THE REACTION BETWEEN C o o AND Pb(N) ACETATES IN ACETIC ACID BY D. BENSON,* P. J. PROLL, L. H. SUTCLIFFE AND J. WALKLEY Dept. of Inorganic and Physical Chemistry, The University, Liverpool Received 20th January, 1960 While the order of the reaction with respect to plumbic acetate is unity under all con- ditions, the order with respect to cobaltous acetate is non-integral and can be varied by the addition of plumbous acetate. These facts are accounted for by a reaction mechanism which requires the postulation of dimeric Co(I1) and the reactive intermediates Pb(II1) and Co(IV). The influence of water on the reaction is consistent with the presence of an unstable intermediate like Co(rV). Added sodium acetate accelerates the reaction probably because of the formation of reactive ionic species of the reactants.Ion-migration experi- ments show that ionic species of Co(1I) and Pb(1V) are present even in pure acetic acid. A survey of oxidation-reduction reactions between metal acetates in anhydrous acetic acid has shown that several occur at measurable rates.1 The kinetically simplest of these, namely, the oxidation of cerous acetate by plumbic acetate, has been reported in detail? In this reaction, ionic species were shown to be present even in the pure solvent and may take part in the rate-determining steps : in the presence of sodium acetate the reacting species are probably Ce(0Ac); and Pb(0Ac)j or Pb(OAc):-. Furthermore, it seems likely that Pb(II1) takes part in the reaction as an intermediate. The purpose of this paper is to present the more complicated results of the Co(II)+Pb(IV) reaction in acetic acid.EXPERIMENTAL MATERIALS ACETIC ACID.-A.R. acetic acid was purified by refluxing with A.R. finely divided chromium irioxide along with a calculated amount of A.R. acetic anhydride to remove water.3 The amount of anhydride required was estimated from freezing-point measure- ments.4 After distillation the excess water or acetic anhydride was determined both from the freezing-point and by a spectrophotometric method.5 The acetic acid usually pro- duced had a melting point of 16.6"C and was estimated to be 99.98 % pure; the remaining 0.02 % was probably acetic anhydride. PLUMSIC ACETATE.-This was prepared by the method of Dimroth and Schweizer,6 then recrystallized from anhydrous acetic acid, pumped dry and stored under a vacuum.The purity was found to be 100 % by titration with hydroquinone using quinalizarh as indicator.7 COBALTOUS ACETATE.-SohtiOnS of known concentration were prepared by refluxing 99-95 % pure cobalt sponge (Johnson Matthey) with purified anhydrous acetic acid. COBALTIC AcETAn.-The method adopted was basically that described by Sharp and White.8 Cobaltous acetate in acetic acid was oxidized electrolytically to cobaltic acetate giving a maximum conversion of about 50 %. The resulting solution was diluted to about 2 x 10-3 M and passed four times through a column 30 cm long and 1.5 cm &am. filled with Amberlite resin IR--120H. This procedure enabled the ratio [Co(III)l/[Co(II)] to be increased to 4911. The concentration of Coon) in the final solution remained con- stant to & 1 % for 24 h.Other reagents were of A.R. quality but before use they were dissolved in anhydrous acetic acid and then dried under a vacuum for 10 h. The procedure was repeated at least once. * present address : Department of Chemistry, College of Further Education, Widnes. 