1811
V O L U M E 2 3 , NO. 1 2 , D E C E M B E R 1 9 5 1 mean values ranged from iz0.0007 to *0.0040%. Results may be duplicated on the same sample with an average precision of about zkO.0016%. Some of the observations of others (5’) regarding the nature of the reagent have been confirmed. Thus a t 90” C. maximum color developed after 3 hours, while a t 80” C. a t least 5 hours were required. The colored complex is stable for a t least several hours. The e~clusionof moisture from the reaction medium is necessary. Although this situation is not ideal, it did not present any serious difficulties. The reagent is both selective and sensitive to boron and a maximum concentration of 10 micrograms of boron per 10 ml. should not be exceeded in the solution used for final photometric measurement. The normal time required to complete a determination is approximately 4 to 5 hours. CONCLUSIONS
1,l-Dianthrim:de may be used as a colorimetric reagent for the determination of boron in aluminum alloys. The average recovery of added boron from representative wrought aluminum alloys vas 104yob. This indicates that the
method may be applied to various aluminum materials without danger of interferences from major alloying elements. Boron was not lost by volatilization during initial solution of the sample. The colorimetric procedure gives results that compare favorably with the mannite titration method ( 2 ) and quantitative spectrographic techniques as applied in this laboratory. The average difference in values for boron determined by the mannite and colorimetric methods is zk0.0065% for aluminum alloys containing from 0.0220 to 0.2200~0boron. This difference is dewhen the colorimetric method is compared creased to =t0.0016~0 to results obtained by spectrographic analysis. Results may be duplicated on the same sample with a precision of about +0.0016 %. LITERATURE CITED
(1) Berger, K. C., and Truog, E., IND.EKG.CHEM.,ANAL. ED.,11, 540-5 (1939); Soil Sci., 57, 25-31 (1914). (2) Churchill, H. V., “Chemical Analysis of riluminum.” 3rd ed., p. 30, Pittsburgh, P a . , Aluminum Co. of America, 1950. (3) Ellis, G. H., Zook, E. G., a n d Baudisch, O., ANAL.C H E M . ,21, 1345-8 (1949). (4) Hatcher, J. T., and Kilcox. L. V.. I t i d . , 22, 567-9 (1950). ( 5 ) Naftel, J. A., IND.EKG.CHEXI..- 4 h - a ~ ED.. . 11, 407-9 (1939). d p r i l 16, 1951. RECEIVED
Fractionation of Lanthanum-Cerium(Ill) and Lanthanum-Praseodymium Mixtures By Precipitation f r o m Homogeneous Solution LOUIS GORDON AND R. A. BRANDT, Syracuse University, Syracuse, N . Y., LAURENCE L. QUILL AND MURRELL L. S..ILUTSICY’, Kedrie Chemical Laboratory, Michigan State College, East Lansing, JMich. A comparison is made of the efficiency of homogeneous and heterogeneous fractional precipitation methods using standard lanthanum and cerium mixtures. In the conventional method for the fractional p;ecipitation of rare earth oxalates, oxalic acid is added directly to a rare earth solution. Heterogeneous precipitations of this type are not so efficient as methods in which the precipitant is homogeneously produced within the solution. In
T
HE principle of precipitation from homogeneous solution ( 2 , 7 ) has been successfully employed in the fractional separa-
tion of pairs of similar chemical elements, such as zirconium and hafnium as phosphates ( 8 ) and praseodymium and lanthanum as carbonates ( 4 ) . This principle is utilized in this investigation for the fractional separation as oxalates ( 1 , 3, 9) of lanthanum and praseodymium and of lanthanum and cerium. A comparison is made of homogeneous and heterogeneous precipitation methods using lanthanum and cerium mixtures. Because the solubility differences of rare earth oxalates (6) are very slight, local interference is unavoidable and a poor separation is obtained when oxalate ion is added directly to a rare earth solution. If the reagent is added internally by the hydrolysis of dimethyl oxalate, this interference is reduced and the separation per fractionation step is improved. The precipitation of rare earth oxalates (11 = rare earth) from a chloride 1
Present address, I\lound Laboratory, .\Tonsanto Chemical Co., Miamis-
burg, Ohio.
