34
F. H. SPEDDING, J. E. POWELL AND E. J. WHEELWRIGHT
[CONTRIBUTION X O . 413
FROM THE
AMESLABORArORY
O F THE
VOl. 78
ATOMICE N E R G Y COMMISSION,
IO\\A S T A T E
COLLEGE]
The Stability of the Rare Earth Complexes with N-Hydroxyethylethylenediaminetriacetic Acid BY F.H . SPEDDJNG, J. E. POWELL AND E. J. WHEELWRIGHT RECEIVEDJUNE 17, 1955 The stability constants of the complexes formed between the rare earth metal ions and the anion of N-hydroxyethylethylenediaminetriacetic acid, abbreviated HEDTA, have been measured a t a temperature of 25” and a n ionic strength of L, = 0.1 by two independent methods. I n the first method, the rare earth-HEDTA stability constants were calculated from measurements of the equilibrium constants of reactions involving competition between HEDTA and a polyamine chelating agent, triaminotriethylamine, for the rare earth ions and cupric ions. The equilibrium constants were calculated from pH measurements of the various reaction mixtures. The second method involved the polarographic measurement of the amount of free copper ions liberated when equal molar quantities of the copper(I1)-HEDTA complex and rare earth ions are mixed together.
Introduction The stability constants of the complexes formed between N-hydroxyethylethylenediaminetriacetic acid, abbreviated HEDTA, and C U + ~ ,Nif2, C O + ~Zn+?, , Cd+z, F e f 2 and &In+? have been measured by Chaberek and Martell.l The complexes are all about a factor of ten less stable than the complexes formed between the same metal ions and ethylenediaminetetraacetic acid. The stability constants of the rare earth-EDTA complexes have been measured by a number of people2-4 and are quite large. In addition, EDTA has been used successfully to separate macro quantities of individual pure rare earths from each other by ion-exchange method^.^ Because of the similarity in the structures of EDT.4 and HEDTA and because of the much greater solubility of the free acid form of HEDTX, it seemed desirable to measure the stability of the complexes formed between each of the rare earth metal ions and the anion of HEDTA%. Such a series of constants would give a good indication of the effectiveness of HEDTA as an eluent in the ion-exchange separation of various adjacent rare earths. The methods used t o measure the rare earthHEDTA stability constants are the same as those used by the authors to measure the rare earthEDTA stability constants. The symbol H3V is used in this paper t o designate the organic acid HEDTX. The apparent stability constants of the rare earth complex is defined K\rv = [ M V ] / [ M ] [ V ]
The brackets denote concentrations and all data were obtained from aqueous salt solutions of ionic strength = 0.1 and a temperature of 25’. The Potentiometric Method Because of the great stability of the rare earth-HEDTA complexes, a direct potentiometric titration method is not desirable. A mixture, containing equal molar amounts of rare earth salt and the acid H3\‘, behaves very much like a strong acid. The hydrogen ion concentration of such a mixture is only slightly less than three times the total concentration of rare earth. Only very approximate values for the (1) S . Chaberek, J r . , and A. E. Martell, THISJ O U R N A L , 77, 1477 (1955). ( 2 ) R. C. Vickery, J . Chem. Soc., 1895 (1952). (3) E . J. Wheelwright, F. H. Speddinfi and G , Schrvarzenbach, THISJ O U R N A L , 75, 4196 (1953). (4) G. Schwarzenbach, R. G u t and G. Anderegg, Hdv. C h i m . A c t o , 37,937 (1954). ( 5 ) F. H . Spedding, J , IC. Powell and E . J . b’heelwright, THISJ O U R N A L , 76, 2557 (1954).
equilibrium concentrations of [MV] and [MI could he obtained from PH measurements of such a solution. The indirect potentiometric method used in this research was first used by Schwarzenbach and Freitag6 t o measure some very stable ethylenediaminetetrdacetate complexes. If equal molar quantities of the copper(I1) complex of HEDTA, NaCuY, a rare earth salt, MC&, and the trihydrochloride of P,P’,P”-triaminotriethylamine, S(CH2-CH,NH2)3, designated as “tren,” are mixed together, reaction I takes place.
