 0.3411 0.1258 0.0037 0.3129
None
3 3 3
0.0437 0.0410 0.0382
2 2 2 2
0.3164 0,4527 0,2275 0.0893
Error, Gram -0.0021 -0.0028
+o. 0002 - 0.0030 -0.0031 - 0.0034 -0.0061 -0.0014
-0.0004 +O. 0003
h-one
-0.0004
Common elements omitted.
phates. T h e maximum error for rare earth plus thorium oxides was +0.0006 gram. The quantities of thorium and rare earth elements were selected to represent samples ranging from low grade to high grade ores. Quantities of thorium and rare earth elements less than 0.0037 gram were not considered in t h e present study, but can be assumed to be quantitatively recovered according to Waring and Mela (14). The largest amount taken, 0.4524gram, is more than will be found in 0.5 gram of any cerium earth mineral. The over-all procedure, although somewhat long, appears to be more reliable than conventional methods employing oxalate and ammonium hydroxide precipitations. T h e results in Table V are of thoria and rare earth oxide determinations in unknown simulated ores prepared b y another member of the laboratory. The low value for thoria obtained in the first experiment probably illustrates the critical nature of the pH adjustment before precipitation with sebacic acid and is the product of overcaution in not adding sufficient ammonia. The loss is compensated b y a commensurate positive error in the value for rare earth oxides. Comparison of determinations of thoria in five monazite s a w ples b y using sebacic acid, b y other chemical methods, and by a spectrographic method ( 3 ) are given in Table TI. Although the proposed sebacic acid method generally seems to give slightly lower values than other chemical methods, all the results are in good agreement. DISCC'SSION
Solubility of thorium, rare earth, and yttrium oxalates (4,12) was confirmed b y the authors. Elperirnents shom ed consistent
1061
V O L U M E 2 7 , NO. 7, J U L Y 1 9 5 5 Table 1V. Determination of Rare Earth plus Thorium Oxides in Known Simulated AZinerals and Ores 0.2; gram of 1:1 CaaiPO&-feldspar mixture added t o Taken, Gram RE, Th CEO?, Oxides Found, Yzoa ThOz Gram LazOa 0.0330 0.4524 0,4098 0.0096 0.2927 0.0096 0.0330 0.3359 0.0048 0.0165 0.1388 0.1171 O.OlG5 0.0683 0.0471 0.0048 0.0017 0.0010 0.0008 0.0037
each
~
~~
Error, Gram Sone + O ,0006 t0.0004 - 0.0001 +0.0002
Table V. Separate Determination of Rare Earth and Thorium Oxides in Unknown Simulated Ores 0.25 gram of 1:1 ThOl, Gram Taken Found 0 0312 0 0296 0.0624 0 OGPG 0 0952 0 0943 0 0040 0 0040
Table VI.
Caa(P0a)z-feldspar mixture added to each __ R E Oxides, Gram Error Taken Found Error -0 OOlG 0 2337 0 2355 + O 0018 0 2337 0 2337 Sone +O 0002 -0 0009 0 2418 0 2421 + O 0003 None 0 2418 0 2414 -0 0004
Determination of Thorium Oxide in Monazites In Per Cent ~
Chemical NO.
Sebacic acid
Other methods
Spectrographic
losses of 2 to 3 mg. where double precipitations were made at p H 2. The losses are eliminated for small initial amounts of thorium and the rare earth elements by addition of calcium as a carrier before precipitation ( 1 , l 4 ) , b u t for large amounts there are still losses of 2 t o 3 mg. Repeated addition of calcium ion is therefore necessary. Experiments with repeated additions of calcium ion as a carrier revealed that the rate of precipitation as oxalate of the added calcium depended upon the quantity of cerium (and presumably other rare earth elements) in the solution-the greater the quantity of dissolved cerium, the longer the period of time for the appearance of the calcium oxalate precipitate. I n a series of experiments 5 mg. of cerium oxide in solution prevented the formation of calcium oxalate (calcium ion added as calcium nitrate equivalent t o 0.05 gram of calcium oxide) for about 4 hours, 3 mg. for 2 hours, and 1 mg. for half an hour; whereas in a solution containing no cerium, the calcium oxalate precipitated within 2 minutes. Thus, the appearanre of precipitated calcium oxalate can serve as an indicator of the completeness of precipitation of cerium and other rare earth oxalates. By decanting the clear supernatant liquid after the previous precipitations, the liquid can be tested for rare earth elements in solution by adding a solution of calcium nitrate. Appearance of the calcium oxalate precipitate within 2 minutes indicates the solution to be essentially free of rare earth elements. Two additions of calcium nitrate a t intervals of 1 hour after the oxalate precipitation usually were found to be sufficient to indicate complete precipitation of the rare earth oxalates. T h e ability of other rare earth elements to act as inhibitors has not been investigated. A solution of anhydrous oxalic acid dissolved in methanol was used t o precipitate thorium and t h e rare earth oxalates from homogeneous solution. This reagent performs equally as well as the more expensive solid methyl oxalate Tvhich was first introduced by Willard and Gordon (17’) as a precipitant from homogeneous solution. Although the prepared alcoholic solution hydrolyzes faster than the solid reagent, it produces easily filterable crystalline oxalates.
