Analytical Chemistry of the Rare Earths J
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Average Atomic Weight of the Rare Earths in a Mixture G. L. BARTHAUERI, R. G. RUSSELL2,AND D. W. PEARCE, Purdue University, West Lafayette, Ind. The oxalate to oxide, oxalate to permanganate, method for determination of the average atomic weight of the rare earth in a mixture has been studied, using pure materials and mixtures of known composition. A n important source of error lies in the standardization of the permanganate solution. To minimize the error this operation should employ as a standard that rare earth which is the principal component of the mixture.
A
KNOWLEDGE of the average atomic weight of the
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2La 48 -= 216.06
o'1863 X 0.1218 0.3208 0.02526 X 7 2 . 0 2 X 5 1 . 4 0 2 x 1000 La = 139.45
While the precision of the experimental results is satisfactory for the desired purpose, the accuracy is not. I n all cases the atomic weights are slightly high. As a result of searching the method for errors leading to high values it appeared likely that the oxalate titer of the permanganate against National Bureau of Standards sodium oxalate (dried overnight a t 140' C.) was not the same as that obtained against the oxalate content of rare earth oxalate dried a t 110' C. for a similar period of time. A possible explanation of this, in turn, might be that some nitratooxalate (6) contaminates the oxalate of the rare earths when the latter is precipitated in the usual may (4). A comparison was therefore made between the normality of potassium permanganate solutions as determined by standardization against Bureau of Standards sodium oxalate on the one hand and against the pure rare earth oxalate itself on the other. Table I1 lists the results obtained in this series of determinations. It is evident that the oxalate titer of the permanganate solution as determined against the oxalate of the rare earth concerned is greater than that determined against sodium oxalate except in the case of gadolinium. The trend is consistent and is toward closer agreement of the two normalities with decrease in basicity of the rare earth (increase in atomic number) and the change in this difference may be significant.
rare earths in a mixture allows the composition to be calculated if only two earths are present or provides sufficient data for such a calculation if the percentages of all but two of the components may be determined in other mays. Analytical methods suitable for routine determinations of the atomic weights of the rare earths are few. The oxide to sulfate method of Schutzenberger (7) is of historical interest only, since the many possible errors in the conversion to anhydrous sulfate render its use unreliable. The choice is apparently among the method of Feit and Przibylla (3) in which a weighed amount of oxide is dissolved in excess dilute sulfuric acid and the excess titrated with standard base; the oxalate to oxide, oxalate to permanganate, method of Gibbs (6); and the chloride t o silver method, first used for a rare earth by Baxter and Chapin ( 1 ) . Of these the first-named is unreliable for mixtures of the earths of weaker basicity, while the last is too time-consuming for routine application. The oxalate method is probably intermediate with regard to both the time reauired for the determination and the accuracy obtained. The method involves the following steps: TABLE I. ATOMICWEIGHTSOF SOMECERIUM GROUPEARTHS precipitation, washing, and oven-drying of the Interoxalate; determination of the oxalate to oxide Weight Volume Weight Weight national of of A' KMnOd of of Calcd. Atomic ratio by the ignition of weighed portions of the ;:g Oxa,atea KhIn04 OS. Oxalate Oxide Atomic Weight, Oxide Titrated Csed NaZCzOc Ignited Obtained W-eight 1941 partially dehydrated oxalate, followed by disM l . Gram Gram solution of weighed portions of the same sample Gram of oxalate in sulfuric acid and titration of the La208 0.1218 51.40 0.02526 0.3208 0.1863 139.45 0,1090 45.80 0.02826 0.3953 0.2294 140.03 oxalate content with standard permanganate Av. 139.74 138.92 solution. The average atomic weight is talNd208 0.1022 40.50 0.02515 0.2214 0.1247 145.54 0,0860 34.12 0.02515 0,2338 0,1321 145.88 culated from the formula, A / B = (2 R. E. Av. 145.71 144.2i 30)/3C203, where A is the weight of rare earth SmiOs 0.1132 43.15 0.02467 0.2099 0.1153 151.25 0.0896 34.10 0.02467 0.3056 0.1673 150.93 oxide, R. E.*Oa which corresponds to B , the Av. 1 5 1 . 0 8 150.43 weight of C2O3 found in the titrated samples. a I t follows from the data in Table I that the weight of water remaining in the oxalate
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Experimental
after partial dehydration a t 110' C. could he calculated readily. Such materials are no longer hydrated with 9 or 10 molecules of water although they may have been precipitated as ennea- or decahydrates. Further investigation of this point is planned.