+
MYli3 CuV-
+ H~tren’~
MV
+ Cutren+2 + 3 H +
(1) If such a mixture is titrated with NaOH, three formula weights of the base are used in a buffer region between p H 4 and 6. The tri-(6-aminoethy1)-amine, “tren,” is a strong triprotonic base with pK values of 10.29, 9.59 and 8.56.’ These constants can be expressed as RHstren =
[H~trenl/[H13[trenl=
KHtrcn
X K
HX ~
~
KHatren =
The amine, “tren,” forms a very stable complex with the copper(I1) i0n.8~9
The equilibrium constant for reaction I can be expressed as
Before Kr can he calculated, the equilibrium concentratioti of five of the six concentration terms expressed in equation 1 must he evaluated. The hydrogen ion concentration is measured directly. Five material balance equations which describe the concentrations of all important species involved in the reactioii I can be written as [Cu], = c = [CuY] (~[Cutren] (2a) c [MV] [MI (2b) [M]t [ \ * I t = c = [CUV] [MY] (2c) [trenlt = c = [Cutren] [Hatren] (2d) [HI, = c ( 3 - a) = [HI 3[Hatren] (2e) The term “c” is used to indicate the initial concentration of the reacting species and “a” denotes the number of equivalents of sodium hydroxide added to the reaction per mole of rare earth in solution. Equation 2a is valid if 110 complexes are formed in which both “tren” a n d “Y” arc bound to the same copper ion. The last term in equation !a represents the coticentratioii of Cutren +z as well as Cu +2 since ( ~ [ C q t r e n= ] [Cutren] [CUI
+ + + + +
+
(6) G. Schwarzenbach and E. Freitag, Heiu. Chi7u. A c l n , 34, 1503 (1951). (7) H. Ackermann and G. Schwarzenhach, ibid., 32, 1513 (1949). (8) J. E. Prue and G . Schwarzenbach, i b i d . , 38, 963 (lQ.50). (9) T. C. Loomis, “Metal Chelates of Cyclohexenediaminetctraacetic Acid,” unpublished P h . D . Thesis. Iuwa State College Library, Ames, Iowa, 1 B S .
~
~
~
Jan. 5, 1956
RAREEARTHS WITH N-HYDROXYETHYLETHYLENEDIAMINETRIACETIC ACID 35
amount of ceric oxide, even with the help of hydrogen peroxide. Consequently, a larger amount of oxide was dis(3) solved in acid, the solution filtered, and diluted to volume. Then the cerium stock solution was standardized. Once the Equation 2b is true only if “tren” does not form a very concentration of the solution was known, 0.005 of a mole stable complex with any of the rare earths. However, since was removed and prepared in the same manner as the other no one has been able to measure any very stable rare earthrare earth salt solutions. ammonium type complexes, it seems very unlikely that any Experimental Procedure .-When equal molar quantities rare earth-tren complexes that might exist would be more of NaCuV, MC13 and Hatren+3are mixed together, reaction stable than the Mntren+2 or Fetren+2 complexes which have I takes place. If the reaction mixture is potentiometrically stability constants of 105.8 and lo8.*, respectively. It can titrated with standard sodium hydroxide, a normal acidbe shown that under the experimental conditions involved, base neutralization curve is obtained with a large pH inK Mwould ~ have ~ to ~ be ~larger than loi2 in order to make the crease occurring a t ‘‘a” = 3. If the reaction proceeded as concentration of the complex M t r e ~ exceed ~+~ which is fast as the neutralization of a strong acid with a strong base, 1% of [Mlt. a direct potentiometric titration could be carried out. In Equations 2c and 2d are valid because the term [HaV], such a case, the rare earth stability constants could be cal[H*V], [HV], [VI, [Hntren], [Htren] and [tren] are so small culated from any value of “a” and PH picked from the that they can be neglected in the pH range concerned. buffer