60BENSON, PROLL, SUTCLIFFE A N D WALKLEY 61 ION-MIGRATION EXPERIMENTS The migration cell consisted of a W-shaped vessel having three compartments isolated from one another by two sintered-glass discs. The solution under investigation was placed in the central compartment and the solvent was placed in the outer compartments. A potential of 500 V d.c. was applied to platinum foil electrodes situated in the two outer limbs of the cell.Current was passed for 20 h then the polarity was reversed and the current passed for a further 20 h. SPECTROPHOTOMETRY All measurements were made by means of a Unicam SP 500 spectrophotometer fitted with a thermostatted cell compartment which enabled solutions to be maintained to within &0-02"C. KINETIC MEASUREMENTS Since both plumbous and plumbic acetates dissolved in acetic acid do not absorb appreciably in the visible region 9 and the absorption of cobaltous acetate is a good deal less intense than that of cobaltic acetate (see fig. 1) in this region, the wavelength of 400 mp is a convenient one for measuring the rate of appearance of Co(II1). Final optical density readings DK,, corresponding to complete reaction, were obtained after leaving reaction mixtures to stand overnight.wavelength FIG. 1.-The absorption spectra of cobaltous (A) and cobaltic (B) acetates in anhydrous acetic acid at 25OC. RESULTS REACTANT AND PRODUCT SPECIES IN ACETIC ACID LEAD Successive complexes up to Pb(0Ac); formed by plumbous perchlorate with acetate ions in water have been detected and their formation constants determined.16 The ion-migration experiments showed that both negatively and positively charged62 THE CO(I1) f P b(1V) REACTION species of Pb(l1) are present in pure acetic acid : the likely species are PbOAc+, Pb(0Ac)z and Pb(OAc),. The addition of 10 % v/v water increased the amount of migration to the cathode thus indicating increased dissociation in favour of PbOAc+ with a possibility of Pb2+ being formed, When the solvent used was anhydrous acetic containing 1 M sodium acetate, the greatest transference was to the cathode which means that dissociation is then in favour of Pb(0Ac); and possibly higher complexes. Ion-migration results for plumbic acetate under the above conditions and kinetic studies on its decomposition9 and on its reaction with tert.-butyl hydro- peroxide 10 led to the conclusion that dissociation is very small and that the most reactive species is Pb(OAc)t-.COBALT Cobaltic acetate appears to dissociate fairly readily in acetic acid, there being considerable migration in the approximate ratio 3/1 for the anode : cathode compartments. The addition of 10 % v/v water or 1 M sodium acetate reduced migration to the cathode almost to zero while that to the anode remained sub- stantial.It seems that cobaltic acetate occurs in acetic acid as Co(OAc)$, Co(0Ac) and Co(0Ac)Z with the concentration of the latter being increased by the addition of water or sodium acetate. Sharp 11 has estimated the dissociation constant of cobaltic acetate in acetic acid to be 4 . 4 ~ 10-8 at 25°C. Cobaltous acetate behaves like plumbous acetate when subjected to electrolysis, there being four likely species, namely, CoOAc+, Co(OAc),, Co(OAc), and Co(OAc)i-. The first is favoured by the addition of 10 % v/v water and the last two by 1 M sodium acetate. Cobaltous acetate is different from the acetates of Co(III), Pb(1V) and Pb(I1) considered above, in that its absorption spectrum is affected markedly by the medium changes described thus permitting a detailed spectrophotometric investigation to be made, as described elsewhere.12 The effect of water on the spectrum can be interpreted as a dielectric effect on the equilibrium, Co( OAc), + CoOAc' + OAc- .The addition of sodium acetate promotes the formation of Co(0Ac);- via the equilibria, Ki KZ Co( OAc), + OAc- + Co( OAc), , Co( OAc); + OAc- + Co(OAc)i-. The type of relationship obtaining between the observed extinction coefficient and the sodium acetate concentration indicates that Co(0Ac)Z- predominates over Co(0Ac)J. K3 STOICHIOMETRY The stoichiometry was determined under various conditions by measuring the optical density at 400 mp of the cobaltic acetate produced from known approxim- ately equivalent amounts of plumbic and cobaltous acetates. The concentrations of the reactants were varied in the range 10-3 to 10-2 M.ANHYDROUS SOLVENT An average of six determinations gave a value of 2-01 f0-01 at 23°C for Co(III)/Pb(IV) which corresponds to the overall reaction, Pb(1V) + 2Co(IT)+Pb(TI) + 2Co(liII).63 BENSON, PROLL, su*rcLIi;pE AND WALKLEY 10 % v/v ACETIC ANHYDRIDE Co(III)/Pb(IV) = 2-02 0.02 at 23°C. Co(III)/Pb(IV) = 2.01 3-0.02 at 23°C. Co(IIl)/Pb(lV) = 2.02 & 0.02 at 23°C. Co(III)/Pb(lV) = 1-96 & 0-02 at 23°C. Co(III)/Pb(IV) = 1.39 & 0.01 and 1.06 & 043 at 23°C respectively. 