1453
V O L U M E 2 4 , N O . 9, S E P T E M B E R 1 9 5 2 The sample should not be ignited directly in contact with porcelain, owing to the danger of action of calcium oxide on the material of the crucible to form silicates insoluble in acid, It is unnecessary to hydrolyze the calcium oxide to calcium hydroxide before the titration, as the water present in the acid being standardized will effect this transformation. PRIMARY STANDARD FOR VERSENATE CHELATOMETRY
Calcium acid malate hexahydrate is also suggested as a primary standard for chelatometry, involving calcium disodium dihydrogen ethylenediamine tetraacetate (Versenate) solutions (1).
As in other instances cited, the acid malate is a superior primary standard for chelatometry because of its unusually high equivalent weight of 207.143. The method used followed closely those standardizing with calcium chloride derived from calcium carbonate as a primary standard. One modification consisted in including the eriochrome black T, the magnesium ion required, and buffer, ammonium hydroxide-ammonium chloride, pH 10, in a single solution, 10 nil. of which was always added to the calcium solution being titrated. This constituted a time saver and afforded a constant amount of magnesium ion for all titrations in place of the variable amount introduced if the essential magnesium ion is included Rith the versenate titrant. The blank deducted from all volumes of reagent required amounted to 0.65 ml. of titrant. The normality of the Versenate solution established by calcium carbonate standard was 0.22 A' but
owing to the small equivalent weight of the calcium carbonate, 50.037, ana the dilutions required, is believed less accurate than that established by direct weighing of calcium acid malate hexahydrate crystals. Six different samples of calcium acid malate hexahydrate were used in determining the normality of a solution of Versenate found to be 0.022 N by calcium carbonak. The normality established by the acid calcium malate hexahydrate was 0.0211 f 0.0002. CONCLCJSIOH
A method has been developed for preparing pure calcium acid malate hexahydrate from crude sugar sand, an impure calcium malate by-product of the maple sugar industry. Three different methods for establishing its purity are given. Data are offered to prove the value of the calcium acid malate hexahydrate as a primary standard in sat,urated solution for calibrating pH meters, in alkalimet,ry, in acidimetry, in calcium c helatometry with disodium dihydrogen ethylenediamine tetracet'ate dihydrate (Versenate) solutions, and its use is suggested for standardizing the Karl Fisnher rea.gent,(3). LITERATURE CITED
(1) Biedermann, W., and Sohwartzenhach, G., Ch,imiu, 2, 56 (1948). (2) Lingane, ,J. J., ANAL.CHEM.,19,810 (1947). (3) Neuss, J. D.. O'Rrien, M. G.. m r l FrPrIiani, H. A , , Ibid., 23, 1332
(1951). RECEIVED for review Dpremhor 8 , 19.51.
.\rrf,r,t,rd .Jiine 23, 19.52.
Separation of Praseodymium from Lanthanum By Fractional Carbonate Precipitation in Trichloroacetate Solution LAURENCE L. QUILL
AND
MUHRELL L. SALUTSKY
Kedaie Chemical Laboratory, Michigan State College, East Lansing, Mirh. Precipitation from homogeneous solution appeared to be a potentially rapid means of separating mixtures of the rare earth elements. The separation and purification of priseodyniium from lanthanum-praseodymium mixtures were effected by the homogeneous precipitation of rare earth carbonates in trichloroacetate solution. Praseodymiuni concentrated in the carbonate precipitates, which upon repeated fractionation yielded a final product of spectroscopic purity. The praseodymium enrichment was independent of the temperature at which the precipitation was made, but seemed to be somewhat greater in dilute solutions. The procedure was applied to the fractionation of a crude yttrium group concentrate with good results. The trichloroacetate method is simpler and more rapid than most fractional methods for separating rare earths.
P
RASEODYMIUM and lanthanum salts are difficult to separate because of their similar chemical and physical properties. Slight differences in solubilities and basicities are important criteria in such separations. Full advantage of these slight differences is not realized by ordinary fractional precipitation methods, which depend upon the immediate formation of a precipitate after addition of a reagent. Interferences caused by local action are minimized if the precipitating rcagent is formed within the solution. Precipitations of this type which have been used for the fractional separation of the rare earthe art: summarized b y Moeller and Kremers ( 5 ) in a review article on the basicity characteristics of the rare earths. Sonic of the more common ones involve the separation of cerium by oxidation to the ceric form followed by hydrolysis, and the use of rare earth nitrites phthalates, lactates, sulfites, citrates, tartrates, m-nitrobenzoates, phenoxyacetates, etc., to form basic salts. In most of these methods the desired anion Present address, Mound Laboratory. hlonssnto Chemical Co.. Miramisburg, Ohio.