region of the neutralization curve (roughly from This is easily shown from the known ionization constants of “a” = 0.4 to 1.9) by the use of equations 1 , 2 and 3. Such H3tren+3, the dissociation constants of H3V, and the sta- a direct approach cannot be used because the reaction is bility constant of the copper( 11)-HEDTA complex. slow. Usually, 25 to 30 minutes are required for the PH The total amount of available acid hydrogen is summarized of the reaction mixture t o reach a stationary value after in equation 2e. each addition of base. It was found necessary t o prepare Preoaration of Solutions. A. 0.01 A[ Hatren+3.-A five individual solutions, each one with the same equal s a m p 6 of triaminotriethylamine trihydrochloride, obtained molar amounts of KaCuV, MC13 and H d ~ e n + ~ enough , from Chemicals Procurement Co., was carefully purified by a KCl to adjust the ionic strength of the solution to fi = 0.1, double recrystallization. T h e amine salt was first dissolved and different amounts of standard sodium hydroxide so as in a minimum amount of warm water and then crystallized t o correspond to five different positions across the buffer from the water with 95% ethanol. Finally, the crystals were region of a normal neutralization curve. This was accomdried a t 100” under a reduced pressure for 24 hours and a plished by pipeting 10 ml. of solution A, 10 ml. of solution 0.0100 M aqueous solution of the amine was prepared. C, 10 ml. of solution D, 9.10 ml. of 1.00 iM KCl, and difB. 0.01 M H3V.-The H E D T A , secured from Geigy ferent predetermined amounts of 0.1 N NaOH into 100-ml. Chemical Co., was further purified by two operations. volumetric flasks and diluting to the mark. The five soluThe acid was first dissolved in a small amount of warm water tions were allowed to equilibrate for 24 hours in a constant and filtered. Then 95% ethanol was added and the solu- temperature water-bath adjusted to 25.00 It 0.02’. At tion allowed t o cool. The acid crystals were filtered on a the end of that period, the p H of each solution was measured sintered glass funnel, washed three times with cold absolute with a Beckman Model GS pH meter using a fiber type ethanol, then once again dissolved in warm water, recrys- saturated calomel electrode and a “General Purpose” glass tallized, filtered and washed. A triple leaching with cold electrode. The PH meter was standardized with Beckman water followed by drying a t 100” under a reduced pressure PH 4 buffer before each set of determinations. Since the for 24 hours completed the purification. A solution of the PH thus obtained is defined in terms of the hydrogen ion purified product was tested for sulfate with barium chloride activity, while hydrogen ion concentrations are needed for and for sodium with a flame spectrophotometer. No the calculations. it was necessarv to correct each DH meter barium sulfate precipitate could be detected. The flame reading. Three solutions were‘ independently prepared, test indicated 0.1 p.p.m. of sodium. each containing 0.000802 M HC1 and 0.100 M KCl. These A 0.1 M aqueous solution of H E D T A was prepared and solutions should have pH values of 3.096 based upon constandardized by three independent methods: titration centrations a t an ionic strength of = 0.10. The pH against a standard zinc(I1) chloride solution using Erio- meter was standardized in the conventional manner using chrome-Schwartz T a s the indicator, titration against a Beckman PH 4 buffer and then two separate determinastandard copper chloride solution using Murexide as the in- tions were made upon the three solutions. The pH was dicator, and a potentiometric titration of the free acid with found t o be 3.167 f 0.001, indicating that a correction of standard sodium hydroxide. T h e three methods gave -0.071 is necessary. values of 0.0992, 0.0989 and 0.0994 for the molar concentraFive values for KIwere calculated for each rare earth from tion of the H E D T A stock solution. the five PH and “a” values by use of equations 1, 2 and 3. C. 0.01 M NaCuV.