Kinetic radiochemical study of cobalt(II) ion substitution in zinc(II) EDTA in aqueous buffer

.I. inorg, nuel. Chem.. 1967, Vol. 29. pp. 1973 to 1981. Pergamon Press Ltd. Printed in Northern Ireland

KINETIC RADIOCHEMICAL STUDY OF COBALT(II) ION SUBSTITUTION IN ZINC(II) EDTA IN AQUEOUS BUFFER S. S. K_rUSrINAN* a n d R. E. JERVlS Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto 5, Canada (First received 18 July 1966; in revisedform 27 February 1967) Al~traet--The kinetics of Co(II) ion substitution in Zn(I1)-EDTA chelate was studied experimentally at concentrations of zinc ion and zinc-EDTA chelate of the order of 10-3 M, considerably in excess of cobalt ion at 5 × 10-s M, and over a pH range of 4"4 to 6"4 at 20, 25, and 30°C. The reaction was followed by the rate of incorporation of radiotracer s°Co ion in the EDTA chelate. Under these conditions and in a medium of 0.05 M acetate buffer, the reaction was found to be quasi first order in cobalt ion and the dependence of the rate of the forward reaction of the solution variables was given, to within 6.6 per cent on the average, by the rate equation: v+ ~ kl [C°~+][ZnL~-] - + k~[CoZ+][ZnL2-] + k3 [H+][C°Z+][ZnL2-] mole/1. [H+1 [Zn~+]

per ~ c

where L represents the ligand EDTA and the rate constants have the values at 20°C of: kl =5.15 × 10-8/sec, ks = 0.1221./mole per sec, and k8 = 20.7 l./mole per see. Activation energies corresponding to the three rate constants were obtained by repeating some experiments at 25 and 30°C and assuming that the same reaction mechanism applied at the higher temperatures and the experimental values were 20, 16.5 and 6.4 keal/mole. The direct substitution of cobalt(II) ion in Zn-EDTA chelate contributed a major part of the overall process except at lower pH where protonation of chelate and subsequent dissociation was appreciable. Reaction involving the species CoOH + was also indicated. Comparison of the kinetics of the Co(II)-ZnEDTAreaction with previous studies of Co(II)-Co(II)EDTA and Zn(II)-Zn(II)EDTA isotopic exchange showed that the cross-substitution was much slower than other reactions involving t h e zinc-EDTA chelate and occurred at a rate comparable to Co(II) exchange. It seems possible that the mechanism of the cross exchange reaction is similar to that involved in the Co(II)-Co(II)EDTA and Zinc(II)-Zinc(II)EDTAexchange reactions. INTRODUCTION SUBSTITUTIONreactions o f the type: M(1) + M(2)L = M(2) + M(1)L where M(1)L a n d M ( 2 ) L are chelates f o r m e d by the ligand, L, with two different metal ions, M(1) a n d M(2) have b e e n studied previously by a n u m b e r of researchers. C1-11) KATO, TANAKA et al. reported
1974

S.S. K~rINAN and R. E. JERvls

in Z n E D T A by streaming mercury electrode. I n these studies, it was f o u n d that in the general case where b o t h M(2) ions and M ( 2 ) L chelate are present in a large excess over M ( I ) ions, the substitution reaction can be treated as a simple first order process in which: --d/dt[M(1)] ~- k+[M(1)] - - k_[M(1)L], where square brackets signify m o l a r concentrations. Therefore the kinetics o f the process can be examined, within the conditions o f this assumption, by determining the variation o f k+ with respect to the concentrations o f the reacting species, p H and temperature. I n previous papers f r o m this laboratory, the isotopic exchange o f Co(II) and Zn(II) ions with their respective E D T A chelates was reported. {a'm It was considered o f interest therefore, to determine the kinetics o f the cross-reaction between C o 0 I ) and Z n E D T A for comparison with the isotopic exchange data. Further experiments o f this type would allow a comparison with other reactions involving a series o f divalent cations o f differing electronic structure reacting with the same Z n E D T A complex. EXPERIMENTAL Materials. The materials used were cobalt metal (Fisher Certified grade), zinc metal (Baker