(:alciuni ltnd sodium salts !$-ere successfully removed b y prceipitating thorium and rare earth hydroxides from solution a t rooin temperature by adding aninioniuin hydroxide to pH > 10. This precipitation is customarily done at boiling tenperatwe, which results in volatilization of ammonia and consequent lack of control in maintaining the p H needed for complete precipitation. The high temperature also was found to be faulty in promoting the precipitation of calcium as carbonate a t the high p H required. 1:ickery ( 1 3 ) states that unpublished investigatioris showed that with a rare earth t o calcium ratio of 3 to 1, at least five precipitations as hydroxide, presumably frcm hot solut.ion, are n e c e s s q - to reduce the ratio to 200 t,o 1. From cold solutions, however, the authors found that 0.4 gram of calcium oxide could be quantitatively separated from 0.2 gram of cerium and lanthanum oxides bj- two precipit,ations with carbonate-free ammonium hydroxide a t p H > 10. -4single precipitation of the hydroxides at, room temperature suffices to remove all but small quantities of calcium; subsequent precipitation Tvith ammonium hydroxide a t p H > 10 or ammonium sebacate (15, 16) at p H 9 removes the remainder. Quantitative separations of varying amounts of thorium oxide up to 0.1560 gram from 0.2 grain of cerium oxide mere obtained by a single precipitation with sebacic acid ( 5 , 6, I O ) in boiling solution a t about p H 2.5. The reagent does not lend itself easily t o double precipitations because of its insolubility in acid solution. The rare earth elements in the cool filtrate of the thorium precipitate are recovered with ammonium hydroxide and ammonium scbacat,e at p H 0. Thus the sebacate ion can be used as a precipitant for thorium a t a low pH and for the rare earth elements a t a higher pH. Although other organic reagents for precipit,ating thorium may act similarly for the rare eart’h elements, they have iiot been reported. Some of the more recently investigated organic reagents for precipitating thorium are m-cresosyacetic acid ( I I ) , cinnamic acid ( 8 ) , and vanillic acid ( 7 ) . ACKNOW‘LEDG.\IENT
T h e authors are indebt,ed to their colleagues: K. J. Murata and H. J. Rose, Jr., for many spectrographic examinations of reagents and experimental precipitates, C. R. Naeser for suggestions, and J. M. axelrod and J. J. Fahey for many helpful comments. Thanks are also due H. E. Kremers, L. F . Yntema, and Therald hloeller for their advice and suggestions. LITERATURE CITED
Borneman-Starynkevich, I. D., Borovik, S. A., and Bororskii, I. B., Dokladv A k a d . X a u k S.S.S.R., 30, 227-31 (1941). Brauner, B., Monatsh. Chem., 3, 1 (1882). Dutra, C. V., and hlurata, K. J., Spectrochim. Acta, 6, 373 (1954).
Kall, H. L., and Gordon, L., 4 ~ a t CHEM., . 25, 1256 (1953). Kaufman, L. E., J . Applied Chem. (U.S.S.R.),8, 1520 (1935). Kaufman, L. E., Traa. inst. Btatradium (U.S.S.R.), 4,313 (1938). Krishnamurty, K. V. S., and Purushottam, A , , Rec. trav. chim., 71, 671 (1952).
Krishnamurty, K . T’. S., and 1-erkateswirlu, Ch., Ibid.,71, 668 (1952).
Rose, H. J., Jr., Murata, K. J., and Carron, If.K., Spectrochim. Acta, 6, 161 (1954).
Smith. T. 0..and James. C.. J . Am. Chem. SOC..34. 281 (1912). Venkataramaniah, M., Rao, B. S. V. R., and Rao, C. L.,‘ANAL. CHEM.,24, 747 (1952). Vickery, R. C . , “Chemistry of the Lanthanons,” pp. 92, 219, Academic Press, Kew York, 1953. Ibid.,p. 189. Waring, ‘2. L., and Mela, H., Jr.. ANAL.CHEM.,25, 432 (1953). Whittemore, C. F., and James, C., J . Am. Chem. SOC.,34, 772 (1912).
Ibid.,35, 127 (1913). Willard, H. H., and Gordon, L., ANAL. CHEM.., 20, 165 (1948). R E C ~ I V Efor D review November 10, 1954. Accepted March 7, 1955. Publication authorized by the Director, U. s. Geological Survey. Presented at the Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1954.