this investigation rare earth oxalates are precipitated from homogeneous solution by the addition of oxalate ions internally through the hydrolysis of dimethyl oxalate. For the experimental conditions described, 10 fractionation steps by the homogeneous method are equivalent to 17 steps by the heterogeneous method. The homogeneous method is more efficient as a greater yield of product of desired purity is obtained with fewer fractionation steps.
solution hy the hydroll-sis of dimethyl oxalate is shown by the following reaction: 211CI3
+ 3(CH,)sCiOi + 6HOH +
+
hf,(c,o,)3 6CHaOH
+ 6HC1
EXPERIMENTAL
Three procedures were used in this investigation: Procedure -4 for the lanthanum-praseodymium mixtures, and Procedures B and C for lanthanum-cerium mixtures. Dimethyl oxalate was used in Procedures A and B as an internal precipitating reagent, whereas oxalic acid was added directll- in Procedure C. Procedure A. Five grams of a mixed oxide of lanthanum and praseodymium were converted to a chloride mixture and then dissolved in 600 ml. of 1 N hydrochloric acid. A solution containing 2.8 grams of dimethyl oxalate dissolved in 400 nil. of 1 N hydrochloric acid was added to this rare earth chloride solution a t a rate of 1 drop every 2 to 3 seconds. The reaction mixture was stirred continuous1,y during the addition of t,he ester solution and for 1 hour afterwards. Under these conditions
1812
ANALYTICAL CHEMISTRY
approximately one third of the total quantity of rare earths was precipitated. The rare earth oxalates, which were very dense and crystalline, were filtered and ignited a t 925" to oxides. The rare earths in the filtrate were precipitated with oxalic acid. These oxalates were also filtered and ignited to oxide?. The original oxide mixture and that obtained from the precipitate and the filtrate were analyzed iodonietrically (6) for praseodymium. Lanthanum, in each case, was determined by difference. Procedure B. Mixtures of standard cerium and lanthanum perchlorate solutions in varying ratios were prepared, so that in each case there was a total of 1 gram of mixed oxides. These mixtures were diluted to 200 ml., the pH was adjusted to 1.0 with hydrochloric acid, and a sufficient quantity of dimethyl oxalate dissolved in 10 ml. of ethyl alcohol was then added to
Additional experiments indicated a somewhat poorer enrichment if all the dimethyl oxalate solution mere added a t one time instead of dropnise and if 1 .\-sulfuric acid were used as the solvent for the sample instead of 1 S hydrochloric acid. The results of these experiments showed large increases in the percentage of praseodymium for a single fractionation step using the homogeneous method. To prove the homogeneous method more efficient than the heterogeneous method, standard mixtures of lanthanum and cerium were used, because lanthanum and cerium of high purity were easily obtainable and the cerium was readily determined volumetrically. The data in Table I compare the homogeneous method (Procedure B) and the heterogeneous method (Procedure C) for the fractional precipitation of lanthanum and cerium. In each case approximately 50% of the total rare earths was precipitated; cerium concentrated in these precipitates. io It is noted that the homogeneous method gives a greater enrichment per fractionation step. A graphic comparison of the two methods is shown in Figure 1. Two curves, one representing homogeneous separations and the other heterogeneous separations, are obtained when the data for Samples A to E (Table I) are plotted as points A to E. Line I,, a straight line connecting 50 and 100 mole yo cerium, represents the condition of no separation and is given for reference. At any point along line L the mole per cent cerium in the precipitate is equal to the mole per cent cerium in the initial sample. The numbered arrows indicate the consecutive precipitations required to obtain a precipitate of composition given by the ordinate a t that point, assuming