10 % V/V BENZENE 1 M SODIUM ACETATE 0.25 M PLUMBOUS ACETATE 10 % V/V ETHANOL OR 10 % V/V METHANOL WATER The addition of water reduced the stoichiometry as shown in fig. 2 ; temperature Corrections were made for the dependence of the has little effect on the values.molar extinction coefficient of Co(II1) on the water concentration. 2.0 40 w 2 0 1 FIG. 2.-The effect of water on the stoichiometry of the Co(II)+Pb(IV) reaction at temperatures of 23°C (denoted by circles) and 37°C (denoted by crosses). The only additives which affected the stoichiometry are methanol, ethanol and water: the other additives listed clearly do not prevent the reaction from going to completion. However, methanol, ethanol and water do not bring about the decomposition of plumbic acetate 10 or of cobaltic acetate 8 under the conditions used here, therefore these additives must be reacting with a transient intermediate and thus cause the final concentration of Co(II1) to be decreased.64 THE Co(II)+ Pb(1V) KEACTION KINETICS THE REACTION IN ANHYDROUS ACETIC ACID Rate data were obtained using concentrations of cobaltous acetate in the range 2-3 x 10-3 M to 2.5 x 10-2 M and concentrations of plumbic acetate in the range 2 .0 ~ 10-4 M to 4.0~ 10-4 M. From fig. 3 it can be seen that the reaction time (min) FIG. 3.-The first-order dependence of the rate on the plumbic acetate concentration at 20.90"C. Cobaltous acetate concentrations : A = 0 . 3 1 2 ~ 10-2 My B = 0.625 x 10-2 My C = 1 . 2 5 ~ 10-2 My D = 1 . 8 7 ~ 10-2 M and E = 2 . 5 0 ~ 10-2 M. is accurately first order with respect to plumbic acetate. The slopes of the lines in fig. 3 were multiplied by 2.303/60 and designated kobs.. From plots of log kobs. against log [CoOI)] the cobaltous dependence was established to be to the power 1.50 f0.05.Fig. 4 illustrates this dependence at four temperatures ; the slopes of the lines give the specific rate constant k. The reaction was also studied at 2540°C with plumbic acetate in excess having concentrations in the range 4 . 3 ~ 10-3 M to 1 . 7 ~ 10-2 M and cobaltous acetate concentrations of about 1 . 5 ~ 10-3 M. The rate law and rate constants obtained were in good agreement with those given already for excess cobaltous acetate. From the Arrhenius relationship the apparent activation energy and entropy of activation at 25°C were calculated to be 17.5 f l . 0 kcal mole-1 (table 1) and - 8 f4 cal mole-1 deg.-l respectively. Reaction products were added to the system in order to detect reversible steps in the reaction mechanism. Concentrations up to 1 .5 ~ 10-3 M cobaltic acetate did not affect the reaction rate but, however, the addition of plumbous acetate in the range 0.05 M to 0.5 M caused a marked retardation. It has already been remarked in the section on stoichiometry that the reaction virtually proceeds to completion when excess plumbous acetate is present. Furthermore, no reaction can be detected when cobaltic acetate is mixed with excess plumbous acetate. The addition of Pb@) also increases the dependence on cobaltous acetate from the power 1.5 f0.05 to 2-3 f0-1 as shown in fig. 5. There is a linear dependence of the reciprocal of kobs. on the plumbous acetate concentration (for high concentra- tions) as may be seen from table 2. Similar measurements were made at other temperatures and from them (see table 1) the energy and entropy of activationBENSON, PROLL, SUTCLIFFE AND WALKLEY 65 at 25°C were found to be l O f l kcal mole-1 and -35 f 4 cal mole-1 deg.-l respec- tively.The first-order dependence on Pb(IV) was marred at the higher [Co(II)] * -5 FIG. 4.-The dependence of kobs. on [Co(II)]1*5 at temperatures : A = 36*52"C, B= 30*92"C, C = 2540°C and D = 20.90"C. [Co(II)12-3 FIG. 5.