-A copper(T1) chloride solution was An average value was then used to calculate K M ~ Check . prepared by dissolving CuC12.2Hz0 in water. Electrolytic determinations, allowing 48-hour equilibration, were made analysis of the solution showed its concentration to be 0.1001 for dysprosium, gadolinium and ytterbium. Since the reM. Therefore, 199.8 ml. of the copper solution was added sults were the same as those obtained with 24-hour equilibrato 201.82 ml. of the stock HEDTA solution and the resultant tion, the practice was discontinued for the rest of the series. acid solution neutralized with sodium hydroxide. The The experimental data and calculated results are presented neutral solution was then diluted to 2000 ml. in Table I. D. 0.01 M MCl2.-Sufficient freshly ignited pure rare Polarographic Method earth oxidela was dissolved in a slight excess of HC1 t o give This method is based upon the fact that it is possible to a solution containing 0.005 of a mole as M + 3 . A part of this solution was then potentiometrically titrated with so- measure the concentration of free uncomplexed copper(I1) dium hydroxide to a point just past the neutral equivalence ions in the presence of the copper complex CULT-, by polaropoint. The PH of a neutral rare earth solution was then graphic means. If equal molar amounts of NaCuV, and a located from a graph of ApH/Aml. against ml. of base added rare earth nitrate, M( N03)3,are allowed to equilibrate, the to the solution. These PH values ranged from 5.2 t o .1.8. rare earth ion competes with the copper ion for the complexing agent, After titration, the two solutions were recombined, adjusted to the equivalence point pH with sodium hydroxCuVM+3 Cu+l MV (11) ide, and diluted to 500 ml. The composition of terbium oxide depends upon the igni- In such a solution, two separate individual polarographic waves are produced by the reduction of Cu+2 and of Cull-. tion temperature. In order to correct this uncertainty, a sample of the oxide was reduced to the sesquioxide by heat- T h e first has the shape of a two-electron wave with El/, = -0.04 v. os. satd. calomel electrode. The second wave has ing it for 8 hours a t 900” in a stream of hydrogen. The sesquioxide was then weighed out and dissolved in HCl as de- an El/, a t about -0.2 v. vs. sat. calomel electrode. Between the two waves the diffusion current, id, remains conscribed above. I t is virtually impossible to dissolve quantitatively a small stant and is proportional t o the concentration of Cu+2 present in the solution. (10) The rare earths used in this research were separated from each The equilibrium constant for equation I1 can be written as and
+
other on ion-exchange columns according to the method of Spedding,
et ol., and each rare earth was spectrographically pure. F. H. Spedding, et at., THIS J O U R N A L , BQ, 2786 (1947); 7 9 , 2354 (1950).
+
F. H. SPEDDING, J. E. POWELL AND E. J. WHEELWRIGHT
36
TABLE I RESULTSFOR THE POTENTIOMETRIC DETERMINATION OF THE RAREEARTH-HEDTA STABILITY COSSTAXTS~ Rare earth
La+3
"a"
