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Determination of stability constants of fixed-site complexes of copper(II) ions and of sorbed copper-l -proline complexes with an asymmetric resin containing l -proline groups
Tnianta, Vol.25,pp.49%504. Q Pergamon PressLtd..1978. Printed inGreatBntain
DETERMINATION OF STABILITY CONSTANTS OF FIXED-SITE COMPLEXES OF COPPER(I1) IONS AND OF SORBED COPPER-L-PROLINE COMPLEXES WITH AN ASYMMETRIC RESIN CONTAINING L-PROLINE GROUPS Yu.
A.
ZOLOTAREV, A.
Institute
A.
KURGANOV,
of Organo-Element
Compounds,
A. V. SEMECHKIN
Academy
and V. A.
DAVANKOV
of Sciences, Moscow, USSR
(Received 30 May 1977. Revised 1 September 1977. Accepted 2 March 1978) Summary-By study of the distribution ofcopper and L-proline (Pro) ions between an aqueous phase and a resin phase with r_-proline groupings on a polystyrene matrix (R), the conditional stability constants of fixed-site R,Cu and sorbed WuPro complexes have been determined and a method for calculation of the
constants for alkaline and neutral media is proposed. The values obtained reflect the actual behaviour of the system, and strongly depend on the pH and the ionic strength of the solution in equilibrium with the resin phase. It was established that formation in comparison with the Donnan distribution ofthe the polyanion. The distribution stability ofindividual fixed-site aqueous phase.
this dependence and the high values of the conditional constants of complex the stability constants of low molecular-weight model compounds are due to low molecular-weight participants and to the effect of the electrostatic field of of the fixed-site ligands on the resin causes considerable differences in the R,Cu complexes and their capacity to enter into reactions with ligands in the
The complex formation constants of copper(H) with the iminodiacetate groups on Dowex-Al resin have been determined by potentiometric titration (log K = 10.54)’ and by measuring the copper-ion distribution between the resin phase and an aqueous solution of a low molecular-weight chelating agent (log K = 16.90).* These values differ considerably but the constant determined potentiometrically is quite close to that for copper-complex formation with the low molecular-weight analogue of the resinbenzyliminodiacetic acid, log K = 10.61.’ In a previous study3 we determined potentiometrically the formation constant of the Cu(I1) complex of an asymmetric resin containing fixed L-proline groups, log K = 12.82. This value was in good agreement with the chelation constant of the low molecularweight analogue of this resin, N-benzyl+proline, log K = 12.39rfr0.10.4 In the present work the formation constants of the copper complexes with the fixed groups of this Lproline resin, and also the constants for sorption of copper-proline complexes on the resin have been calculated from the results of distribution measurements of Cu(I1) and t_-proline ions between the resin and aqueous phases, with the aim of studying the effect of conditions of complex formation on the values of these constants. Factors
affecting
the interphase
distribution
of Cu(II)
and L-proiine ions
In the calculations of ~ncentrational chelation constants it is sometimes assumed that the concentration of the low molecular-weight reaction components in the resin phase is equal to their concentration in the aqueous phase. In our opinion, this
assumption is frequently incorrect. This is indicated, for instance, by the strong dependence of the chelating properties of the sorbent on the ionic strength of the external solution. Thus, reduction in chelating ability with increase in ionic strength was reported for resins containing iminodiacetic acid groups5 and carboxylic groups.6 In the latter work it was shown that the formation constant of the Cu(I1) complex is reduced by two orders of magnitude when the supporting electrolyte concentration is increased from 0 to 2N. In the present work we have studied the chelating properties of an asymmetric sorbent containing Lproline groups, prepared from an isoporous macroreticular matrix with 10% cross-linking.’ A sharp reduction of chelating power in sorption of copper from ammonia solution is also observed for this sorbent if a neutral electrolyte, potassium chloride, is added in 1M concentration (Fig. 1). This apparently refects the dependence of the copper ion distribution on the ionic strength of the solution, in accordance with the Donnan equilibrium. A similar dependence was also observed for the distribution of L-proline (Fig. 2). At pH values around 7 the distribution coefficient of the L-proline zwitter-ion is K, = 0.83, close to that of the giycine zwitter-ion in a carboxylic acid resin, K, = 0.93.’ In this range the distribution coefficient of the L-proline zwitter-ion between the solution and the electroneutral resin phase is practically independent of the ionic strength. In the alkaline region, at pH = 11.5, the distribution coefficient describes the behaviour of the L-proline anion in the negatively-charged resin phase. In the absence of a supporting electrolyte, K, = 0.1, whereas in the presence of 1M potassium chloride, Kd = 0.58. Such a big difference in the distribution coefficients is undoub-
Yu. A. ZOLOTAREV.A.A.KURGANOV,
A.V. SEMECHKIN and V.A. DAVANKOV
proline absence The [Pro’], proline formula
between the two phases in the presence and of 1M potassium chloride. conditional concentration of free L-proline, was calculated from the total amount of Land Cu(II) in solution according to the
(1)
CCu*+l, M Fig. I. Sorption isotherm of copper(l1) from 2M ammonia by an asymmetric resin containing L-proline groupings, in the absence of supporting electrolyte (1) and in the presence of 1M potassium chloride (2).
where m Proand mcu are the numbers of moles of proline and copper in solution respectively, and V is the volume of solution (taking into account the swelling of the resin). The conditional concentration of free copper in solution was calculated as Lcu,3
IO
c
05 -
i
\
01
[Cu(ProM Pro'1
;------h
y:
=
I
I
I
I
7
8
9
10
\ ‘\
\
\
\
\ .L_ I I
/I
I 12
PH Fig. 2. Coefficient of Donnan distribution of L-proline between the aqueous and asymmetric resin phases as a function of the pH of the aqueous phase, in the absence of supporting electrolyte (1) and in 1M potassium chloride (2).