Analysed), tetrasodium salt of ethylene diamine tetraacetic acid (EDTA) (Fisher Certified grade), high specific activity radionuclides of cobalt-60 (,~100 c/g) and zinc-65 (,~50 c/g) obtained from Oak Ridge National Laboratories, and Dowex 50W (x 8) cation exchange resin (Baker Analysed). Other chemicals used for preparation of solutions and buffer were guaranteed reagents. Experimentalprocedure. In preparation for a radiochemical measurement of substitution rate, two separate solutions were made: one containing both zinc-EDTA and excess zinc(II) ion and the other, cobalt(II) ion, both solutions made up in 0.05 M (total) acetate buffer. The pH of the solutions was adjusted to the desired range with sodium hydroxide and they were brought to the appropriate constant temperature 20, 25 and 30°C (:k0.01°C) in a thermostat. The cobalt solution was labelled with high specific activity e°Cot+ radiotracer. The kinetic experiment was initiated by mixing known volumes of the two solutions in the thermostat. At suitable intervals of about 5-10 min, aliquots of the reacting mixture were withdrawn and the uncomplexed cationic cobalt was separated from the complexed anionic cobalt and zinc species by passage onto a Dowex 50 cation exchange column and elution with a predetermined volume of buffer through a procedure described previously. ~11,m The effective time of separation of the reacting species was usually less than I rain and contributed negligible error in the reaction rate determination. Radiocobalt-60 ion was observed to be incorporated in the chelate at an exponential rate extending over several hours. The relative amounts of uncomplexedf and complexed cobalt after reaction equilibrium had been attained was found by re-measuring the radioactivity in the two species after six to eight reaction halftimes had elapsed. These observations were used to compute k+ values by the method of TANAKAta-s~ a n d BRm et al. ~2~ The two solutions were not mixed and allowed to come to reaction equilibrium before addition radiotracer as in the previous experiments, c1~.~8}in order to avoid any contribution from the reaction: *Co8+ + CoEDTA2- ~_ Co~+ + *CoEDTA2-, where (*) represents the e°Co-labelled species. This process being slow {~ would be encountered if radiotracer were added to a solution containing Co(II), Zn(II) and ZnEDTA in equilibrium and would complicate the interpretation of the results. Also as indicated above, in all experiments the concentrations of Zn(II) and ZnEDTA were kept in the range of 10-8 M, considerably higher than the Co(II) concentration of 10-5 M to produce an apparent first order kinetics. f Cationic metal ion species are considered to include any labile aquo and acetato complexes formed under these conditions. cm S. S. KaIsm,;ANand R. E. JERws, J. inorg, nuel. Chem. 29, 87 (1967). tm R. E. JSRVlSand S. S. KRIS.NAN,J. inorg, nuel. Chem. 29, 97 (1967).

Kinetic radiochemical study of cobalt(II) ion substitution RESULTS

AND

1975

DISCUSSION

Over a pH range 4.4 to 6"4 and zinc ion and zinc--EDTA chelate concentrations of the order of 104 M compared to a cobalt concentration of 10-5 M, the substitution reaction: *COS+ q- ZnL~-~__ *CoL2- + Zn ~+, followed the reversible first order relation: --d/dt[Co ~-] = k+[Co z+] -- k_[CoLZ-]. The rate constant for the forward reaction k+ determined from the rate of incorporation of radiocobalt in the chelate varied linearly with chelate concentration, as indicated in Fig. 1 and in a complex manner with respect to hydrogen ion concentration 3O

25 pH = 5 ' 7 8

, /

20

% t

to

// 1

I

0"5

I

I

I'0

I

I

I

/

1'5 2'0 [ZnL= ] x I0 3

I

2 5

3.0

FIG. 1.--Variation of the first order rate constant with zinc E D T A concentration in cobalt-zinc E D T A reaction. ( e represents experimental results and the line represents results calcuhted from the overall rate equation.)

particularly above pH 6 as indicated in Fig. 2. The increase in rate in this region was shown to be proportional to 1/[H+], or to hydroxyl ion concentration. The extent of competition between zinc and cobalt ions is evident from the inverse dependence of the rate of cobalt ion substitution in the zinc-EDTA complex with increasing zinc concentration (Fig. 3), in spite of the fact that zinc was always present in considerable excess over cobalt. As an aid to the postulation of a reaction mechanism, all of the k+ values obtained were fitted to various rate equations of different forms in terms of their dependence on the solution variables. The fitting calculations were done using an IBM 7090 computer and a least squares programme. The best agreement of experimental values (k+ exp.) and those calculated by the several rate equations (k+ calc.) was obtained (to 4-6.6 ~o) with the relation (Table 1): Rate of forward reaction, [znL~-] [ZnL~-][H+]L~ 2+1 v+ = ka [H+-----~ + k2[ZnL'-] + ks ~ Itt~o j,

1976

S . S . KRISHNAN and R. E. JERVIS

I ' z n ++ ] = txlO-SM

/

[ZnL= ] = txlO-3M [Co++ ] = 5xlO'SM

/ /

% x

13

II IC o

I

0m

[

I 0

!

I

I 30

I

I 40

I

I

I

50

I 60

=

[H+]xlO T

FIG. 2.--Variation of the first order rate constant with acidity in cobalt-zinc E D T A reaction. ( 0 represents experimental results and the line represents results calculated from the overall rate equation.)

where v+ is given in the units: mol/l, per see and the rate constants have the values: (at 20°C) kl = 5"15 × 10-8/see, ks -~ 0.122 1./mole/see, and ka = 20.7 L/mole/see. Activation energies corresponding to the three rate constants were evaluated by repeating experiments at other temperatures of 25 and 30°C on the assumption that the same form of rate equation (i.e. same reaction mechanism) applied at the higher

[znt-""J= ) xtO-~U [C~÷]= 5 x IO'~M pH = 5 ' 7 8 17

% ._.~+ 15

I

o

0.5

I

]

T

I °

r

I

vo ].5 2.o (, / [ z . *÷] } x i 6 5

r

[

z.5

i

J

3.0

FIG. 3.--Dependence of the first order rate constant with zinc concentration in cobalt-zinc E D T A reaction. (0 represents experimental results and the line represents results calculated from the overall rate equation.)

Kinetic radiochemical study of cobalt(II) ion substitution

1977

TABLE 1.--COBALT(II)-zINC(II)-EDTA SUBSTITUTIONREACTION. [EXPERIMENTALRESULTSI [Co '+ ] = 5

× 10- ~ M

Experiment

Zn 3+ × 10-3

ZnL 9- × 10-9

pH

k+ exp. × 104 (sec-1)

k+ talc. × 104 (sec-0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

2"0 1"0 0.67 0"50 0.40 0.50 1"0 2.5 1.0 1"0 1"0 1.0 1"0 1.0 1"0 1-0 1"0 1-0 1-0 1-0 1-0 1"0 1-0

1"0 1-0 1"0 1.0 1"0 1.0 1"0 1"0 0"25 0"50 1.50 2"0 2.5 0"50 2"5 1"0 1"0 1"0 1-0 1"0 1-0 1"0 1"0

5.78 5"78 5.78 5'78 5"78 4"70 4-70 4.70 5.78 5.78 5.78 5"78 5.78 4"70 4.70 4"40 4"52 4"86 4"97 5"08 5-30 6"05 6.26

1"83 1"92 2"08 2"10 2-23 9.4 5"4 3"2 0"53 1.10 3'15 4.1 4"6 3"0 13'1 9"7 7"9 3.65 3'10 2-65 2"35 2"05 2"15

1"70 1"85 2-05 2-2 2"4 9"5 5"4 2"9 0"45 0"93 2"8 3"75 4.7 2.7 13.4 9-5 7.5 4-1 3"5 3"0 2"35 2"0 2-25