A preliminary investigation of the accuracy of the method was made, using certain spectrographically pure rare earths of the cerium group. The data are shown in Table I. The calculation: 2La+.. 3GOs
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I n view of these results it was decided that the pernianganate solution in all cases should be standardized against a pure sample of the oxalate of that rare earth which appears t o be most abundant in the niivture concerned.
Recommended Procedure
N of KMn04 x m. eq. of Cz08X ml. of KMn04used
The experimental procedure is the same for the standardization of the permanganate solution against the oxalate of a rare earth of known atomic weight as it is for the use of the permanganate thus standardized in the determination of the
Present address, Magnolia Petroleum Co., Dallas, Texas. 2 Present address, Aluminum Research Laboratories, New Hensington, Penna. 1
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A N A L Y T I C A L EDITION
September 15, 1943
TABLE 11. COMPARISON OF STANDARDIZATION METHODS Rare Earth Oxide La208
Xddh
SniG
Weight of Oxalate Titrated Gram 0.1195 0.0782 0.0982 0.1303 0,1421 0.1275 0.1301 0.1562 0.1595 0.1251 0.2386 0.1043 0.1528 0.1182 0.1784 0.1487
Volume of KMnOd Used
M1. 51.20 33.55 41.00 54.50 55.35 49.60 51.70 62.00 60.70 47.65 89.40 39.20 56.95 44.10 64.60 53.85
Weight of Oxalate Ignited Gram 0.2005 0.2017 0.4747 0.6367 0.1410 0.1912 0,5301 0.4457 0,2544 0.2530 0.4198 0.6214 0.5778 0.8658 0.5724 0.4277
Weight of Oxide Obtained Gram 0.1178 0.1183 0.2761 0.3703 0.0775 0.1049
0.3034 0.2551 0.1410 0.1405 0.2347 0.3468 0.3238 0.4851 0.3205 0.2394
V . KMnOl
R. E. Oxalate
NKh.1n04 vs. Na2C204
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02516 02514
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0":!!E:
0"
0 02505 0 02508 0 02566
0 0 0 0
02494 02494
02494 02494 02545 02545 02494
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Accurately weighed amounts of pure lanthanum oxide and samarium oxide were taken and dissolved in nitric acid, the solutions were combined, and the oxalates were precipitated together. The average atomic weight of the rare earth in the mixture was then determined according to the recommended procedure. The permanganate used was standardized against lanthanum oxalate, the principal constituent of the mixture, as well as against Bureau of Standards sodium oxalate, and both of the resulting normalities were used in the calculation. The results are shoim in Table IV. It must be concluded that the accuracy obtained is much greater if standardization against the rare earth oxalate is
The oxalates of cerium, praseodymium, terbium, and possibly of neodymium yield on ignition various oxides containing Some or all of the rare earth in a higher oxidation 0 02485 02559 0 02557 0 02567 state. Their exact behavior in mixtures is not yet certainly defined nor the influence of cerium certainly understood. It is therefore advisable TABLE 111. STANDARDIZATION AGAINST LANTHANUM OXALATE to remove cerium before the atomic weight is Volume Wei ' Weight Weight Oxide Weight of of Equivalent M , Eq. determined; this may be accomplished quantiof of 0 KkfnOc Oxalate 0x1 Oxalate KMnOc Oxalate Oxide to Oxalate of S of tatively by the use of the iodate method (4). .sample Titrated Used Ignited Obtained Obta,..,, Titrated C ~ 0 8 K\ino4 Gram Gram Praseodymium and terbium cannot be thus reGram Ml. Gram I 0.1792 47.80 0.2005 o 1178 0.1053 o 03601 0 , 0 4 0 5 ~ moved, so that their behavior on ignition must I1 0.1564 41.70 0.2017 0.1183 0 0917 0 03601 0 04052 be accepted, for the present, as resulting in an unavoidable error. These elements are rare, howerer, and for usual mixtures the error nil1 he small. average atomic weight of a n unknown mixture. The calFor mixtures rich in these elements the ignition product may culations involving data from the former operation are shown be assumed to be Pr6011( 2 ) and Tb407 (8). The behavior of below. neodymium under the prescribed ignition conditions is still incompletely known, but it is certain that it is not oxidized About 1 gram of the pure rare earth oxide standard (or of the to a valence higher than 3 when cerium alone is present. unknown oxide mixture) is dissolved in 50 ml. of water containing The influence of cerium and praseodymium together has not 7 ml. of 15 N nitric acid. After complete dissolution has occurred, the solution is diluted to 100 ml. and heated to boiling yet been fully investigated, but the behavior of these elements and the rare earth oxalate is precipitated by the slow addition of a in the ignition of various mixtures is being studied and will hot solution of 10 grams of oxalic acid dihydrate in 50 ml. of form the subject of a later paper in this series. water. Since the presence of even small amounts of oxalic acid in the precipitate will seriously affect the final result, it is important that the oxalate be thoroughly washed. This operation should TABLEIT. AXALYSISOF A MIXTUREOF Ksonrx COXPOSITION consist of two washings by decantation with 100-ml. portions of Btomic Weight hot water followed by a Tvashing on a filter, removal to a beaker, of La Sm Actual Actual and another series of decantations. After a final washing on a Standard S KMnOh Mixture Calcd. Calcd. La Sm filter the net oxalate is removed t o a beaker, the supernatant % % % % liquid decanted, and the precipitate dried at 110" C. for 8 to 12 xanCz0a 0.02300 1 4 3 . 4 ~ ~ f10.4 39.6 69.2 30.8 hours. T,az(C20P)a 0,02580 142.2ob r1 5 28.5 69.2 30.8 From the partially dehydrated oxalate are removed two sam0 Average of 143.78 and 143.17. ples, each of about 0.2 gram, and these are accurately weighed b Average of 142.48 and 141.91. and ignited in an electrically heated muffle furnace to constant weight at 900" C. In this ignition it is advisable t o put the oxalate into a cold furnace and to allow the temperature to rise slowly, since the dry powder is very easily blown from the crucible .4clinowledgment by the escaping gases i f rapid heating next t o the walls is allowed It is a pleasure to acknoivledge the aid given by B. S. to occur. Two other oxalate samples, each of about 0.15 gram, are reHopkins of the University of Illinois to this and other inmoved, accurately weighed, and dissolved in 20 m]. of warm 10 S vestigations soon to he reported. He has generously loaned sulfuric acid. These solutions are diluted with 100 ml. of water, several rare earths of high purity; without these the authors heated to go", and titrated with standard potassium permangaTyould have been unable to proceed. nate solution. The normality of this oxidant is conveniently either 0.04 or 0.025 A- (ca. 1.26 or 0.79gram of potassium permangaLiterature Cited nate per liter of solution), depending upon the amount of pure material available for Standardization as well as upon the weight (1) Baxter, G . P., and Chapin, H. C., J . Am. Chem. SOC.,33, 1-27 of sample available for the desired study. (1911). (2) Brinton, P. H., and Pagel, H. il., Ibid., 45, 1460-5 (1923). The data in Table I11 illustrate the standardization of such (3) Feit, W., and Przibylla, K., Z . anorg. allgem. Chem., 43, 212-13 (1905). a permanganate solution against lanthanum oxalate. (4) Furman, K.H., "Scott's Standard Methods of Chemical Analysis", New York, D. Van Nostrand Co., 1939. 2La 30 0.1053 -= ( 5 ) Gibbs, W., Am. Chem. J.,15,546-51 (1893). rNKMnO4 X 47.80X 0.03601 3C2OS (6) Meyer, R. J., and Marckwald, E., Be?., 33, 3003-13 (1900). 2 X 1 3 8 . 9 2 + 3 X 16 (7) Schutzenberger, P., Compt. rend., 120, 962-6 (1895). 3 X 72.02 (8) Urbain, G., Ibid., 141,521-4 (1905). z = 0.04056 Gd208
0 02554 o 02488
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I Iv.mLTu
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Results and Discussion In order to examine the usefulness of the method, a binary iriixture of known composition was prepared and studied.
PRESENTED before the Division of Physical and Inorganic Chemistry at the 105th Sleeting of the AMERICAN CHEMICAL S O C I E T Y , Detroit, hlich. Abstract of a section of a thesis presented by G . L. Barthauer to the Graduate School of Purdue University in partial fulfillment of the requirements for the degree of doctor of philosophy, August, 1943.