tedly connected with ion-exclusion of the L-proline anion, which in this case acts as a co-ion. It is known that exclusion is greater the larger the volume of the ion and the lower the ionic strength of the solution.’ The greatest reduction of K, occurs at the transition of the L-proline zwitter-ion into the anion in the region corresponding to deprotonation of amino groups, pK,, = 10.6. Therefore, at pH > 11.5 in a Donnan equilibrium the amount of L-proline in the resin phase is only a tenth (in the absence of a supporting electrolyte) of that in the aqueous solution at equilibrium with it. Addition of a supporting electrolyte in 1M concentration increases the amount of proline anion in the resin phase by a factor of 5.8. Evidently, the changes observed in the interphase distribution coefficients of low molecular-weight components with variation in the ionic strength and pH should considerably affect the values of complex formation constants determined by this method. Calculation qfchelation
constants in alkaline solutions
To determine the dependence of the chelating and sorption properties of asymmetric sorbents on ionic strength, we studied the distribution of Cu(II) and L-
‘h
where [Cu(Pro),] is the concentration of the bisproline copper complex (assumed equal to the total concentration of copper in solution, as [Cu(Pro),] >> [Cu]) and b2 is the overall formation constant of Cu(Pro),, logp, = 16.66.9 The amount of water and the total quantities of copper and L-proline in the resin phase were determined. Naturally, the values obtained include certain amounts of non-absorbed r_-proline and copper bis-prolinate. Taking the distribution coefficients as 1 for the copper bis-prolinate, 0.58 for the L-proline anion in the presence of a supporting electrolyte and 0.1 in its absence, and accounting for the volume of water in the resin phase, we calculated the actual quantities of copper, M,,, and proline, MPro, participating in the formation of complexes with the stationary phase. The quantity of copper-proline complexes sorbed on the resin phase, MR~“~~,,,is then equal to the corrected quantity of sorbed L-proline in the resin phase: MRCuPro = MPro
(3)
and the quantity of copper complexed with the fixed sites in the resin phase, MR,&s equal to the difference between the amounts of copper and proline sorbed: MR,c~ = Mcu - M~ro
(4)
The equilibrium number of moles of fixed resin ligands (MR) that did not enter into the chelation reaction with copper was determined as MR = MR~- MRC~P~-~MR,C~
(5)
where MR, is the total amount of fixed groups in the resin phase. The concentration of components in the resin phase was calculated with respect to the volume of water (VW) in the phase:
[X] = : w
The formation constants of the fixed-site complexes with copper and for sorption of the copper-proline
8.34 7.92 8.24 8.14 9.02 8.86
15 16 17 18 19 20
68.1 170.0 357.0 535.0 679.0 842.0
9.8 70.7 120.0 166.0 174.0 175.0
202.0 306.0 403.0 499.0 596.0 772.0 904.0 1018.0
1928 1918 190 1869 1850 1806
490 461 437 408 370 356
1954 1949 1940 1936 1924 1901 1860 2045
/~mole
M~rw
12.20 12.30 12.71 12.21 13.76 14.22
9.99 9.98 9.96 9.95 9.94 9.93
13.75 13.76 13.76 13.76 13.77 13.78 13.80 14.82
V, ml
15-20,
0.878 0.714 0.535 0.349 0.252 0.058
47.0 32.0 19.7 7.6 2.2 4.0
115.0 100.0 83.0 67.0 52.0 26.0 4.2 6.0 --
IM KCI.
146.0 130.0 91.3 60.7 35.8 85.8
--
-
-
[ProH], mM
of the aqueous
[Pro], mM
electrolyte;
5.6 14.0 27.7 40.9 48.8 59.7
0.98 7.07 12.0 16.6 17.4 17.6
22.0 29.5 36.6 43.6 56.5 66.0 12.0
15.0
mM
-[CuPro,],
* l-8, 1M KCl; Q-14, without supporting t Based on formation of R&u.
11.95 11.85 11.80 11.60 11.20 10.60
9 10 11 12 13 14
11.62 11.60 11.55 11.50 11.45 11.40 11.20 10.65
l 2 3 4 5 6 7 8
~.
pH
No.*
MC,, nmole
1. Composition
Solution
Table
12.81 12.16 11.66 11.13 10.76 9.41
17.01 15.82 15.17 14.20 12.11 10.62
16.60 16.32 16.03 15.75 15.45 14.74 13.09 11.36
pCu
Resin
and resin phase and effective constants
32.3 39.7 63.5 95.9 166.0 216.0
41.4 92.1 134.0 189.0 261.0 303.0
11.2 14.8 24.9 35.3 44.3 84.2 169.0 247.0
--
20.7 24.3 42.8 65.7 109.0 156.0
33.2 58.4 19.6 107.0 141.0 169.0
6.0 8.2 13.3 15.9 24.5 47.3 85.0 135.0
324 322 313 308 306 298
362 346 333 321 304 296
366 364 360 358 354 343 319 302
64.1 75.5 133.0 206.0 356.0 523.0
22.6 97.4 165.0 255.0 395.0 453.0
22.3 30.4 48.2 68.1 86.5 160.0 272.0 405.0
33.4 47.8 62.0 98.0 186.0 201.0
91.7 16.9 239.0 333.0 464.0 571.0
19.5 24.3 41.4 57.2 70.4 129.0 268.0 336.0
185 102 1.56 130 301 221
1400 1240 1100 885 618 355 1260 1320 1220 1110 644 570
-
-
12.79 12.78 12.06 11.85 11.07 9.92
15.68 14.86 14.46 13.83 12.19 11.27
14.59 14.41 14.37 14.25 14.05 13.69 12.62 11.30
[RH], mM hKrt,cU 1%
formation
1400 1390 1370 1340 1330 1210 890 617
of complex
MPro, Swelling, [RCuPro], [RiE],.-if$ MC,, pmole @mole mg H,O mM
solution
15.40 15.18 14.86 14.83 14.29 14.02
16.55 16.21 16.05 15.78 14.57 14.12
15.74 15.65 15.65 15.65 15.55 15.45 14.95 14.40
hwu~ro
12.5 16.0 25.0 37.0 65.0 84.3
14.3 31.8 46.2 65.2 90.0 104.0
4.4 5.8 9.1 13.8 17.3 32.9 65.9 96.6
%t
Degree of saturation of resin with copper
Ku. A. ZOLOTAREV,A. A. KURGANOV, A. V. SEMECHKINand V. A. DAVANKO~
502
complexes
may now be determined
as
sample; n2 = 1.6 and pK_ = 9.70 for an asymmetric resin, in the absence of complex formation.3 Equation (10) was solved for MR by the method of iteration at each point. The values of MR were then substituted into equation (9) to obtain the values of MIH.
this calculation of the effective concentrational constants of complex formation, presented in Table 1 and Fig. 3, the concentrations of unchelated copper [Cu] and proline [P ro ] ions in the aqueous phase were used. Since all tests of both series (with and without a supporting electrolyte) were carried out in alkaline solutions (pH > 11.2, except for samples with maximal copper concentration) it was assumed that all the proline and resin-site ligands were in the anionic form and capable of complex formation (the dissociation constants, pK_, of their amino groups are 10.6,” and 9.70,3 respectively). In
The concentrations of the sorbed and fixed-site complexes as well as of the resin groups in zwitter-ion and anionic forms were determined with due regard to the amount of water in the resin phase, according to equation (6). The number of moles of copper bis-proline complex in solution, mCuPro,, was taken as equal to the number of moles of copper determined by analysis of the solution; the amount of unchelated proline, mpro +mProH, in this case is equal to the difference between the total amount ofproline in solution and the amount of its complex with Cu(II). From the equation for the dissociation constant of proline:
,y, =
Culculution qf’chelution constants in neutrul und weukly alkaline solutions Use of neutral solutions in the study of interphase equilibria makes it possible to minimize the effect of the electrostatic field of the sorbent. However, the acid&base equilibria of both the resin fixed-site ligands and the mobile ligands should be taken into account, since both species are capable of co-ordination with Cu(II) ions when they are in the anionic form, but not when they are in the zwitter-ion form. In the consideration of the acid-base equilibria in the resin phase we proceeded as follows. The dissociation constant of the amino-acid group in the weak-acid carboxylic acid cation-exchanges is practically the same as the constant for the amino-acid in solution,7 and for a resin on which no electrostatic charge is present, the distribution coefficient of charged and neutral molecules tends to unity.’ For the fixed-site groups on resins it is assumed that the acid-base equilibrium is not shifted by the presence of proline zwitter-ions in the resin phase, and that the apparent dissociation constant depends only on the total charge of the polyelectrolyte chain.6,10 The amount of sorbed (MaCuPro) and fixed-site (MR,& complexes was determined, as previously, according to equations (3) and (4). The quantity of sorption groups in zwitter-ion form, M,,, was obtained from M,, =
Miicu~r<>+
~~~~~~ + MR + MR,,
M is the number
of moles
of a species
rPro]‘LH1; pi = [ProH]
I4
lo.6
’
”
we found separately the amounts of proline in the zwitter-ion and anionic forms. The concentration of unchelated copper was determined from equation (2). Finally, the formation constants of the fixed-site stationary and sorbed complexes were calculated from equations (7) and (8). Results of the calculations are given in Table 1 and Fig. 3.
I8 0 I 160
3
I4 0
t
12.0
$ IO 0
8.0 2.0
(9)
and substituted into the equation for polyelectrolyte dissociation under conditions of complex formation :*
where
‘I>
in the
0 Degree
20
40
of saturation
80
60 of resin
with
Cu’:
100 %
Fig. 3. Stability constants with Cu(II) ) and of sorbed CumPro complexes (---) on an (asymmetric resin, as a function of the degree of saturation of the resin with copper(H) (to form R,Cu). (1)-without supporting electrolyte, pH > 11 .O ; (2).-1M potassium chloride, pH B 11.0; (3)-l A4 potassium chloride, pH - 8.
Copper-proline-resin The dependence of conditional chelation constants on .‘he degree of sorbent saturation with copper ions, pH and the ionic strength of the solution Three pairs of conditional stability constants of fixed-site (fi,Cu) and sorbed (iiCuPro) complexes, determined under different conditions, are shown in Fig. 3. As can be seen, they decrease with increase of sorbent saturation with copper ions. The decrease of KQ-” is especially rapid. Earlier,” we explained the gradual reduction in stability of fixed-site complexes formed by two ligands bonded to the polymer matrix as due to deterioration of the steric conditions for chelation, as the copper ions must react with less and less favourably arranged pairs of fixed-site ligands. Evidently, the stability of individual fixed-site fi,Cu complexes varies over a very wide range. This is confirmed by the fact that the average stability constant of these complexes drops by at least 3 orders of magnitude with increase in the degree of saturation with copper ions from 0 to 100%. The stability of sorbed complexes (RCuPro) depends to a considerably smaller extent on the copper content of the system. This is to be expected, since the mobile ligand (Pro) in these complexes is not bonded to the polymer chains of the matrix and may arrange itself favourably in space. The fact that the stability of the sorbed complexes nevertheless shows a tendency to reduction with increase of copper content in the resin phase, may be explained by the increase of steric hindrance to formation of bulky complexes in the resin phase and also by variation in the activity coefficients of mobile ions. (The exchange constants have been reported to be dependent on the ionic composition of carboxylic lz and diallyl phosphate’ 3 resins.) Figure 3 demonstrates graphically the dependence of the conditional stability constants of the fixed-site and sorbed complexes on the pH and ionic strength of the equilibrium solution, which, as shown above, considerably affect the distribution coefficients of copper and proline ions. The conditional stability constants of complexes in the resin phase should, undoubtedly, depend on such characteristics of the polymer matrix as the degree of cross-linking and concentration of fixed ligands. The fact that the constants for a similar sorbent with 6% cross-linking and 1.68 meq/g stationary L-proline groupings’ ’ are slightly higher than those obtained in the present work, and depend to a smaller degree on the amount of copper ions in the sorbent, we also associate with these factors. The calculated conditional stability constants (Fig. 3) are in good agreement with the actual behaviour of the system under static and dynamic conditions and reflect the following important features of the system. (i) Sharp reduction of the sorption capacity of the resin for copper ions on addition of a neutral electrolyte to the equilibrium solution (see Fig. 1) and on transition from alkaline to neutral solutions. (ii) Considerable increase of resin affinity for mobile ligands with increase of the degree of resin saturation
complexes
503
with copper ions. Indeed, the sorption constant of Lproline is determined by the difference between the stability constants of the sorbed complex and of the initial fixed-site complex, ~~~~~~~- KQ-“; this difference rapidly increases with the amount of copper in the system (see Fig. 3). (iii) Noticeable increase of resin affinity for the mobile amino-acid ligand (Pro) with increase in the ionic strength of the equilibrium solution and, in particular, on transition from alkaline to neutral solutions. Donnan distribution While the dependence of the conditional constants for chelation with the resin phase on the degree of its saturation is, apparently, primarily associated with steric features of the process, the dependence of the calculated constants on pH and the ionic strength, undoubtedly reflects changes in the distribution coefficients of low molecular-weight components between the two phases. Evidently the latter factor should always be taken into account when comparing stability constants for complexation in resin phases with the stability constants of low molecular-weight model compounds. Experimental determination of the distribution coefficients of unchelated copper and proline ions in a real system that contains all the components participating in the process is impossible. However, as mentioned above, we determined the distribution coefficients of L-proline (I&,) in the absence of copper ions. The distribution coefficients of unchelated copper ions (KS,) may be estimated from these values, if we assume, according to Helfferich,14 that formation constants of low molecular-weight complexes in the resin phase do not differ from the same constants in solution and that the distribution coefficient of the electroneutral CuPro, complex is equal to unity. In this case, at equilibrium K
Cu + 2Pro 2
CuPro,
and K& = (Kg,)‘. The calculated values of the distribution coefficients of unchelated copper (K&) are 1.49 for the neutral pH range, and 2.97 and 100 at pH = 11.5 in the presence and absence of 1M potassium chloride, respectively. On the basis of the K$,Oand K& values the stability constants, ~~~~~~~ and KQ-,, corresponding to 50% and 20% sorbent saturation with copper ions may be corrected. The corrected values obtained are given in Table 2 together with KQ-” determined potentiometrically3 and the stability constants of the corresponding model complexes bis(N-benzyl-L-prolinato)copper(II)4 and (L-prolinato)(N-benzyl-r=prolinato)copper(II).ls As can be seen from the data presented in Table 2, the corrected values of the stability constants for the sorbed and fixed-site complexes obtained in alkaline solutions in the presence and absence of a supporting electrolyte are in good agreement with each other. Therefore, the differences in the corresponding effec-
504
Yu. A. ZOLOTAREV, A. A. KURGANOV, A. V.
SEMECHKIN and V. A. DAVANKOV
Table 2. Stability constants offixed-site and sorbed complexes in the resin phase and oftheir low molecularweight analogues Test conditions
logK~cu~ro
log ht2Cu
Conditional
Corrected
Conditional
Corrected
pH > 11;5O%Cu
14.2
12.2
16.0
15.0
pH> ll~lMKCl~50”/,Cu pH-8;iMKCl;;O%Cu PH -8;lMKCl;20%Cu Potentiometry3 Model complexes4~15
13.0 11.5 12.7
12.5 11.35 12.55
15.2 14.6 15.2
15.0 14.5 15.1
12.8 12.4+0.1
tive chelation constants arise from the high values obtained in the absence of a supporting electrolyte and caused by the Donnan distribution of low molecularweight participants in the process. Both the corrected and conditional chelation constants obtained for feebly alkaline 1M potassium chloride medium have lower values than those for alkaline solutions. This is evidently connected with the decrease in the charge on the polymer chain on transition to the neutral pH-region, i.e. with decrease in the electrical potential of the polymer chain. This conclusion conforms with the results obtained by Gregor who showed for carboxylate linear and crosslinked polymers that the chelating properties of a polyanion significantly exceed the chelating properties of low molecular-weight model compounds. Evidently the polyanion field affects the stability of fixed-site complexes to a considerably greater extent than that of sorbed complexes. The corrected values of the constants obtained for sorbed and fixed-site complexes in alkaline solutions at 50% saturation of the sorbent with metal-ion are close to the stability constants oflow molecular-weight model compounds. Apparently, this is due to compensation by two opposite effects, namely, the effect of the polyanion field and steric hindrance to complex formation. Both effects should be insignificant in pH-regions close to neutrality and at low degrees of sorbent saturation with copper ions (10-200/0). Under these conditions, the values obtained are indeed close to the stability constants of the corresponding low molecular-weight analogues and to the stability constant of the fixed-site complex determined potentiometrically. EXPERIMENTAL
The asymmetric resin containing L-proline groupings was synthesized on a macro-reticular isoporous polystyrene matrix with 10% crosslinking. The sorbent capacity was 2.71 meq/g. Distribution of the components in the system was determined under static conditions in 15-ml columns.3 The airdried resin was brought to equilibrium with a solution of copper and L-proline by agitation in sealed tubes for 72 hr. The resin phase was isolated by centrifuging at 2000 y (3500 rpm) for 15 min. For analysis of the resin phase, copper and L-proline were desorbed by washing the resin in the column with 5M hydrochloric acid. The amounts of copper and L-proline were determined in the aqueous and resin phases. The results obtained agree within experimental error with the amounts of copper and L-
14.9kO.2
proline introduced into the system. Copper was determined spectrophotometrically with diethylcarbamate at 440 nm. L-Proline was determined in 5M hydrochloric acid by polarimetry at 436 nm. The pH of the solutions was measured to 0.01 pH unit. A0.300-g portion ofair-dried resin containing 0.512 meq of functional groups was placed in a column. For studies in alkaline and weakly alkaline media, 2.00 meq of L-proline and 0.2-2.4 meq of Cu(II) were added if a 1M potassium chloride medium was used, and 0.53 meq of r.-proline and 0.1-l .Omeq of Cu(II) if the supporting electrolyte was omitted. For studies in alkaline solutions, potassium hydroxide was also added, in quantities equivalent to the total quantity of functional groups in the resin and added proline. In the case of weakly alkaline solutions potassium hydroxide was added in amounts that would not exceed by more than 5-10% the quantity of protons formed during complex formation. The volume of equilibrium solution was corrected for the water introduced with the air-dried resin and extracted by the swollen resin. The amounts of occluded copper bis-prolinate and L-proline were taken into account in the determination of sorbed copper and L-proline. The distribution coefficient of copper bis-prolinate was taken as equal to unity. For proline in alkaline solution in the presence of I M potassium chloride, Kg,, = 0.58, and 0.1 in its absence. In the weakly-alkaline range it was assumed that K& = 0.83 (see Fig. 2). REFERENCES
1. C. Eger, W. M. Anspach and J. A. Marinsky, J. Inorg. Nucl. Chrw., 1968, 30, 1899. 2. H. Loewenschus and G. Schmuckler, Tulunta, 1964, II, 1399. 3. Y. A. Zolotarev. A. A. Kurganov and V. A. Davankov, ibid., 1978, 25, M/S 124. 4. A. A. Kurganov, V. A. Davankov and S. V. Rogozhin, Zh. N@orgcm. Khim., 1972, 17, 2163. 5. E. Leyden and A. L. Underwood. J. Phys. Chem., 1964, 68, 2093. 6. H. P. Gregor, L. B. Luttinger and E. M. Loeble. ibid.. 1955, 59, 34, 336. I. P. S. Nys and E. M. Savitskaya, Z/z. Fir. Khim.. 1969, 43, 1536. Russian). IL. Moscow, 8. F. Helfferich. I~)n-EuchunRrrs(in 1962. 9. A. A. Kurranov. V. A. Davankov. S. V. Roeozhin and Yu. D. Koreshkov, Koord. Khim., i977,3,66?. 10. K. Gartner and H. Wanjek, Z. Ph!,.s. Chrm., 1962, 221, 391. II. A. V. Semechkin. S. V. Rogozhin and V. A. Davankov, J. Chromatoa., 1977. 131. 65. 12. H. P. Greg;;. H. J. Hamilton and R. J. Oza. J. Pity.?. Chem., 1956, 60, 263. 13. J. Kennedy, R. V. Davies and H. Small. J. Appl. Chem.. 1959, 9, 32. 14. F. Helfferich, J. Am. Chem. Sot., 1962, 84, 3237. and P. R. Mittchel, J. Chrm. Sot., 15. V. A. Davankov Ddton, 1972, 1012.
DETERMINATION OF STABILITY CONSTANTS OF FIXED-SITE COMPLEXES OF COPPER(I1) IONS AND OF SORBED COPPER-L-PROLINE COMPLEXES WITH AN ASYMMETRIC RESIN CONTAINING L-PROLINE GROUPS Yu.
A.
ZOLOTAREV, A.
Institute
A.
KURGANOV,
of Organo-Element
Compounds,
A. V. SEMECHKIN
Academy
and V. A.
DAVANKOV
of Sciences, Moscow, USSR
(Received 30 May 1977. Revised 1 September 1977. Accepted 2 March 1978) Summary-By study of the distribution ofcopper and L-proline (Pro) ions between an aqueous phase and a resin phase with r_-proline groupings on a polystyrene matrix (R), the conditional stability constants of fixed-site R,Cu and sorbed WuPro complexes have been determined and a method for calculation of the
constants for alkaline and neutral media is proposed. The values obtained reflect the actual behaviour of the system, and strongly depend on the pH and the ionic strength of the solution in equilibrium with the resin phase. It was established that formation in comparison with the Donnan distribution ofthe the polyanion. The distribution stability ofindividual fixed-site aqueous phase.
this dependence and the high values of the conditional constants of complex the stability constants of low molecular-weight model compounds are due to low molecular-weight participants and to the effect of the electrostatic field of of the fixed-site ligands on the resin causes considerable differences in the R,Cu complexes and their capacity to enter into reactions with ligands in the
The complex formation constants of copper(H) with the iminodiacetate groups on Dowex-Al resin have been determined by potentiometric titration (log K = 10.54)’ and by measuring the copper-ion distribution between the resin phase and an aqueous solution of a low molecular-weight chelating agent (log K = 16.90).* These values differ considerably but the constant determined potentiometrically is quite close to that for copper-complex formation with the low molecular-weight analogue of the resinbenzyliminodiacetic acid, log K = 10.61.’ In a previous study3 we determined potentiometrically the formation constant of the Cu(I1) complex of an asymmetric resin containing fixed L-proline groups, log K = 12.82. This value was in good agreement with the chelation constant of the low molecularweight analogue of this resin, N-benzyl+proline, log K = 12.39rfr0.10.4 In the present work the formation constants of the copper complexes with the fixed groups of this Lproline resin, and also the constants for sorption of copper-proline complexes on the resin have been calculated from the results of distribution measurements of Cu(I1) and t_-proline ions between the resin and aqueous phases, with the aim of studying the effect of conditions of complex formation on the values of these constants. Factors
affecting
the interphase
distribution
of Cu(II)
and L-proiine ions
In the calculations of ~ncentrational chelation constants it is sometimes assumed that the concentration of the low molecular-weight reaction components in the resin phase is equal to their concentration in the aqueous phase. In our opinion, this
assumption is frequently incorrect. This is indicated, for instance, by the strong dependence of the chelating properties of the sorbent on the ionic strength of the external solution. Thus, reduction in chelating ability with increase in ionic strength was reported for resins containing iminodiacetic acid groups5 and carboxylic groups.6 In the latter work it was shown that the formation constant of the Cu(I1) complex is reduced by two orders of magnitude when the supporting electrolyte concentration is increased from 0 to 2N. In the present work we have studied the chelating properties of an asymmetric sorbent containing Lproline groups, prepared from an isoporous macroreticular matrix with 10% cross-linking.’ A sharp reduction of chelating power in sorption of copper from ammonia solution is also observed for this sorbent if a neutral electrolyte, potassium chloride, is added in 1M concentration (Fig. 1). This apparently refects the dependence of the copper ion distribution on the ionic strength of the solution, in accordance with the Donnan equilibrium. A similar dependence was also observed for the distribution of L-proline (Fig. 2). At pH values around 7 the distribution coefficient of the L-proline zwitter-ion is K, = 0.83, close to that of the giycine zwitter-ion in a carboxylic acid resin, K, = 0.93.’ In this range the distribution coefficient of the L-proline zwitter-ion between the solution and the electroneutral resin phase is practically independent of the ionic strength. In the alkaline region, at pH = 11.5, the distribution coefficient describes the behaviour of the L-proline anion in the negatively-charged resin phase. In the absence of a supporting electrolyte, K, = 0.1, whereas in the presence of 1M potassium chloride, Kd = 0.58. Such a big difference in the distribution coefficients is undoub-
Yu. A. ZOLOTAREV.A.A.KURGANOV,
A.V. SEMECHKIN and V.A. DAVANKOV
proline absence The [Pro’], proline formula
between the two phases in the presence and of 1M potassium chloride. conditional concentration of free L-proline, was calculated from the total amount of Land Cu(II) in solution according to the
(1)
CCu*+l, M Fig. I. Sorption isotherm of copper(l1) from 2M ammonia by an asymmetric resin containing L-proline groupings, in the absence of supporting electrolyte (1) and in the presence of 1M potassium chloride (2).
where m Proand mcu are the numbers of moles of proline and copper in solution respectively, and V is the volume of solution (taking into account the swelling of the resin). The conditional concentration of free copper in solution was calculated as Lcu,3
IO
c
05 -
i
\
01
[Cu(ProM Pro'1
;------h
y:
=
I
I
I
I
7
8
9
10
\ ‘\
\
\
\
\ .L_ I I
/I
I 12
PH Fig. 2. Coefficient of Donnan distribution of L-proline between the aqueous and asymmetric resin phases as a function of the pH of the aqueous phase, in the absence of supporting electrolyte (1) and in 1M potassium chloride (2).
tedly connected with ion-exclusion of the L-proline anion, which in this case acts as a co-ion. It is known that exclusion is greater the larger the volume of the ion and the lower the ionic strength of the solution.’ The greatest reduction of K, occurs at the transition of the L-proline zwitter-ion into the anion in the region corresponding to deprotonation of amino groups, pK,, = 10.6. Therefore, at pH > 11.5 in a Donnan equilibrium the amount of L-proline in the resin phase is only a tenth (in the absence of a supporting electrolyte) of that in the aqueous solution at equilibrium with it. Addition of a supporting electrolyte in 1M concentration increases the amount of proline anion in the resin phase by a factor of 5.8. Evidently, the changes observed in the interphase distribution coefficients of low molecular-weight components with variation in the ionic strength and pH should considerably affect the values of complex formation constants determined by this method. Calculation qfchelation
constants in alkaline solutions
To determine the dependence of the chelating and sorption properties of asymmetric sorbents on ionic strength, we studied the distribution of Cu(II) and L-
‘h
where [Cu(Pro),] is the concentration of the bisproline copper complex (assumed equal to the total concentration of copper in solution, as [Cu(Pro),] >> [Cu]) and b2 is the overall formation constant of Cu(Pro),, logp, = 16.66.9 The amount of water and the total quantities of copper and L-proline in the resin phase were determined. Naturally, the values obtained include certain amounts of non-absorbed r_-proline and copper bis-prolinate. Taking the distribution coefficients as 1 for the copper bis-prolinate, 0.58 for the L-proline anion in the presence of a supporting electrolyte and 0.1 in its absence, and accounting for the volume of water in the resin phase, we calculated the actual quantities of copper, M,,, and proline, MPro, participating in the formation of complexes with the stationary phase. The quantity of copper-proline complexes sorbed on the resin phase, MR~“~~,,,is then equal to the corrected quantity of sorbed L-proline in the resin phase: MRCuPro = MPro
(3)
and the quantity of copper complexed with the fixed sites in the resin phase, MR,&s equal to the difference between the amounts of copper and proline sorbed: MR,c~ = Mcu - M~ro
(4)
The equilibrium number of moles of fixed resin ligands (MR) that did not enter into the chelation reaction with copper was determined as MR = MR~- MRC~P~-~MR,C~
(5)
where MR, is the total amount of fixed groups in the resin phase. The concentration of components in the resin phase was calculated with respect to the volume of water (VW) in the phase:
[X] = : w
The formation constants of the fixed-site complexes with copper and for sorption of the copper-proline
8.34 7.92 8.24 8.14 9.02 8.86
15 16 17 18 19 20
68.1 170.0 357.0 535.0 679.0 842.0
9.8 70.7 120.0 166.0 174.0 175.0
202.0 306.0 403.0 499.0 596.0 772.0 904.0 1018.0
1928 1918 190 1869 1850 1806
490 461 437 408 370 356
1954 1949 1940 1936 1924 1901 1860 2045
/~mole
M~rw
12.20 12.30 12.71 12.21 13.76 14.22
9.99 9.98 9.96 9.95 9.94 9.93
13.75 13.76 13.76 13.76 13.77 13.78 13.80 14.82
V, ml
15-20,
0.878 0.714 0.535 0.349 0.252 0.058
47.0 32.0 19.7 7.6 2.2 4.0
115.0 100.0 83.0 67.0 52.0 26.0 4.2 6.0 --
IM KCI.
146.0 130.0 91.3 60.7 35.8 85.8
--
-
-
[ProH], mM
of the aqueous
[Pro], mM
electrolyte;
5.6 14.0 27.7 40.9 48.8 59.7
0.98 7.07 12.0 16.6 17.4 17.6
22.0 29.5 36.6 43.6 56.5 66.0 12.0
15.0
mM
-[CuPro,],
* l-8, 1M KCl; Q-14, without supporting t Based on formation of R&u.
11.95 11.85 11.80 11.60 11.20 10.60
9 10 11 12 13 14
11.62 11.60 11.55 11.50 11.45 11.40 11.20 10.65
l 2 3 4 5 6 7 8
~.
pH
No.*
MC,, nmole
1. Composition
Solution
Table
12.81 12.16 11.66 11.13 10.76 9.41
17.01 15.82 15.17 14.20 12.11 10.62
16.60 16.32 16.03 15.75 15.45 14.74 13.09 11.36
pCu
Resin
and resin phase and effective constants
32.3 39.7 63.5 95.9 166.0 216.0
41.4 92.1 134.0 189.0 261.0 303.0
11.2 14.8 24.9 35.3 44.3 84.2 169.0 247.0
--
20.7 24.3 42.8 65.7 109.0 156.0
33.2 58.4 19.6 107.0 141.0 169.0
6.0 8.2 13.3 15.9 24.5 47.3 85.0 135.0
324 322 313 308 306 298
362 346 333 321 304 296
366 364 360 358 354 343 319 302
64.1 75.5 133.0 206.0 356.0 523.0
22.6 97.4 165.0 255.0 395.0 453.0
22.3 30.4 48.2 68.1 86.5 160.0 272.0 405.0
33.4 47.8 62.0 98.0 186.0 201.0
91.7 16.9 239.0 333.0 464.0 571.0
19.5 24.3 41.4 57.2 70.4 129.0 268.0 336.0
185 102 1.56 130 301 221
1400 1240 1100 885 618 355 1260 1320 1220 1110 644 570
-
-
12.79 12.78 12.06 11.85 11.07 9.92
15.68 14.86 14.46 13.83 12.19 11.27
14.59 14.41 14.37 14.25 14.05 13.69 12.62 11.30
[RH], mM hKrt,cU 1%
formation
1400 1390 1370 1340 1330 1210 890 617
of complex
MPro, Swelling, [RCuPro], [RiE],.-if$ MC,, pmole @mole mg H,O mM
solution
15.40 15.18 14.86 14.83 14.29 14.02
16.55 16.21 16.05 15.78 14.57 14.12
15.74 15.65 15.65 15.65 15.55 15.45 14.95 14.40
hwu~ro
12.5 16.0 25.0 37.0 65.0 84.3
14.3 31.8 46.2 65.2 90.0 104.0
4.4 5.8 9.1 13.8 17.3 32.9 65.9 96.6
%t
Degree of saturation of resin with copper
Ku. A. ZOLOTAREV,A. A. KURGANOV, A. V. SEMECHKINand V. A. DAVANKO~
502
complexes
may now be determined
as
sample; n2 = 1.6 and pK_ = 9.70 for an asymmetric resin, in the absence of complex formation.3 Equation (10) was solved for MR by the method of iteration at each point. The values of MR were then substituted into equation (9) to obtain the values of MIH.
this calculation of the effective concentrational constants of complex formation, presented in Table 1 and Fig. 3, the concentrations of unchelated copper [Cu] and proline [P ro ] ions in the aqueous phase were used. Since all tests of both series (with and without a supporting electrolyte) were carried out in alkaline solutions (pH > 11.2, except for samples with maximal copper concentration) it was assumed that all the proline and resin-site ligands were in the anionic form and capable of complex formation (the dissociation constants, pK_, of their amino groups are 10.6,” and 9.70,3 respectively). In
The concentrations of the sorbed and fixed-site complexes as well as of the resin groups in zwitter-ion and anionic forms were determined with due regard to the amount of water in the resin phase, according to equation (6). The number of moles of copper bis-proline complex in solution, mCuPro,, was taken as equal to the number of moles of copper determined by analysis of the solution; the amount of unchelated proline, mpro +mProH, in this case is equal to the difference between the total amount ofproline in solution and the amount of its complex with Cu(II). From the equation for the dissociation constant of proline:
,y, =
Culculution qf’chelution constants in neutrul und weukly alkaline solutions Use of neutral solutions in the study of interphase equilibria makes it possible to minimize the effect of the electrostatic field of the sorbent. However, the acid&base equilibria of both the resin fixed-site ligands and the mobile ligands should be taken into account, since both species are capable of co-ordination with Cu(II) ions when they are in the anionic form, but not when they are in the zwitter-ion form. In the consideration of the acid-base equilibria in the resin phase we proceeded as follows. The dissociation constant of the amino-acid group in the weak-acid carboxylic acid cation-exchanges is practically the same as the constant for the amino-acid in solution,7 and for a resin on which no electrostatic charge is present, the distribution coefficient of charged and neutral molecules tends to unity.’ For the fixed-site groups on resins it is assumed that the acid-base equilibrium is not shifted by the presence of proline zwitter-ions in the resin phase, and that the apparent dissociation constant depends only on the total charge of the polyelectrolyte chain.6,10 The amount of sorbed (MaCuPro) and fixed-site (MR,& complexes was determined, as previously, according to equations (3) and (4). The quantity of sorption groups in zwitter-ion form, M,,, was obtained from M,, =
Miicu~r<>+
~~~~~~ + MR + MR,,
M is the number
of moles
of a species
rPro]‘LH1; pi = [ProH]
I4
lo.6
’
”
we found separately the amounts of proline in the zwitter-ion and anionic forms. The concentration of unchelated copper was determined from equation (2). Finally, the formation constants of the fixed-site stationary and sorbed complexes were calculated from equations (7) and (8). Results of the calculations are given in Table 1 and Fig. 3.
I8 0 I 160
3
I4 0
t
12.0
$ IO 0
8.0 2.0
(9)
and substituted into the equation for polyelectrolyte dissociation under conditions of complex formation :*
where
‘I>
in the
0 Degree
20
40
of saturation
80
60 of resin
with
Cu’:
100 %
Fig. 3. Stability constants with Cu(II) ) and of sorbed CumPro complexes (---) on an (asymmetric resin, as a function of the degree of saturation of the resin with copper(H) (to form R,Cu). (1)-without supporting electrolyte, pH > 11 .O ; (2).-1M potassium chloride, pH B 11.0; (3)-l A4 potassium chloride, pH - 8.
Copper-proline-resin The dependence of conditional chelation constants on .‘he degree of sorbent saturation with copper ions, pH and the ionic strength of the solution Three pairs of conditional stability constants of fixed-site (fi,Cu) and sorbed (iiCuPro) complexes, determined under different conditions, are shown in Fig. 3. As can be seen, they decrease with increase of sorbent saturation with copper ions. The decrease of KQ-” is especially rapid. Earlier,” we explained the gradual reduction in stability of fixed-site complexes formed by two ligands bonded to the polymer matrix as due to deterioration of the steric conditions for chelation, as the copper ions must react with less and less favourably arranged pairs of fixed-site ligands. Evidently, the stability of individual fixed-site fi,Cu complexes varies over a very wide range. This is confirmed by the fact that the average stability constant of these complexes drops by at least 3 orders of magnitude with increase in the degree of saturation with copper ions from 0 to 100%. The stability of sorbed complexes (RCuPro) depends to a considerably smaller extent on the copper content of the system. This is to be expected, since the mobile ligand (Pro) in these complexes is not bonded to the polymer chains of the matrix and may arrange itself favourably in space. The fact that the stability of the sorbed complexes nevertheless shows a tendency to reduction with increase of copper content in the resin phase, may be explained by the increase of steric hindrance to formation of bulky complexes in the resin phase and also by variation in the activity coefficients of mobile ions. (The exchange constants have been reported to be dependent on the ionic composition of carboxylic lz and diallyl phosphate’ 3 resins.) Figure 3 demonstrates graphically the dependence of the conditional stability constants of the fixed-site and sorbed complexes on the pH and ionic strength of the equilibrium solution, which, as shown above, considerably affect the distribution coefficients of copper and proline ions. The conditional stability constants of complexes in the resin phase should, undoubtedly, depend on such characteristics of the polymer matrix as the degree of cross-linking and concentration of fixed ligands. The fact that the constants for a similar sorbent with 6% cross-linking and 1.68 meq/g stationary L-proline groupings’ ’ are slightly higher than those obtained in the present work, and depend to a smaller degree on the amount of copper ions in the sorbent, we also associate with these factors. The calculated conditional stability constants (Fig. 3) are in good agreement with the actual behaviour of the system under static and dynamic conditions and reflect the following important features of the system. (i) Sharp reduction of the sorption capacity of the resin for copper ions on addition of a neutral electrolyte to the equilibrium solution (see Fig. 1) and on transition from alkaline to neutral solutions. (ii) Considerable increase of resin affinity for mobile ligands with increase of the degree of resin saturation
complexes
503
with copper ions. Indeed, the sorption constant of Lproline is determined by the difference between the stability constants of the sorbed complex and of the initial fixed-site complex, ~~~~~~~- KQ-“; this difference rapidly increases with the amount of copper in the system (see Fig. 3). (iii) Noticeable increase of resin affinity for the mobile amino-acid ligand (Pro) with increase in the ionic strength of the equilibrium solution and, in particular, on transition from alkaline to neutral solutions. Donnan distribution While the dependence of the conditional constants for chelation with the resin phase on the degree of its saturation is, apparently, primarily associated with steric features of the process, the dependence of the calculated constants on pH and the ionic strength, undoubtedly reflects changes in the distribution coefficients of low molecular-weight components between the two phases. Evidently the latter factor should always be taken into account when comparing stability constants for complexation in resin phases with the stability constants of low molecular-weight model compounds. Experimental determination of the distribution coefficients of unchelated copper and proline ions in a real system that contains all the components participating in the process is impossible. However, as mentioned above, we determined the distribution coefficients of L-proline (I&,) in the absence of copper ions. The distribution coefficients of unchelated copper ions (KS,) may be estimated from these values, if we assume, according to Helfferich,14 that formation constants of low molecular-weight complexes in the resin phase do not differ from the same constants in solution and that the distribution coefficient of the electroneutral CuPro, complex is equal to unity. In this case, at equilibrium K
Cu + 2Pro 2
CuPro,
and K& = (Kg,)‘. The calculated values of the distribution coefficients of unchelated copper (K&) are 1.49 for the neutral pH range, and 2.97 and 100 at pH = 11.5 in the presence and absence of 1M potassium chloride, respectively. On the basis of the K$,Oand K& values the stability constants, ~~~~~~~ and KQ-,, corresponding to 50% and 20% sorbent saturation with copper ions may be corrected. The corrected values obtained are given in Table 2 together with KQ-” determined potentiometrically3 and the stability constants of the corresponding model complexes bis(N-benzyl-L-prolinato)copper(II)4 and (L-prolinato)(N-benzyl-r=prolinato)copper(II).ls As can be seen from the data presented in Table 2, the corrected values of the stability constants for the sorbed and fixed-site complexes obtained in alkaline solutions in the presence and absence of a supporting electrolyte are in good agreement with each other. Therefore, the differences in the corresponding effec-
504
Yu. A. ZOLOTAREV, A. A. KURGANOV, A. V.
SEMECHKIN and V. A. DAVANKOV
Table 2. Stability constants offixed-site and sorbed complexes in the resin phase and oftheir low molecularweight analogues Test conditions
logK~cu~ro
log ht2Cu
Conditional
Corrected
Conditional
Corrected
pH > 11;5O%Cu
14.2
12.2
16.0
15.0
pH> ll~lMKCl~50”/,Cu pH-8;iMKCl;;O%Cu PH -8;lMKCl;20%Cu Potentiometry3 Model complexes4~15
13.0 11.5 12.7
12.5 11.35 12.55
15.2 14.6 15.2
15.0 14.5 15.1
12.8 12.4+0.1
tive chelation constants arise from the high values obtained in the absence of a supporting electrolyte and caused by the Donnan distribution of low molecularweight participants in the process. Both the corrected and conditional chelation constants obtained for feebly alkaline 1M potassium chloride medium have lower values than those for alkaline solutions. This is evidently connected with the decrease in the charge on the polymer chain on transition to the neutral pH-region, i.e. with decrease in the electrical potential of the polymer chain. This conclusion conforms with the results obtained by Gregor who showed for carboxylate linear and crosslinked polymers that the chelating properties of a polyanion significantly exceed the chelating properties of low molecular-weight model compounds. Evidently the polyanion field affects the stability of fixed-site complexes to a considerably greater extent than that of sorbed complexes. The corrected values of the constants obtained for sorbed and fixed-site complexes in alkaline solutions at 50% saturation of the sorbent with metal-ion are close to the stability constants oflow molecular-weight model compounds. Apparently, this is due to compensation by two opposite effects, namely, the effect of the polyanion field and steric hindrance to complex formation. Both effects should be insignificant in pH-regions close to neutrality and at low degrees of sorbent saturation with copper ions (10-200/0). Under these conditions, the values obtained are indeed close to the stability constants of the corresponding low molecular-weight analogues and to the stability constant of the fixed-site complex determined potentiometrically. EXPERIMENTAL
The asymmetric resin containing L-proline groupings was synthesized on a macro-reticular isoporous polystyrene matrix with 10% crosslinking. The sorbent capacity was 2.71 meq/g. Distribution of the components in the system was determined under static conditions in 15-ml columns.3 The airdried resin was brought to equilibrium with a solution of copper and L-proline by agitation in sealed tubes for 72 hr. The resin phase was isolated by centrifuging at 2000 y (3500 rpm) for 15 min. For analysis of the resin phase, copper and L-proline were desorbed by washing the resin in the column with 5M hydrochloric acid. The amounts of copper and L-proline were determined in the aqueous and resin phases. The results obtained agree within experimental error with the amounts of copper and L-
14.9kO.2
proline introduced into the system. Copper was determined spectrophotometrically with diethylcarbamate at 440 nm. L-Proline was determined in 5M hydrochloric acid by polarimetry at 436 nm. The pH of the solutions was measured to 0.01 pH unit. A0.300-g portion ofair-dried resin containing 0.512 meq of functional groups was placed in a column. For studies in alkaline and weakly alkaline media, 2.00 meq of L-proline and 0.2-2.4 meq of Cu(II) were added if a 1M potassium chloride medium was used, and 0.53 meq of r.-proline and 0.1-l .Omeq of Cu(II) if the supporting electrolyte was omitted. For studies in alkaline solutions, potassium hydroxide was also added, in quantities equivalent to the total quantity of functional groups in the resin and added proline. In the case of weakly alkaline solutions potassium hydroxide was added in amounts that would not exceed by more than 5-10% the quantity of protons formed during complex formation. The volume of equilibrium solution was corrected for the water introduced with the air-dried resin and extracted by the swollen resin. The amounts of occluded copper bis-prolinate and L-proline were taken into account in the determination of sorbed copper and L-proline. The distribution coefficient of copper bis-prolinate was taken as equal to unity. For proline in alkaline solution in the presence of I M potassium chloride, Kg,, = 0.58, and 0.1 in its absence. In the weakly-alkaline range it was assumed that K& = 0.83 (see Fig. 2). REFERENCES
1. C. Eger, W. M. Anspach and J. A. Marinsky, J. Inorg. Nucl. Chrw., 1968, 30, 1899. 2. H. Loewenschus and G. Schmuckler, Tulunta, 1964, II, 1399. 3. Y. A. Zolotarev. A. A. Kurganov and V. A. Davankov, ibid., 1978, 25, M/S 124. 4. A. A. Kurganov, V. A. Davankov and S. V. Rogozhin, Zh. N@orgcm. Khim., 1972, 17, 2163. 5. E. Leyden and A. L. Underwood. J. Phys. Chem., 1964, 68, 2093. 6. H. P. Gregor, L. B. Luttinger and E. M. Loeble. ibid.. 1955, 59, 34, 336. I. P. S. Nys and E. M. Savitskaya, Z/z. Fir. Khim.. 1969, 43, 1536. Russian). IL. Moscow, 8. F. Helfferich. I~)n-EuchunRrrs(in 1962. 9. A. A. Kurranov. V. A. Davankov. S. V. Roeozhin and Yu. D. Koreshkov, Koord. Khim., i977,3,66?. 10. K. Gartner and H. Wanjek, Z. Ph!,.s. Chrm., 1962, 221, 391. II. A. V. Semechkin. S. V. Rogozhin and V. A. Davankov, J. Chromatoa., 1977. 131. 65. 12. H. P. Greg;;. H. J. Hamilton and R. J. Oza. J. Pity.?. Chem., 1956, 60, 263. 13. J. Kennedy, R. V. Davies and H. Small. J. Appl. Chem.. 1959, 9, 32. 14. F. Helfferich, J. Am. Chem. Sot., 1962, 84, 3237. and P. R. Mittchel, J. Chrm. Sot., 15. V. A. Davankov Ddton, 1972, 1012.