an initial equal mole per cent mixture of lanthanum and cerium and a precipitation of 50% of the mixture for each step. D HOYOOEMEOUS For example, sample A (50 mole % cerium) was 0 HETEROBENEOUS treated bv the homogeneous method so that 50% of the sample !;as precipigted; the precipitate cdGtained LINE "L' ( N O SEPARATION) 62.4% cerium. This composition is represented by arrow 1 on the curve for precipitation from homogeneous solution. If the precipitate containing 62.4% cerium is taken as the initial sample, and one half of it is precipitated from homogeneous solution, the precipitate, arrow 2, which results will contain 73.3% cerium. This precipitate, if taken as the initial 40 50 60 70 80 90 100 sample for the third step, will yield a precipitate conCERIUM I N I N I T I A L SAMPLES, M O L E % taining 81.1% cerium, arrow 3. In turn, huccessive fractions indicated by the arrows may be obtained. Figure 1. Homogeneous and Heterogeneous Fractional The number of fractionation steps required to obPrecipitation of Lanthanum and Cerium tain a component of desired purity and t h e resultant yield of this purified component are criteria by which fractional processes are to be compared. Extrapolation of the precipitate one half of the total rare eaiths. Thesc solutions curves in Figure 1 indicates that ten fractionation steps by the were heated for 3 hours at 80" C. and cooled, and the resulting homogeneous method, compared with seventeen steps by .the precipitate was filtered and washed with ice water. The rare heterogeneous method, are required to produce a 99% cerium earth oxalates were ignited a t 1000' C. and the oxides weighed. mixture from an original 50% mixture. A4stlie weight of the The filtrate was then evaporated with nitric-perchloric acid earth sample is decreased by one half for each fractionation, mixture to fumes of perchloric acid, and the cerium was deterthere will also result a much greater yield of the desired prodmined volumetrically by titration with a standard ferrous sulfate uct when the homogeneous method is employed. It is probsolution after oxidation of the cerium with ammonium persulfate. able that this mode of precipitation is applicable to the fracProcedure C. Precipitations were carried out as under Protionation of other rare earth mixtures. cedure B, except that oxalic acid was used as the precipitating reagent. After the pH of the lanthanum-cerium perchlorate solutions had been adjusted to 1.0 with hydrochloric acid, a ACKNOW LEDGM E S T sufficient quantity of oxalic acid dissolved in 10 ml. of water mas added to precipitate one half of the total rare earths. The authors wish to acknowlrdge the assistance furnished by George Podoliak of Syracuse University i n the early stages of this RESULTS AND DISCUSSION investigation. The homogeneous method as outlined under Procedure A was LITERATURE CITED used for a mixture of lanthanum and praseodymium oxides which originally contained 64% PrsOil. Duplicate experiments yielded (1) Caley, E. It.. Gordon, L., and Simmons, G. A , . J r . , AXAL.CHEW, oxalate precipitates containing, on an oxide basis, 85% PrsO,,.
qL-4-y
22, 1060 (1950). L., I b i d . , in press. (3) Gordon, L., and Caley, E. R., Ibid.. 20, 560 (1948). (4) Quill, L. L., and Salutsky, 11.L., paper presented a t 117th Meeting of AMERICAS CHEMICALSOCIETY.. Detroit, hlich., April ( 2 ) Gordon,
Tahle I.
Comparison of Homogeneous and Heterogeneous Methods % of Total Rare Mole % Cerium in Initial $.Iole %
Cerium HomoHeteroBainple geneous geneous
Earths Precipitated HomoHeterogeneous geneous
Precipitate HomoHeterogeneous geneous
1960. (5) Salutsky, 51. L., Ph.D. thesis, hlichigan State College, 1950. (6) Sarver, L. A, and Brinton, P. H. M.-P., J . Am. C h e m Soc., 49, 943 (1927). ( 7 ) millard, H. H., AKAL.CHEW,22, 1372 (1950). (8) Willard, H. H., and Freund, H., IND.ENQ.CHEM.,ANAL.ED.,18, 195 (1946). (9) Killard, H. H., and Gordon, L., ANAL.CHEY.,20, 165 (1948).
RECEIVED April 9, 1951.