0.697 0.930 1.162 1.394 1.627 0.697 0.930 1.162 1.394 1.627 0.697 0.930 1.162 1.394 1.627 K d t 3 0.697 0.930 1.162 1.394 1.627 Sm+s 0.697 0.930 1.162 1.394 1.627 E U + ~ 0.697 0.930 1.162 1.394 1.627 G d + 3 0.697 0.930 1.162 1.394 1.627 T b +s 0.697 0.930 1.162 1.394 1.627 D y f s 0.697 0.930 1.162 1.394 1.627 H O + ~ 0.697 0.930 1.162 1.394 1.627 E r + 3 0.697 0.930 1.162 1.394 1.627 T m f 3 0.697 0.930 1.162 1.394 1.627 Y b + 3 0.697 0.930
Cor. pH
5.112 5.257 5.364 5.482 5.611 4.847 4.970 5.087 5.196 5.316 4.730 4.859 4,980 5.097 5.226 4.637 4.762 4.876 4.991 5.122 4.495 4,616 4.739 4.850 4.977 4.480 4.600 4.718 4.832 4.959 4.517 4.636 4.754 4.885 4.998 4.517 4.637 4.755 4.863 4.989 4.517 4.640 4.758 4.873 5.001 4.524 4.645 4.764 4.881 5.008 4.489 4.608 4.729 4,842 4.971 4.432 4.549 4.664 4.778 4.903 4.358 4.474
-log K1v
13.24 13.30 13.27 13.29 13.34 12.44 12.42 12.43 12.43 12.45 12.07 12.08 12.11 12.13 12.18 11.78 11.78 11.59 11.81 11.86 11.31 11.32 11.35 11.38 11.42 11.26 11.27 11.30 11.32 11.35 11.39 11.39 11.41 11.38 11.47 11.39 11.39 11.41 11.41 11.45 11.39 11.40 11.42 11.44 11.49 11.41 11.41 11.44 11.47 11.52 11.29 11.30 11.33 11.35 11.40 11.09 11.10 11.12 11.15 11.19 10.81 10.84
Av. -log KIV
13.29
log K\ir
1.162 1.394 1.627 L u + ~ 0.697
13 22 Y+3
12,43
14.08
0.930 1.162 1.394 1.627 0.697 0.930 1.162 1.391 1.627
4.584 4.697 4.823 4,322 4.429 4.539 4.652 4.775 4,701 4.827 4.952 5.066 5,194
Vol. 75
10.86 10.89 10.95 10.68 10.68 10.71 10.75 10.80 11.98 11.99 12.02 12.03 12.08
10.87
15.64
10.72
15.79
12.02
14.49
Supporting electrolyte = KC1; temperature = 25.005
0.02";ionic strength 12.12
14.39
11.80
14.71
11.36
15.15
11.30
15.21
11.41
15.10
11.41
15.10
11.43
15.08
11.45
0.10.
Preparation of Solutions.-Previous measurements3 indicated that chloride ions may interfere with the polarographic measurements. Therefore, 0.0100 111 solutions of SaCuT' and M(N03)a were prepared with nitrate rather than chloride ions in the same manner as previously described. Experimental Procedure.-The polarograms were taken from solutions prepared b y mixing 10 ml. of 0.0100 hf M(N03)3,10 ml. of 0.0100AT NaCuV, 10 ml. of 0.10 M acetic acid, 10 ml. of 0.10 A1 sodium acetate, and enough 1.00 M K N 0 3 to adjust the final ionic strength to = 0.10, and diluting the solution to 100 ml. in 100-ml.volumetric flasks. After mixing, the solutions were allowed to equilibrate for 24 hours in a constant temperature water-bath a t 25.00 f 0.02". The acetic acid-sodium acetate buffer buffered the solutions to a pH of approximately 4.65. The measurements were made with a Sargent Model XXI polarograph. An H type cell was used, with a dropping mercury electrode in one side and a saturated calomel electrode in the other side, contact being made through a porous plug. Two drops of basic fuchsin (0.2%) were added to the solutions in the electrolytic cell and pure nitrogen was bubbled through the solutions for 20 minutes before each polarogram was taken. Reference i,l values were obtained for loo% uncomplexed copper and for 100% complexed copper by obtaining polarograms from solutions containing C U ( K O ~and ) ~ NaCuV, respectively, but no rare earth nitrate. The stability constants were calculated from the reference and experimental id values by use of equation 5. The results are given in Table 11.
TABLE I1 POLAROGRAPHIC DETERMISATIOX OF THE RARE EARTHCON ST ANTS^ HEDTA STABILITY Rare earth
15.06
=
where "c" is the initial concentration of both reactants. c ~desired ~, stability constant can Since K I I = K M ~ ~ / Kthe be calculated from equation 5.
Uncomplexed copper,
%
log
KMV
11.13
15.17
15.38
%
Sm'3 Gd+3 Dy'3
7.7 9.0 8.1
15.3 l5,4 15.3
Tm+3 Yb'3 Lu+~
Ho-3
8.6
15.4
YC3
Er+3 9.5 15.4 a Temperature = 25.00i 0.02'; supporting electrolyte = KSOs.