is added in the form of an alkali salt,. The resulting high concentration of alkali ions in the reaction mixtures is disadvantageous because of double salt formation. Willard and Gordon (9) separated thorium and the rare eart,hs from monazite by a homogeneous precipitation procedure involving the hydrolysis of dimethyloxalate to precipitate insoluble thorium and rare earth oxalates. Recently, dimethyloxalate has also been employed for the fractional separation of 1ant.hanum and cerium(II1) and of lanthanum and praseodymium ( 2 ) . In this investigation the separation of praseodymium from lanthanum was affected kip fractional precipitation as the carbonate in trichloroacet,at,esolution, The trichloroacetate ion under the influence of heat yields, according to Verhoek (7, 8) and Hall and Verhoek (3), carbon dioxide and the strongly basic trichloromethyl ion, whirh in tnrn reacts with water to form chloroform.
c(:13c:oo-
1
CXIq-
+ HyO
-~
4
C C k -t c : o 1 CHClr
-~-+
+ OH-
1454
ANALYTICAL CHEMISTRY
The chloroform is evolved from the hot solution. Part of the carbon dioxide reacts with water and any cation present to form a carbonate and the remainder escapes from the solution. I n the case of the rare earths (M = rare earth) the insoluble normal carbonates (6) are formed by the following reaction:
2M(CzC1302)3
+ 3HOH +3C02 + GCHCI, + Mz(C0s)s
The rare earth carbonates are not quantitatively precipitated. If the reaction is continued until all the trichloroacetate ion has decomposed, the filtrate will contain a small quantity of rare earth chloride The chloride ion is apparently formed during the reaction by the secondary oxidation of chloroform in the hot solution. Verhoek in his studies of the rate of decomposition of the trichloroacetate ion haa shown that the ion decomposes and not the free acid or its salts. In water the rate of decomposition of the trichloroacetate ion is dependent upon its concentration and the temperature of the solution. Hall and Verhoek claim that for a fixed temperature and salt concentration there is very little difference in the rate of decomposition of sodium, ammonium, barium, and even tetraethylammonium trichloroacetates in water. I t is probable that lanthanum and praseodymium trichloroacetates decompose at essentially the same ra.e; and, therefore, the separation of these two rare earths depends primarily on the solubility difference of their carbonates. EXPERIMENTAL
In separations involving fractional precipitation two methods are frequently utilized. Procedure A consists of precipitating several successive small fractions from the mother liquor. In turn each of these small fractions may be dissolved and reprecipitated to give additional fractions. A second method (Procedure B) consists of precipitating the greater portion of the material as a single fraction, leaving only a small amount in the mother liquor. This precipitate is dissolved and treated in such a manner that again the greater part of the material is precipitated. The procedure may be repeated until the desired purity is attained. A comparison of these two methods was made by fractionating mixtures of lmthanum and praseodymium. In the first case carbonate samples were removed after short time intervals. In the second case the trichloroacetate decomposition reactions were run for long periods of time t o precipitate most of the rare earth in a single sample. The amount of praseodymium in the samples w m determined by the usual absorption spectrum methods as outlined by Moeller and Brantley (4). Both the 444.5 and 589.0 mp absorption bands were used. PROCEDURE A
The lanthanum and praseodymium oxide mixture was dissolved in the calculated quantity of hot 25% trichloroacetic acid solution by sifting the oxide gradually into the acid solution. For oxide mixtures of high praseodymium content a slight excess of acid was necessary to effect complete solution. The trichloroacetate solution waa diluted with water until the rare earth concentration ww approximately 10 grams of oxide per liter. The solution wzb~heated to 90" C., and maintained a t this temperature for 20 minutes after the appearance of turbidity. During the heating the solution was stirred continuously. The reaction was stopped by cooling the mixture in an ice water bath. The carbonate precipitate, containing about 30% by weight of the original rare earth, was filtered in a Buchner funnel and washed with a small quantity of water. The filtrate was reheated to 90" C., and the decomposition reaction carried on for an additional 35 minutes. The mixture was cooled and the precipitate (a second 30% fraction) filtered as before. The filtrate from the second precipitate was treated with oxalic acid to recover the remaining rare earth. PROCEDUREB
The mixed oxides of lanthanum and praseodymium were dissolved in 25% trichloroacetic acid. The solution, diluted with
Table I.
Concentration of Praseodymium by Fractional Carbonate Precipitation
Original sample First carbonate Second carbonate Filtrate
(Prooedure A) Separation I Mixed Weight % oxides PrsOli 72 36 22 53 21 46 29 21
Separation I1 Mixed Weight % oxides PrsOu 22 53 6.5 72 6.5
62
9
36
Table 11. Concentration of Praseodymium by Fractional Carbonate Precipitation (Procedure B) fraction hlixed Oxides, G . Weight % PrsOli Original 1 2 3 4 5 6
7 8
0
26.2 20.3 16.7 12.4 10.4 8.9 7.7 5.5 4.5
67 74 79
Rfi -.