-The effect of plumbous acetate on the reaction between cobaltous and plumbic acetates in anhydrous acetic acid at 25.00"C. Plumbous acetate concentrations : A = 0.050 M, B = 0.125 M, C = 0-250 M, D = 0-330 M and E = 0.500 M. temperature, there being a departure from linearity of the first-order plots after 70 % reaction. CTHE Co(I1) + Pb(1V) REACTION TABLE 1 .-TEMPERATURE DATA FOR VARIOUS CONDITIONS apparent activa- apparent entropy of kcal mole-1 cal mole-1 deg.-1 additive temp., "C k, M-1.5 sec-1 tion energy. activation at 25"C, none 20.90 1 *20 17-5 f 1.0 -8f4 25.00 1 *70 30.92 3.00 36.52 5-38 NaOAc 25.00 2.37 30.74 4.20 35.23 6.6 1 5.15 x 10-2 M 21.02 1.81 17.0f 1.0 -954 3.70 M H2O 20.42 0.44 17-0 f 1.0 -11&4 25-00 0.68 3049 1.1 3 36.08 1.90 k[Pb(II)] M-~*~;scc-~ Pb(0Ac)z 20.90 7.14 lO&l 25.00 9.09 30-92 12.5 -35f4 TABLE 2.-THE LINEAR DEPENDENCE OF kit)s.ON [Pb(II)] AT 2540°C PW>l M 0.500 0.333 0.250 0-125 0.050 0.500 0.333 0.250 0.125 0.050 0.500 0.333 0250 0.1 25 0.050 0.455 0341 0.227 0.1 13 0.500 0.333 0.250 0.125 0.050 0.500 0.250 0.125 0.050 -1 kobs, 479 384 248 174 164 787 582 483 290 238 1450 1090 935 543 3 84 1470 1150 863 581 2180 1590 1300 725 62 1 4000 2220 1 200 1000 A k k .A[Pb(II)] sec M-* 570 920 820 700 - - 1210 1230 1320 1220 - 2150 2060 2420 2370 - 2800 2660 2600 I 3530 3 520 3880 3470 - 7100 7500 6700BENSON, PROLL, SUTCLIFPE AND WALKLEY 67 THE EFFECT OF SODIUM PERCHLORATE In a previous investigation,z sodium perchlorate has been shown to have a retarding influence on the Cc(III)+Pb(IV) reaction in acetic acid. It was pointed out that a combined primary and secondary salt effect, as would be expected, cannot in principle give rise to an observed ncgative total salt effect. A similar retarding effect was encountered when sodium perchlorate was added to the Co(II)+Pb(IV) reaction up to concentrations of 0.125 M at 2540°C (see table 3).TABLE 3.-THE EFFECT OF SODIUM PERCHLORATE AT 25.00"C [Co(II)] = 1.98 x 10-2 M [NaC1041 M [NaClO.Jt Ma kobs. sec-l log /Cobs. A log kobs./d[NaC104]f 0.125 0.59 1 2-87 x 10-3 3.458 - 0.0938 0-553 3.10 8,491 -0.87 0.0625 0.500 3.22 3.508 - 0.56 0.03 13 0.42 1 3.61 8.558 -0.59 0.023 5 0.392 3.84 3.584 - 0.68 0.01 56 0.354 4.1 5 3-618 - 0.68 0~0000 0000 4.53 3.656 - 0.34 - - The linear dependencc of log kobs. on [NaCIO& (a function of ionic strcngth) gives a value of -0-6 for the apparent charge product ZAZB of the reactants. 1.0 [NaOAc] acetate concentrations : A = 9-80 X 10-3 M and B = 6.55 x 10-3 M. FIG. &-The variation of kobs. with sodium acetate concentration at 2500'C. Cobaltous THE EFFECT OF SODIUM ACETATE The fact that sodium acetate accelerates the reaction, in contrast jvith sodium Fig.6 perchlorate, is indicative of a spccik cffect rather than an incrt salt cflect.68 THE Co(II)+ Pb(IV) REACTION shows the variation of /Cobs. with sodium acetate concentrations up to 0.25 M at two concentrations of cobaltous acetate. The shape of the line resembles that obtained for the reaction between cerous and plumbic acetates 2 under similar conditions. The order with respect to cobaltous acetate was found to be unchanged at 1.5 when determined at several temperatures in the presence of 5.15 x 10-2 M sodium acetate (see fig. 7). From these data was obtained an apparent activation I I 1 1 [CO(II)]'*5 FIG. 7.-The 1 +order dependence on the cobaltous acetate concentration in the presence C = 2540°C and D = 21.02"C.of 5.15 x 10-2 M sodium acetate. Temperatures : A = 35*23"C, B = 30*74"C, energy of 17.0fl.0 kcal mole-1 and an entropy of activation of -9 rt4 cal mole-1 deg.-1 at 25°C (see table 1). Sodium perchlorate caused acceleration when added to reaction mixtures con- taining 4 . 1 2 ~ 10-2 M sodium acetate. THE EFFECT OF ACETIC ANHYDRIDE Normally the solvent used in this investigation contained about 0.02 % V/A acetic anhydride therefore its effect was tested by adding 1 % v/v to reaction mixtures when it was found not to affect the observed rate constant. THE EFFECT OF BENZENE The addition of benzene had an accelerating influence on the reaction up to the limit of 25 % v/v which was attempted. The experimental data could be interpreted in terms of a dielectric effect, there being a linear dependence of log kobs.on D-1 (table 4). In the calculations, a value of 2-28 for the dielectric constant D TABLE 'I.