Mean Diff: k+ exp. -- k+ talc. = 6"6~o t e m p e r a t u r e s (Fig. 4). T h e v a l u e s o b t a i n e d a r e l i s t e d in T a b l e 2 a n d r a n g e f r o m 6 to 20 k c a l [ m o l e , s i m i l a r to t h o s e f o u n d f o r i s o t o p i c e x c h a n g e in t h e c o b a l t ( I I ) a n d z i n c ( I I ) - E D T A systems. ~12a8~ F r o m t h e b e s t fit r a t e e x p r e s s i o n t h e f o l l o w i n g d e d u c t i o n s m a y b e d r a w n concerning the nature of the parallel reactions contributing to the substitution TABLE 2.--ACTIVATION ENERGIES Rate equation: k+[Co 9+] = v+ =

kl ~

+ kdZnL ~-] + k3

Temperature (°C)

kl (per sec)

ks (L/mole per sec)

20 25 30

5"2 :x: 10-s 12 :,< 10-s 28 × 10-s

0.122 0.210 0.351

Slope:

log kl/1/T = --4"37 × 10a; Ex 1 = 20"x kcal/mole log kg/1/T = --3'59 × 103; Ex 3 = 16.~ keal/mole logk3/1/T= --1"38 × 10a; E~_3 = 6.48keal/mole

[Co g+] k8 (l./mole per see) 20"7 25"6 31 "5

1978

S . S . KRISrrNAN and R. E. JEgws

reaction: the first term is of the form that could be accounted for, by postulating the reaction: *Co(OH) + + ZnL2- ~-- *CoL~ + Zn(OH) + since the rate term: const. [ZnL2-][CoOH+] can be equated to: const, x khya

[ZnL~-][Co~+1 [H+]

or,

kl

[ZnL~-][Co 2+] [H+] (say),

where khra = [CoOH+][H+]/[Co~-] is the hydrolysis constant of cobalt ion. It should be noted that, according to this interpretation, the rate constant kl includes the hydrolysis constant kh. In the previous study of Co(II)-Co(II)EDTA exchange, (12) participation of the CoOH+ species was not detected. However, in the present study,

5"5

5"(:

i

4"633.0

I

i

I

I

I

I

I

33.5 (I/T'K}

I

I

I

I

i

34.0 x 10 4

FI6. 4.1Variation of rate constants with temperature in the cobalt-zinc EDTA reaction. A ((log k,) + 10.4)vs. l/T; © ((log k,) + 4.0)vs. I/T; × ((log ks) + 2.0)vs.

1/T.

experiments were extended to a higher pH region in which the contribution of CoOH + becomes appreciable, as is made evident in Fig. 2. The second term in the overall rate equation indicates the direct reaction of cobalb (II) ion and zinc EDTA chelate: *Cos+ + ZnL~-,~-~ *CoL2- + Zn2+, and the third term is consistent with the following two reactions in a quasi-steady state: ZnL2- + H + ~- Zn~ -t- HL aand HL a- + *Co~+~_ *CoL~ + H +, in which the protonation and dissociation of zinc chelate catalyses the substitution of cobalt. This mechanism is similar to that postulated by TANAKAet aI. ta-a) and by BRIL ¢2) for cross-reaction involving divalent metal EDTA chelates.

1979

Kinetic radiochemical study of cobalt(II) ion substitution TABLE 3.--RELATIVE CONTRIBUTIONS OF PARALLEL REACTION PATHS

Experimental conditions: [Zn g+] = [ZnL ~-] = 10 -8 M; [Co S+] = 5 × 104 M; pH=5"78

or

[H + ] = 1 . 6 6 × 10-eM

CoOH + + ZnL ~-

Co ~+ + ZnL*-

Co 2+ + HL 8-

(term 1)

(term 2)

(term 3)

Total reaction rate

(mole/l. per see)

(mole/l. per see)

(mole/1. per see)

(mole/l. per see)

1.60 × 10-*

6"1 × 10-~

1"68 × 10-g

9.33 × 10-°

(17 %)

(65 %)

(18 %)

T h e r e l a t i v e c o n t r i b u t i o n s t o t h e o v e r a l l process by t h e a b o v e t h r e e p a r a l l e l r e a c t i o n p a t h s is g i v e n i n T a b l e 3, f o r a r e p r e s e n t a t i v e e x p e r i m e n t a l c o n d i t i o n . I t will b e seen t h a t t h e c o n t r i b u t i o n o f t h e * C o ~+ + Z n L ~ r e a c t i o n r e p r e s e n t s a m a j o r proportion of the overall substitution process under these conditions, although at a l o w e r p H o f 4.7, t h e t h i r d p a t h i n v o l v i n g p r o t o n a t i o n o f c h e l a t e c o n t r i b u t e s a b o u t

80%. I t is o f i n t e r e s t t o c o m p a r e t h e results o f t h e s e studies w i t h o t h e r w o r k i n v o l v i n g t h e s e a n d r e l a t e d d i v a l e n t e l e m e n t s : i n T a b l e 4 a r e listed r a t e c o n s t a n t s a n d a c t i v a t i o n e n e r g i e s o f s u b s t i t u t i o n r e a c t i o n s o f d i v a l e n t m e t a l i o n s a n d t h e i r E D T A chelates. T h e r a t e c o n s t a n t s are, in g e n e r a l , a f u n c t i o n o f t h e c o n c e n t r a t i o n o f t h e a c e t a t e i o n s ( O A t - ) . T h e s t a b i l i t y c o n s t a n t s o f a c e t o c o m p l e x e s o f c o b a l t a n d z i n c a r e n o t large. Therefore taking into consideration the formation of acetocomplexes and reaction TABLE 4.--COMPARISON OF SUBSTITUTIONREACTIONSOF DIVALENTMETAL-EDTA CHELATES Reaction

Rate constant (L/mole per see)

Activation energy, Ex (kcal/mole) 16

Reference

(a)

Co *+ + COL*-

7.0 × 10-~ (20°C)

(12)

(b)

Zn s+ + ZnL*-

4"1 (20°C)

29

(13)

(c)

Co *+ + C o H L -

1 × 104 (20°C)

10.6

(12)

(d)

Co *+ + ZnL*-

12 × 10-* (20°C)

16'5

This study

(e)

CoOH + + ZnL 2-

5.2 x 10 -a (20°C)

20

This study

23 x 10-* (25°C,/z = 0"2)*

--

(17)

~ 5 0 × 10-* (25°C,/~ = 0.2)*

--

(17)

(f)

Zn *÷ + CoL a-

(g)

Zn ~+ + C o H L -

0a)

Cu ~+ + CoL 2-

15 (15°C,/z = 0"2)

--

(8)

(i)

Cu *+ + C o H L -

1.5 × 102 (15°C, # = 0"2)

--

(8)

(j)

Cu *+ + ZnL 2-

19 (15°C, tt = 0-2)

12

(3)

(k) (1)

Cu 2÷ + Z n H L Pb 2+ + CoL ~-

60 (15°C, # = 0.2) 10 × 10-2 (15°C, # = 0.2)

(m) (n)

Pb *+ + C o H L Pb 2+ + ZnL 2-

1.2 × 102 (15°C, # = 0.2) 5 (25°C, # = 0"2)

(o)

Pb *+ + Z n H L -

3 × 10' (25°C, # = 0-2)

*/~ = ionic strength.

5.3 --11 6-7

(3) (6) (6) (7) (7)

1980

S.S. KmSnNANand R. E. JF.~VlS

through these complexes would not appreciably change the values of the rate constants reported especially under the experimental conditions involved in this work, viz. 0.05 M total acetate buffer. (la) The rate constant of the cross-reaction (d) of Co(II) ion with ZnEDTA is similar in magnitude to that for the Co(II)--Co(II) EDTA isotopic exchange reaction (a) and is appreciably lower than the value for Zn(II)Zn(II)EDTA exchange reaction (b). This result would not be expected if a SEI dissociation mechanism were prominent and the kinetics were little affected by the nature of the incoming metal ion. Also, these studies show a rate dependence of metal ion concentration in the substitution reactions of cobalt and zinc EDTA chelates indicating a possible SE2 displacement mechanism. Possibly Co 2+ can be considered to be capable of forming stronger complexes than Zn 2+ due to the presence of incomplete 3d shell in the former. In that case, reaction (b) involves two weaker complexes viz. Zn 2+ aq. and ZnL 2-. Therefore, this reaction might be expected to be faster than reaction (a) which involves the two stronger complexes (Co s+ aq. and COL2-). Reaction (d) involving one stronger and one weaker (Co s+ aq. and ZnL ~-) complex could be expected to react with an intermediate rate between reactions (a) and (b). It seems therefore possible that the cross exchange reaction (d) takes place by a mechanism similar to that in the case of the other two pure systems (a) and (b).(12,18) Although all rate constant values in Table 4 are not given at the same experimental conditions, a rough comparison of the pairs of reactions involving cobalt and zinc EDTA; (b) and (d); (a) and (f); (a) and (b) indicates that substitution of Zn(II) ions in a given chelate proceeds much more rapidly than the substitution of Co(II) ion. Comparison of the reactions: (a) and (d), (b) and (f), (h) and (j), (1) and (n) indicates that substitution of a given metal ion in ZnEDTA proceeds somewhat more rapidly than with Co(II)EDTA. The same trend is not apparent between the reactions: (i) and (k), (m) and (o), which involve the corresponding protonated chelates of Co(II) and Zn(II). In these cases the rate constants tabulated include the equilibrium constants of the protonated species. This is not expected to alter the relative magnitudes of the rate constants due to presumably similar values for the equilibrium constants for the protonated cobalt and zinc EDTA complexes. Even taking this into account, the rate constant for reaction (c) appears to be ordinately high. Further, notwithstanding the above trends in relative lability, the rate constants for the substitution of Co(II), Zn(II) and Pb(II) ions in Co(II)EDTA chelates in reactions: (a), (f) and (1) respectively, are of similar magnitude, although the values for Cu(II) reaction (h) is anomalously high. Similarly, the rates of substitution of Zn(II), Cu(II) and Pb(II) in Zn(II)EDTA are comparable in magnitude (cf. reactions: (b), (j) and (n)), but that of Co(II) ion reaction (d) is much lower. On the basis of crystal field splittings, (16) and the assumption of octahedral configuration of the zinc-EDTA chelate and weak field metal-EDTA interaction, it is to be expected that the lability of zinc chelates with the d 1° configuration would be greater than or equal to that of cobalt(II) chelates involving d 7 cobalt(II) ion electron structure, for dissociative reaction mechanism, as discussed previously. (13) It will be noted that Cu(II) reacts a little more rapidly than Zn(II) but crystal field predictions (14)N. TANAKAand H. OGINO. Private communications. as) F. BASOLOand R. G. PEARSON,Mechanisms of Inorganic Reactions, p. 108. John Wiley, New York (1958).

Kinetic radiochemical study of cobalt(II) ion substitution

1981

are not unique in this case. Reactions of Pb(II) with E D T A chelates do not involve inner d orbitals presumably and hence can be expected to occur relatively more rapidly than divalent transition elements. Apart f r o m considerations of the kinetics of the C o ( I I ) - Z n E D T A substitution reaction, some information can be obtained from the equilibrium distribution of radiotracer cobalt which is determined in such a system by the ratio of the chelate formation constants for the two competing metal ions. The ratio of the formation constants: Kzr~/KcoL was determined in this work to be 1.52 at a temperature of 20°C, compared to a value of 1.55 reported by SCHWARZENBACHet aL tl°) and a value determined polargraphically by OGINO et al. ~17) of 1"70 at 25°C,/z = 0-2. Acknowledgements--The financial support to this research and the award of a research Fellowship to

one of the authors (S. S. K.) by the National Research Council of Canada is gratefully acknowledged. ~xe~G. SCHWARZENBACI~,R. Gtrr and G. ANDEREGG,Helv. chim. Acta 37, 937 (1954). ~17~H. OorNO, Bull. Chem. Soc. Japan 38, 771 (1965).