91 94
97 99 99
+
water so that the rare earth concentration was 25 grama of oxide er liter, was heated to and maintained at 90" C. for 2 hours. uring the heating the solution was stirred continuously. The carbonate mas filtered and dissolved in the minimum quantity of 5% trichloroacetic acid. This solution was heated for 2 hours in the same manner as the original solution and a second carbonate fraction precipitated. The carbonate was filtered, and dissolved in the 5% acid, and the decomposition was again effected. This procesE of precipitating a carbonate fraction, dissolving, and reprecipitating it was repeated until the desired purity was attained.
fb
RESULTS AND DISCUSSION
The data of Table I illustrate the concentration of praseodymium by Procedure A. A 72-gram sample of an oxide mixture containing 36% PreOll was treated to give two carbonate precipitates and a liltrate. In Separation I the first carbonate precipitate was enriched in Pr601, by 17%; when this precipitate was retreated an additional enrichment of 19% resulted. The praseodymium enrichment was doubled in two steps from 36 t o 72% PreOll. The analyses of the different fractions obtained in one typical separation study by Procedure B are given in Table 11. The original oxide sample weighed 26.2 grams and contained 67% PrsOll; fraction 8, consisting of high purity praseodymium, weighed 4.5 grams and represented a yield of approximately 25% with respect to the original praseodymium content. For both types of separation it is observed that the carbonate precipitate is enriched in praseodymium. Each of the two procedures may be utilized to advantage. For example, Procedure A is the better for concentrating small amounts of praseodymium from samples that contain less than 80% praseodymium. Procedure B is better for samples containing more than 80% praseodymium; although the enrichment per step is less, the major part of the sample remains intact. Procedure B was utilized for the purification of some prmeodymium oxide (about 16 grams) which contained about 5% Ianthanum oxide. The reaction time waa increased to 3 hours in order to precipitate a larger portion of the rare earth. After 10 fractionations the resulting carbonate precipitate was ignited and about 8 grams of oxide was obtained. The purity of this final praseodymium fraction was checked by two independent spectrum analyses. One analysis was made by F. S. Tompkins and his associates at the Argonne National Laboratory using the well-known copper spark method. The other, made a t Michigan State College by the authors, was the umial arc method using graphite electrodes to which the d u t i o n s
V O L U M E 2 4 , NO. 9, S E P T E M B E R 1 9 5 2
1455
of rare earth nitrates was added. Both analyses showed the following elements to be present as impurities: lanthanum, less than 0.02%; iron, less than 0.02%; silicon, 0.04%. S o n e of the other rare earth elements was detected. The effect of temperature on praseodymium enrichment was investigated by heating rare earth trichloroacet,ate solutions a t 60“)70”: 80°,and 90’ C. Each solutioii contained the equivalent of 10 grams of lanthanum-praseodymium oxide per liter. Two fractions of carbonates (Carbonatc I and 11),each containing about 30y0 of the original oxide sample, were removed and analyzed for prascodyniiuni. Thc reaction t,inies and analyses are listed in Table 111. The reaction time in each case was calculated by employing the usual firseordcr reaction equation and the rate constants reported by Verhoek (7) for the decomposition of sodium trichloroacetate in water. From these results it is evident that the separation of praseodyniiuni from lanthanum is not improved by carrying out, the reaction at a lower temperature and a dower rate. The effcet of rare earth concentration on praseodymium enrichment was investigated by heating a t 90” C. trichloroacetate solutions containing an equivalent of 10 and 20 grams of rare earth oxide per liter. Thc firfit carbonate fraction was removed 20 minutes after the appearance of a precipitate; the second after an additional 35-minute period. Each fraction contained approximately 3070 by weight’ of the original rare earth. The carbonates and the filial filtrates were analyzed for praseodymium, and thc average results from several experiments are lieted in Table IV. The results indicate that a better separation of praseodymium froin lanthanum is obtained in the more dilute solutions. Cerium interferes a i d should be rcnioved before lanthanumpraseodymium separations arc carried out. In hot trichloroacetate solutions cerium is oxidized and precipitates as a yellow basic ceric trichloroacetate, which imparts a gelatinous rharacter to the carbonate precipitate, making filtration difficult. Precipitation from homogeneous solution is a potential nieans of separating other difficultly separable mixtures of rare earth elements. The procedure is being applied to the fractionation of a yttrium group concentrate containing some cerium group earths. The average atomic weight of the rare earth in the original sample was 145.3. The separations process wag analogous t o Procedure A, outlined above: three successive fractions being obtained (Separation I). The first carbonate precipitate of this series was further fractionated to give two additional samples (Separation 11). The average atomic weight of the rare earth in each fraction of both Separation I and Separation I1 was determined ( 1 ) and t,he results are tabulated in Table V. Since the fractions arc rare earth mixtures, changes in composition are indicat,ed by changes in average atoniic weights. In both cases the average at,oniic \wight of the rare earth in
Table 111.
