Fractional Precipitation of Lanthanum-Praseodymium Iodate from Homogeneous Solution Using Double Complexatio n and Replacement F. H. FlRSCHlNG Department o f Chemisfry, University o f Georgia, Athens, Ga.
b An efficient separation of lanthanum from praseodymium is described. Equal amounts of lanthanum and praseodymium are complexed b y equivalent amounts of [ (carboxymethy1imino)bis(ethylenenitrilo)]tetraacetic acid and N-(carboxymethyl)-N'-2-hyd roxyethylN,N'-ethylenediglycine, and iodate is added. A dilute solution of cadmium chloride is added dropwise to the stirred solution, selectively releasing lanthanum from its complex. A precipitation of rare earth iodate, enriched in lanthanum, occurs and continues as cadmium is added, The final precipitate contains about 99% of the lanthanum and 3070 of the praseodymium. The filtrate contains about 70% of the praseodymium and 1% of the lanthanum.
T
HE precipitation of various rare earth compounds from homo geneous solution has been studied by numerous workers ( 5 ) . Even though separation efficiency was increased over conventional precipitation methods, the enrichment in each step was low. With the application of ion exchange methods to rare earth separations, further precipitation work declined. Recently, a new method for homogeneously releasing cations in solution, using complexation and replacement, was applied to the separation of the alkaline earths (,9> 3). -4 modification of this scheme, using double complexation and replacement, has been applied to the separation of lanthanum from praseodymium with a marked improvement in separation efficiency.
EXPERIMENTAL
Reagents. Analytical grade reagents were used, except as described. Lanthanum nitrate, 99.99% pure, Code 548, a n d praseodymium oxide, 99.9% pure, Code 729.9, Lindsay Chemical Division, American Potash and Chemical Corp. Barium-140-lanthanum-140, processed, carrier-free, and praseodymium143, processed, carrier-free, Oak Ridge National Laboratory. A' - (Carboxymethyl) - A" - 2 - hydroxyethyl - A7,N' - ethylenediglycine
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ANALYTICAL CHEMISTRY
(HEDTX) sold as hydroxyethylethylenediaminetriacetic acid and [ (carbosymethylimino)bis(ethylenenitrilo) ]te t r a acetic acid (DTPA) sold as diethylenetriaminepentaacetic acid, K and K Laboratories, Inc., Jamaica, K. Y. Procedure. Add about 100 mg. of lanthanum oxide (0.6 mmole), about 100 mg. of praseodymium oxide (0.6 mmole), about 0.6 mmole of H E D T A , about 0.6 mmole of D T P A , and 10 mmoles of ammonium acetate to a new unscratched 250-ml. beaker. (All reagents should be in a soluble form.) Adjust the p H to 5.0 with acetic acid or ammonia. While stirring, add varying amounts of potassium iodate (0.4N), depending on the fraction of rare earths to be precipitated. (If 16 mmoles of iodate are added, about 50% of the rare earths precipitate; if 24 mmoles of iodate are added, about 65% precipitate.) d d just the volume to about 150 ml. and the final p H to 5.0. (If a precipitate forms during the addition of iodate, heating will remove the turbidity.) Stir the solution and add dropwise (a drop every 5 to 10 seconds) 50 ml. of 0.02X cadmium chloride, p H 5.0. with acetate buffer. After all the cadmium chloride has been added, let the solution stand for several hours to avoid any supersaturation, then filter. Carefully scrub the beaker to remove the tenacious film that tends to form. Filter. RESULTS A N D DISCUSSION
This separation is based on the formation of a complex of definite stability with each cation and each chelating agent. The logarithms of the formation constants for the complex formed with HEDTA and the following cations are: cadmium, 13.0; lanthanum, 13.22; praseodymium, 14.39 ( 1 ) . The logarithms of the formation constants for the complex formed with DTPA and the following cations are: cadmium, 18.93; lanthanum, 19.96; praseodymium, 21.85 (1, 6). If iodate is added to a solution of lanthanum and praseodymium HEDTA complexes, a precipitate will not form because of the low concentration of free cations. If cadmium ion is slowly added t o this solution, a competition for the chelating agent occurs. Each cation will compete with the other