11.33
Uncomplexed copper,
Rare earth
10.5 14.4 17.2 4.8
log KhlV
15.5 15.8 16.0 14.8
ionic strength = 0.10;
Discussion of Results The accuracy of the constants, KMV,depends directly upon the accuracy of Kcuv. This constant was determined a t 29.6' by Chaberek and Martell'. The constant was not re-evaluated a t 25' since it was believed that its magnitude would probably not be changed much more than the experimental error with a temperature change of only 4.6'.
Jan. 5, 1956
RAREEARTHSWITH N-HYDROXYETHYLETHYLENEDIAMINETRIACETIC ACID 37
The 9 H values obtained with the Beckman Model GS p H meter are more accurate than the buffer used to standardize the meter. Since sufficient buffer solution was prepared a t the beginning of the experiments to standardize the 9H meter with the same buffer each time, the p H values have an absolute accuracy of only *0.01 p H unit, but have a relative accuracy of *0.0025 unit, the limit of the PH meter. The errors in determining the p H of the various solutions have an effect upon the stability constant of only AO.03 log unit. Because of the uncertainty of the values of the other constants, Rmren,Kcuv and Kcutren, the values of log Khfv reported in Table I, probably have an absolute uncertainty of about A0.2 unit. The relative values within the rare earth series are probably of the order of 0.06 unit or better. The uncertainty in the values of the polarographically determined stability constants is much larger. This is a direct result of the instability of K M V compared to Kcuv. The equilibrium in equation I1 is shifted predominantly toward the left. As a result, less than 20y0 of the total copper is uncomplexed. Since an error of *2y0 is committed in measuring the copper, the constant KMVis accurate to only f0.2 to f0.4 unit. With errors of this magnitude, a comparison of the values as given in Fig. 1 shows that the values determined by the two independent methods do agree reasonably well. The errors are so great that the light rare earth constants could not be measured by polarography. Two important facts can be observed from Table I and Fig. 1. First, there is no significant variation in the magnitude of the stability of the rare earth-HEDTA constants from samarium to erbium. This means that little or no separation of these elements from one another could be obtained by an ion-exchange elution with HEDTA as the eluting agent. The second important fact to observe is the very dramatic shift of the yttrium constant from its usual holmiumdysprosium region to the neodymium-praseodymium region. Since the Rz03 leached from gadolinite ore contains 60% yttrium, 25% light rare earths and only 15% heavy rare earths, this shift presents the possibility of a very fast removal of yttrium from the more desirable heavy rare earth fraction of the ore. HEDTA has the ad-
vantage of being quite soluble in acid solutions, so it can be used to elute rare earth bands directly onto ion-exchange resin in the hydrogen cycle. 16001
: . : 2--P
.50-,
J
>
I
I
I
lr
5.0c
0
/
*
UNCERTAINTY
0
RANGE
ii
POLAROGRIPYIC V4LUE WTh UNCERTAINTY R A N G E
13 00
I
I 1
1
LO
Cr
Pi
I
l
l
Ud
6'
Sm
ATOMIC
Fig. 1.-Rare
Eu
Gd
I
I
Tb
Dy
I
I
HO Er
I
I
!
Tm
Yb
Lu
NUMBER
earth-HEDTA stability constants.
An experiment was conducted in which a band containing 1223 g. of mixed rare earths (weighed as oxide) was eluted three times its length down a series of columns containing 100 to 200 mesh Dowex-50 resin with an eluent containing 5 g. of HEDTA per liter, buffered to p H 7.68 with ammonium hydroxide. An analysis of the 37 eluate fractions collected showed too much trailing of the heavy rare earths into the yttrium region, and no separation was obtained for any of the rare earths from samarium to lutetium. This separation is inferior to that which can be obtained by the copper-EDTA methode5 The results did show that the yttrium was concentrated between samarium and neodymium, thus giving support to the correctness of the yttrium-HEDTA stability constant. AMES,IOWA