Effect of Temperature on Praseodymium Enrichment
Time from Appearance
I.
leinp.,
c.
90 80 70
GO
of Turbidity, Hours -___
Carbonate Carbonate I I1 0.36 0.62 1 .A 2.5 6.6 11.3 32.4 55.4
Table IV.
Original 53 53 5a .5 3
Weight yo PrsOil Carbonate Carbonate I I1 72 60 70 61 73 63 71 62
Effect of Rare Earth Concentration on Praseodymium Enrichment Weight ?’ & PrsOii
Rare Earth Oxide Concn. G./L.
Original
10 20
53
53
Carbonate I 72 65
Carbonate I1 62 61
Filtrate 36 41
the first carbonate precipitate is larger than that of the original sample, but in the second carbonate precipitate and the filtrate fractions it is 1ws than that of the original saniple. This is the type of change to be expected, since the order of separation of the rare earths follows the order of basicity of these elementsnamely, tht, least basic wrths (highest atomic weights) precbipitating before thti most h q i c (lov est xtoniic weights).
Table V. Fractionation of a Crude Yttrium Group Concentrate by Fractional Carbonate Precipitation (Procedure A ) Earth Averagi. -~ Aroniic .Weight Separation I Separation I1 148 3 152.5 152 5 162 5 111 2 111 8 1ii.i
- Rare
Originnl saiiigle First carbonate Second c a h o n a t r Tilt rate
,
~
Separation I1 illustiates the effectiveness of a single fractional precipitation step. The average atomic weight of thc rare earth in the original sample corresponds roughly to the atomic weight of element 63 (europium), while the carbonate precipitate approximates that of element 66 (dysprosium) and the filtrate that of element 60 (neodymium). Fractionation of this niagnitude is not common for a single precipitation step of rare earth salts. Work is in progress extending this type of separation to other rare earth mixtures. The trichloroacetate method for fractionating rare earth carbonates from homogeneous solution has a definite advantage over most heterogeneous precipitation methods in that no alkali ions are added during the course of the reaction. Accordingly, there is no opportunity for double salt formation and, thus, a reversal of solubility order which occurs in many instances if sodium or ammonium salts are used as the precipitating agents. The trichloroacetate method is more rapid than most fractional methods for separating rare earths becauee the precipitated rarbonates can be dissolved easily in trichloroacetic acid for RUCcessivc steps in the separations process. ACKNO W LEDGiMENT
Credit is due to Harry Ulmer and Robert Berry for assistance in obtaining some of the analytical data. LITERATURE CITED
(1) Fernelius, W. C., “Inorganic Syntheses,” Vol. 11, p. 58, New
York. McGraw-Hill Book Co. 1946 (2)Gordon, L.,Brandt, R. A . , Qu’ill, L. L., and Salutsky, LI. L., A N a L . CBEM.,23, 1811 (1951). (3) Hall, G. .4.,and Verhoek, F. H., J. Am. Chem. Soc., 69, 613 (1947). (4) Moeller, T.,and Brantley, J. C., AsaL. CBEM.,22,433 (1950). (5) Moeller, T.,and Kremers, H. E., Chem. Revs., 37,W (1945). (6) Salutsky, M.L.,and Quill, L. L., J . Am. Chem. SOC.,72, 3306 (1950). (7) Verhoek,‘F.H., Ibid.,56,571 (1934). (8) Ibid.,67, 1062 (1945). (9) Willard, H.H., and Gordon, L., ANAL.CHEM.,20, 165 (1948). RECEIVED for review November 16, 1950. Accepted June 27, 1952. Presented in part before the Division of Physical and Inorganic Chemistry. Bymposiiiin on Cheniistry of the Less Familiar Elements, at the 117th MeetSOCIETY, Detroit, Mich. Abstracted froin a ing of the AMERICAXCHEMICAL doctoral thesis submitted to the Michigan State College, 1950.
Correction I n the article entitled “Volumetric Determination of Milligram Quantities of Uranium” [Sill, C. W., and Peterson, H. E., ANAL. CHEX.,24, 1182 (1952)l in test 7 of Table VI11 the figure 0.761 under “UaOa Added” should read 1.761. C. W. SILL