-THE EFFECT OF BENZENE AT 2540°C [Co(II)] M benzene, % v/v D-1 1 . 9 6 ~ 10-2 25.00 0.192 21.83 0.1 88 18.75 0.184 15.63 0.180 12.50 0.176 6.25 0.1 69 0.00 0.162BENSON, PROLL, SUTCLIFFE AND WALKLEY TABLE 4.--continued 69 1-21 x 10-2 25.00 0. I92 5.60 x 10-3 3-748 - 18.75 0.1 84 4-40 3.644 12.2 12.50 0.176 3.48 3.542 12.6 6.25 0.169 2.96 3.471 11.4 0.00 0.162 2.60 3.415 11.0 of benzene at 25°C was used.13 The dielectric effect gives a negative sign to the apparent value of ZAZB, in agreement with the findings from the neutral salt effect. THE EFFECT OF ETHANOL Ethanol was shown to form a reactive complex with plumbic acetate in the Ce(III)+ Pb(1V) and the Pb(IV)+ t-butyl hydroperoxide reactions 2, 10 hence a similar occurrence was expected for the reaction under discussion.Unfortunately, complicated kinetics were obtained which prevented any conclusion from being drawn about the Pb(IV)+ethanol complex. The overall effect of ethanol addition was to increase the rate of reaction. THE EFFECT OF WATER Since the stoichiometry of the reaction was affected by water the final optical density of Co(II1) was calculated and then used to compute the values of kobs.. TABLE 5.-THE 1'543RDER DEPENDENCE ON COBALTOUS ACETATE IN THE PRESENCE OF 2.78 M WATER AT 25.00"C kObs.9 SW-' [CO(II)]~-~, M k o b s . / [ c o ~ I ) ] l * ~ , M-1.5 s a - 1 6.87 x 10-3 7-42 x 10-3 0.93 4.53 4.8 1 0-94 2.61 2.76 0.94 1.80 1.76 1-02 0.96 0.97 0.99 TABLE THE DEPENDENCE OF kobs.ON THE WATER CONCENTRATION temp. = 25-00"C. [Co(II)] = 1 . 9 6 ~ 10-2 M k0bs.s SCC-l [HzOI. M log kobs. D-1 A log kobs.lAD-l 4.53 x 10-3 4.35 4.10 3.70 3.50 2.61 2.09 1 *42 0-96 0000 0-348 0.695 1.39 1.75 2.78 3.70 4.1 7 5-56 3.655 3.638 3.612 3.568 3.543 3.416 3.320 3.151 4-98 1 0.162 0.151 0.148 0.122 0.108 0.102 0.094 0.086 0.075 - 1.6 3.1 2.2 2.1 4.0 4.7 6.6 7.7 As may be seen from table 5, the addition of water to the system did not change the order of the reaction with respect to Co(I1). Table 6 illustrates the variation of kobs. with water concentration. From rate measurements at four temperatures and a ccqstant water concentra- tion of 3.70 M (table 1) the apparent energy and entropy of activation were found to be 17.0 f 1.0 kcal mole-1 and - 1 1 f4 cal mole-1 deg-1 respectively.70 THE Co(II)+ Pb(IV) REACTION D I § CUS SI ON THE REACTION IN PURE SOLVENT The important features of the reaction in pure solvent are (i) the non-integral order with respect to Co(l1) and (ii) the retardation caused by plumbous acetate.These are the main differences between the Co(II)+Pb(IV) reaction and the simplcr Ce(llI)+ Pb(1V) reaction.2 The following reaction scheme provides an explanation for most of the experimental observations : K Co(I1) + Co(I1) + [CO(II)],, Pb(IV)+[Co(II)],~'-,Pb(Il)+2Co(III), Pb(1V) + [Co(II)],~~Pb(lII) + Co(lI1) + Co(II), Pb(1V) + [Co(I1)I2~+Pb(II) + Co(1V) + Co(II), k4 Pb(1V) + Co(I1) + Pb(I1) + Co(IV), k ; Pb(1V) + Co(IIp+Pb(III) + Co(III), Co(1V) + Co(IIp+2Co(III), Pb(II1) + Co(IIF%Pb(II) + Co(II1).By assuming that the concentrations of Pb(II1) and Co(IV) are stationary and that the cobaltous acetate is mainly in the monomeric form then the following rate law is obtained : kobs. = (k, + k2)K[Co(II)l2 + k5[Co(II)] + 2k6[Co(II)] The tetravalent state of cobalt has had to be introduced in order to account for the retardation brought about on the addition of excess plumbous acetate. The alternative reaction Pb(1V) + Pb(11)+2Pb(III) could not be included because the exchange of radio lead between plumbous and plumbic acetates in acetic acid has not been detected.14 The postulation of a reactive dimer of cobaltous acetate is necessary to the rate law and there may well be higher polymers present in the solutions.We have obtained some evidence for the existence of polymeric species in acetic acid solutions of cobaltous acetate.12 Dimers are well known for solutions of copper alkanoates in solvents of low di- electric constant .15 From the measurement of the lowest concentration of plumbous acetate which gave a linear dependence of k&s. with respect to [Pb(II)]-l a value of about 2 x 10-3 at 25°C was obtained for k;/k6. THE EFFECT OF SODIUM ACETATE An important aspect of the effect of sodium acetate is that the order with respect to cobaltous acetate remains at 1.5 but it should be pointed out that the rate increase is not very large. One may draw the tentative conclusion that bothBENSON, PROLL, SUTCLIFFE AND WALKLEY 71 the monomeric and dimeric forms are affected similarly by the addition of sodium acetate.The following empirical relationship represents the influence of the salt on kobs. : kobs.= A+B[N~OAC]~+C[N~OAC], where A , B and C are constants. The expression is identical with that found for the reaction between cerous and plumbic acetates.2 If, for simplicity, only reaction path 5 is considered then the following detailed reaction mechanism will yield an equation of the same type as the empirical one: Pb(OAc),+Co(OAc), 5 Pb(OAc),+Co(OAc);% Pb(OAc),+Co(OAc)',-~ Pb(0Ac); + CoOAc' 5 Pb( OAc); + Co(OAc), 5 Pb( OAc), + Co( OAc), % Pb( 0Ac):- + CoOAcs k Pb( 0Ac):- + Co( OAc), 5 In setting up this scheme use has been made of the species of Pb(1V) and Co(I1) indicated by the ion-migration and spectrophotometric experiments.Using the cobaltous acetate equilibria mentioned earlier and the equilibria K4 Pb( OAc), + OAc- + Pb( 0Ac)J , Pb( OAc), + OAc- + Pb( OAc);-, Ks KA NaOAc + Na' + OAc- , then k5 = k, + k,K, K4 + (I<&, + k,K, + k,K1K,K5)K~[NaOAc]* On account of the very small dissociation occurring in acetic acid it has been as- sumed in deriving this expression that the total concentrations of Co(1I) and Pb(1V) correspond to Co(0Ac)z and Pb(OAc)4. The above reaction scheme cannot be simplified since there is no means of distinguishing between the various reaction paths. It is clear, however, that ionic species play an important part in the reaction. +(k,K,K, + k&&+ k,,K,K,)K,[NaOAc]. THE EFFECT OF WATER From the experimental data the addition of water was seen to affect the reaction in the following ways : (i) there is probably a primary dielectric effect on the reacting species; (ii) there is a secondary dielectric effect on the ionic equilibria involving the (iii) there is direct reaction of water with a reactive intermediate.reactants and on the Co(I1) dimer equilibrium ;72 THE Co(I1) + Pb(IV) REACTIONS If it is assumed that the following rapid reaction occurs, Co(1V) + H20+ Co(I1) + 2H+ + 3 0 2 , thereby eliminating step 6 in the main reaction scheme then the stoichiometry of the reaction should be 1.2 : from fig. 2 it may be seen that the limiting value is about 1.6. The rate law becomes hence the order with respect to Co(I1) will remain at approximately 1.5 and there will be a reduction in the rate of the reaction upon the addition of excess water. A further reduction in rate occurs due to the increase of dielectric constant of the solvent (see table 6). The interpretation in terms of a dielectric effect is supported by the negligible change in the apparent activation energy (table 1) which results from the addition of water. Both the benzene and water dielectric effects are indicative of the participation of ionic species in the reaction even in the pure solvent. Alcohols probably have an effect similar to that of water but the lower values obtained for the stoichiometric ratio [Co(III)]/[Pb(IV)] may be due to their reaction with Pb(II1). 1 SutclXFe and Walkley, Nature, 1956, 178, 999. 2 Benson and Sutcliffe, Trans. Farday SOC., 1960, 56, 246. 3 Orton and Bradfield, J. Chem. SOC., 1927, 983. 4 de Visser, Rec. trav. chim., 1893, 12, 101. 5 Bruckenstein, Anal. Chem., 1956, 28, 1921. 6 Dimroth and Schweizer, Ber., 1923, 56, 1375. 7 Tomidk and Valcha, Czech. Chem. