OBSERVATIOXS O X THE RARE EARTHS.
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395
OBSERVATIONS Or\’ THE RARE EARTHS. LI1
AN ELECTROMETRIC STUDYOF THE PRECIPITATION OF TRIVALENT HYDROUS RAREEARTH OXIDESOR HYDROXIDES THERALD MOELLER
AND
HOWARD E. KREMERS2
No yes Chemical Laboratory, University o j Illinois, Urbana, Illinois Received June 29, 1944 INTRODUCTION
Theoretical considerations based upon the more or less steady decrease in the molecular volumes of isomorphous compounds of the trivalent rare earth elements, or in the radii of the trivalent ions themselves, with increase in atomic numbers indicate a corresponding decrease in basicity in this series of elements (23). Experimental data obtained in a variety of fashions agree well with this prediction (22), but many of the methods used have been indirect. Probably the most useful comparisons are those based upon the properties of the hydrous oxides or hydroxides themselves, but in most instances such data as are available depend upon results obtained with rare earth mixtures rather than with the individual elements. It has already been shown (6) that the basicities of the rare earth elements can be compared in terms of the solubility-product constants for the hydrous hydroxides, but data are available only for the elements lanthanum, praseodymium, neodymium, samarium, gadolinium, dysprosium, and yttrium (6). Furthermore, basicity comparisons can also be made in terms of the pH values at which the hydrous oxides or hydroxides precipitate (4). Since both of these types of data can be obtained from electrometric titrations of metal salt solutions with alkalies (3), it seemed logical to approach the problem in this fashion. The rare earth elements have thus far been only incompletely investigated in this manner. Thus, Hildebrand (8) found praseodymium and neodymium chloride solutions to yield precipitates in pH ranges close to 7 , whereas the corresponding nitrate solutions gave precipitates a t pH values below 4, a difference which has been ascribed to catalytic reduction of the nitrate ion a t the hydrogen electrode employed (2). Britton (21, working with a hydrogen electrode in solutions approximately 0.01 in rare earth ions, investigated the effects of sodium hydroxide upon salts of the cerium earths (lanthanum through samarium) and yttrium. -4similar investigation, based upon results obtained with a glass electrode, was carried out by Bowles and Partridge (1) upon approximately 0.01 X lanthanum, cerous, ceric, praseodymium, neodymium, ytterbium, and thorium salt solutions. Some additional measurements upon 0.03 lanthanum chloride solutions (21) and upon approximately 0.005 N lanthanum, cerous, and yttrium salt solutions (18) have been reported. 1 For the preceding communication in this series, see Moeller and Kremers: J. Am. Chem. SOC. 66, 307 (1944). 2 Present address: Lindsay Light and Chemical Company, West Chicago, Illinois.
396
THERALD MOELLER .iKD HOT.%RD E. KREMERS
Inasmuch a9 in all these investigations except the last tu-o (18,21) no data upon solubiIity-product constants have been presented, and inasmuch as data upon the yttrium earths are almost entirely lacking, a comprehensive study of the series n-as indicated. This paper presents data obtained by the electrometric method for twelve of the rare earth elements, including yttrium, for nitrate, sulfate, and acetate solutions. These data serve the triple purposes of checking the effects of anions, evaluating the precipitation pH values and the solubility-product conT.4BLE: i Rare earth materials employed DESIGNATIOY
OXIDE
YT-17 LA-24
PrsO11. . . . . . . . . . Xd203. , . . , . . . . .
PR-21 ND-34 SM-19
Gd203. , . . , . . , . .
GD-5
Ei-203. . . . . . . . . . , T m 2 0 3. . . . . , . . ,
ER-34-2d TM-5-R3
Smp03.. . . . . . . . . EU203.. . . , . . . . ,
YB-1-a L u 2 0 3 ... . . . . . . . ,
~
LU-4-Rll
SOURCE
Reference 9 Reference 10 Ignition of G . F. Smith Chemical Co. hexanitrato ammonium cerate Less t h a n 1% Kd103 Unknown Atomic weight purity Unknown Reference 19 * Atomic weight purity Atomic weight purity Ignition of oxalate prepared by McCoy (14) Fraction Gd-7 (reference Atomic weight purity 17) Reference 16 96% Er203, 4% Y z O ~ 78% TmpOB, 11% Lu203, Extraction of Yb from Tm-Yb fractions with < I % Ybz03 sodium amalgam (reference 13) Less than O.lyoother rare Reference 20 earths 95% Lu203, 4% Tm203, Extraction of Yb from Yb-Lu fractions with
t -1 m3
1.00
-
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I!
-
1.1 0
-
1.00
-I
$
LL
0
0 !Q
-0 0
LL
0
2
z - 1.00
a
a
P
-
-
!-
k
'\\
0.90
0
0 -I
40
50
60
70
A T O M I C NUMBER
FIG.4. Relative basicities and ionic radii
relative size of its cation. Independent confirmation of published basicity arrangements (22) is thereby offered. If one assumes n-ith EndreF.: (6) that the relative basicities of the hydroxides
OBSERVATIONS O S T H E RARE EARTHS.