cations present in the solution for the chelating agent. The cations will tend to form complexes according to their respective formation constants. The cation (praseodymium) that forms the most stable complex will tend to remain complesed, while the cations (lanthanum and cadmium) that form about equally stable complexes will tend to compete about equally for the chelating agent. The concentration of free lanthanum ions will gradually increase as the lanthanum is replaced from its complex by the introduced cadmium. Lanthanum forms the more insoluble iodate and selectively precipitates. Hon-ever, a fairly large amount of praseodymium coprecipitates under these conditions. Complexation and replacement using one chelating agent produce separations that are similar to other methods of precipitation from homogeneous solution. A marked improvement in the separation is realized when two chelating agents are used in the same solution. When a combination of DTPA and HEDTA is used to complex lanthanum and praseodymium, with cadmium as the replacement ion, separation is excellent. This increased separation efficiency is due to a combination of slight differences which collectively tend to improve the separation. Lanthanum forms the least stable complex with both chelating agents and also the most insoluble iodate. The formation constants for DTPA and HEDTA indicate that if equal molar amounts of these two chelating agents are added to equal molar amounts of the two rare earth ions, most of the praseodymium is complexed by the DTPA and most of the lanthanum is complexed by the HEDTA. If cadmium ion is slowly added to this solution, a competition for both chelating agents occurs. Cadmium forms a complex with HEDTA that is of about equal stability with lanthanum. However, cadmium forms a complex with DTPA that is considerably less stable than the praseodymium-DTPA complex. This means that as cadmium is added, the chief disturbance is to
the HCDTA equilibrium which contains mostly lanthanum The lanthanum tends to be replaced selectively from its complex and precipitated, while the praseodymium tends t o remain in solution as the DTPA complex. Tlie results using this method were determined using lanthanum-140 and praseodymium-143. Tlie distribution of the radioisotope was used to determine the distribution of the corresponding rare earth. Only one radioisotolle IT a< used in each determination. Tlie lanthanum-140 was separated fioni the barium-140 by forming insolul~lebarium nitrate in concentrated nitric acid and filtering off the soluble lanthnnum nitrate (8). A spectrographic analysis for lanthanum, made on fn-e filtrates, verified the results ohtained using lanthanum-140. Table I presents the analytical results and the ilietliod used in rnaking each determinal ion. Figure 1, prepared by combining the t n o sets of determinations given in Table I. shows the coprecipitation of praseodymium with lanthanum and mnmarizes the results. Slight variations, as would be expected in any coprecipitation study, were not incorporated in the graph. The last t n o ( oIumns, percentage of praseodyniiiun :ind percentage of lanthanum in the filtrate, show a smooth change :is tlic ncight of precipitate decreasep. 'l'hus hy u-ing the n-eig,lit of precipitate a s a source of reference, the fraction of praseodymium cokirecipitated can be plotted with respect to the fraction of lanthanuni p~ecipitated. The ignited precipitates contain material other than lanthanum and praseodyniium oxides. Both cadmium and potassium iodate contaminate the precipitate to some extent. When the precipitate weighs 100 mg., about 10% is not rare earth material; when the precipitate weighs 150 mg., about 15% is not rare earth material. The quantity of rare earth precipitated was controlled by varying the amount of iodate added. When 50% of the lanthanum precipitates, about 6% of the praseodymium coprecipitates; when 99% of the lanthanum precipitates, about 30% of the praseodymium coprecipitates. This means that about 70% of the praseodymium is left in the filtrate with about 1% of the lanthanum in one operation. Figure 1 indicates that this separation can be made more eficient by using a tno-step process. For example, enough cadmium could be added to precipitate about 90% of the lanthanum and about 15y0 of the praseodymium. The precipitate would contain about half the original material with lanthanum t o praseodymium in the ratio of 6 to 1. The solution could
Table I.
Experimental Results
(99.4 mg. of Lasot and 103.7 mg. of PreOll
taken)
176 169 163 152 150 146 138 132 130 126 126 124 118 115 106 106 100 92 86 83 65 63 55 54
Figure 1. Coprecipitation of praseodymium with lanthanum iodate
be filtered and additional cadmium added until 99% of the original lanthanum and about 30% of the original praseodymium are precipitated. This second precipitate contains about 12% of the starting material with lanthanum t o praseodymium in the ratio of 1 to 2. A t this point the filtrate contains about 35% of the original material with lanthanum to praseodymium in the ratio of 1 to 70. The logarithmic distribution coefficient of Doerner and Hoskins, A, can be applied to fractional precipitation from homogeneous solution ( 5 ) . Distribution coefficients calculated from points taken from Figure 1 are given in Table 11. The many combinations that could be used in this separation have not been completely surveyed. OnIy the following combinations have been studied to any significant degree: two precipitants, iodate and oxalate; two replacement ions, cobalt and cadmium; and two chelate combinations, H E D T A with DTPA and E D T A with DTPA. The reagents chosen for the method described are superior to any other combination studied. The effect of p H on this separation was not studied thoroughly. A p H of 5.0 was selected because preliminary experiments indicated that this range would be satisfactory. Furthermore, if the p H becomes too acid, the stability of the complexes is decreased and premature precipitation occurs. If the pH becomes too basic, the precipita-
Table 111.
...
64 69
0.9 1.04 0.8,o 0 . 5
...
70
...
'
...
1.3 1.2:
80 ...
...
'
1.3r 4.44
...
...
82
...
5.4"
... ...
84 85
...
14d
...
85 90
...
...
26d
...
91
...
41d
...
93.5
...
47d 63d
...
...
95
Ignited about 860" C. * Radiochemically using Pr143. Spectrographic analysis. Radiochemically using La1*.
a
Table II.
Calculated Distribution Coefficients
Per cent Per cent in precipitated filtrate 7T-E La Pr 5 10 12 15 18 20 31
43 62 81 90 95 98 99
57 38 19 10 5 2 1
Distribution
95 90 88 85 82 80 69
x 11.0 11.4 13 .O 14.2 15.1 17.4 12.4
tion of hydroxides complicates the separation. Many other combinations of chelating agents, precipitants, replacement ions, etc., are possible. This study has shown that precipitation from homogeneous solution using double complexation and replacement markedly
Comparison of Lanthanum-Praseodymium Separation Efficiency Using Precipitation
Pi-, 70 Original
Final
Yield, yo
Pptns.
64 67 67 50
85 97 99 98.5
33 59 42 70
1 6 7 1
Method Oxalate (4) Carbonate ( 7 ) Carbonate ( 7 ) Described procedure
VOL 34, NO. 13, DECEMBER 1962
1697
A Spectrophotometric Determination of Iodine in Silicate Rocks W. H. CROUCH, Jr.! Deparfmenf o f Chemistry, University o f Arkansas, Fayefteville, Ark.
b A spectrophotometric method for the determination of trace qjantities of iodine in silicate rocks has been developed. The iodine is removed from the rock b y alkaliqe fusiori, precipitatnd as silver icdide, and converted to iodate b y bromine. Iodine i s l i b e i a t d from the iodate solution in CJ qwoi?fity six times that cf the original sample b y the additiorl of excess iodide ion, and i s determined spectrophotometrically as the starchiodine chromogen at a woLrelength of 580 mp. The iodine colitent of a few silicate rock samples analyzed b y this method varied belween 0.Od a r d 0.20 p.p.m.
Ileceased Future co~res~icindence concerning this paper should be addressed to Professor P. K Kurodn, Ilepartment of Chemistrv. Universit:) of .irkansns, Fayetteville, Ark.
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ANALYTICAL CHEMISTRY