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405
qtand in the same ratio as their solubility-product constants, one can then evaluate these relative basicities from the data in table 3 . Basing these calculations upon yttrium hydroxide as a standard, one thus obtains the values listed in table 4. Included also are the corresponding values given b j Endres (G), as n-ell a9 the ratios of the ionic radii of the trivalent ions to the yttrium ion. Agreement between the values obtained in this investigation and those of Endres is good, and the decrease in relative basicities parallels the decrease in the ratios of the ionic radii. The anomalously high value for the basicity of praseodymium hydroxide is again traceable to peculiarities in the titration curves. Inasmuch as Endres (6) pointed out that plots of both the logarithms of the ratios of the solubility-product constants and the ratios of the corresponding ionic radii against atomic numbers gave similar curves, this approach has been carried out in figure 4. Since the similarities in trend are so apparent, Endres’ conclusions as to the relations between basicities and ionic sizes ( 6 ) can be considered substantiated. Consideration of the precipitation pH values and the solubilities of the hydrous hydroxides indicates that only in the separation of lanthanum from the other trivalent rare earth elements can basic precipitation take place with any reasonable degree of efficiency. Furthermore, separations of the elements samarium, europium, and gadolinium and of the elements thulium, ytterbium, and lutecium cannot be effectively carried out in this manner. These indications are in accord with experimental observations. The data given in figures 1, 2, and 3 suggest that, of the anions studied, nitrates are preferable for separations of the cerium earth metals by basic precipitation, acetates are preferable for corresponding separations of the terbium earths, and sulfates are preferable for the yttrium earths. Further investigations along these lines nre indicated. SUMJISRP
1. Electrometric titration data for nitrate, sulfate, and acetate solutions of twelve of the rare earth elements, including yttrium, indicate decreases in the basicities of the hydrous oxide? or hydroxides which parallel decreases in the radii of the trivalent cations. 2. The basicity of yttrium hydroxide is between those of gadolinium and erbium hydroxides, in accordance TI ith size relationships. 3 . Calculation of the solubility-product constants and n-ater solubilities has been carried out on the assumption that all precipitates are hydrous hydroxides. 4. h numerical evaluation of relative basicities based upon solubility-product constant? is given. REFERESCES (1) BOWLES ASD PARTRIDGE: Ind. Eng. Chem., Anal. Ed. 9, 121 (1937). (2) BRIrTON: J. Chem. soc. 127, 2112 (1925).
(3) BRITTOS:Hydrogen Ions, 5’01.11,3rd edition, p. 41. Chapman and Hall, Ltd., London (1942). (4) BRITTON:Reference 3, p. 79. (5) BUSCH:2. anorg. allgem. Chem. 161, 161 (1927).
406
RAYMOND NELSOK AND HAROLD F. W.4LTON
ENDRES:Z. anorg. allgem. Chem. 205, 321 (1932). GRIJIJI.4ND KOLFF: Z.physik. Chem. 119, 254 (1926). HILDEBRASD: J. Am. Chem. soc. 35, 847 (1913). HOPKINS4 S D BALKE:J. Ani. Chem. SOC.38, 2332 (1916). HOPKIXSAND DRIGGS:J. h i . Chem. SOC. 44, 1927 (1922). HUTTIGAXD KANTOR:Z . anorg. allgem. Chem. 202, 421 (1931). KOLTHOFF AND ELhfQUIsT: J. .Im. Chem. Soc. 53, 1217 (1931). KREMERS:Doctoral Dissertation, University of Illinois, 1944. J. Am. Chem. SOC.59, 1131 (1937). ?VICCOY: MOELLER:J. Am. Chem. SOC.63, 2625 (1941). YIOELLER AND I h E v E R S . J. h m . Chem. soc. 66, 307 (1914). S A E S E R .4SD HOPKIAS: J. -kin. Chem. sot. 67, 2183 (1935). OKA:J. Chem. SOC.Japan 69, 971 11938). OWESS, BALKE,AND KREJrERS. J. h n . Chem. SOC. 42, 515 (1920). PEARCE .4XD S A E S E R WITH HOPKINS: Trans. Am. Electrochem. SOC. 69, 587 (1936). SADOLIN: Z. anorg. allgem. Chem. 160, 133 (1927). SHERWOOD WITH HOPKINS:J. Am. Chem. SOC.55, 3117 (1933). VON HEVESY: Die seltenen Erden vom Standpunkte des dtornbaues, pp. 16-30. Verlag von Julius Springer, Berlin (1927). (24) WALDEN:Handbuch der allgemeinen Chemie, Band I X , “Hydroxyde und Oxydhydrate,” (Fricke und Hdttig), pp. 114-129. Akademische Verlagsgesellschaft m.b.h., Leipzig (1937). (25) WEISERAND MILLIGAN: J. Phys. Chem. 42, 673 (1938). (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23)
CATIOS EXCH-lSGE ,AT HIGH pH RBYhlOSD SELSOS
~ N HAROLD D
Department o j Chemistry, -\-orthuestern
F. W‘ALTOS
LTniverszty, Evanslon, Illinois
Received J u l y 10, 1944
It was found by one of the authors (4) that the uptake of calcium ions from a calcium salt solution by the carbonaceous cation exchanger Zeo-Karb rose continuously with pH and showed no signs of levelling off a t the highest p H recorded. It v a s suggested (private communication from Professor John A. Bishop) that the increased uptake in alkaline solution might be due to absorption of an ion, CaOHf, similar to the ZnOH+ postulated by Elgaby and Jenny (1) to explain their results on ion exchange in bentonite. To test this suggestion, measurements have been made on the uptake of potassium ions, which can form no such complex cations, by Zeo-Iiarb from solutions of pH 3.8 to 12.5. .\gain an almost linear increase of uptake x-itli pH u-ats found, riqing at high pH t o over 4 milliequivalents per gram. This abnormal ion-exchange capacity in alkaline solutions may ha\-e some utility,-for example, in the absorption of heavy metals as metal-ammonia complex ions. Tests Jvere therefore made to determine the uptake of nickel, copper, and zinc from ammoniacal salt solutions, and to find whether the metal was absorbed as an ammonia complex or aq the free ion.
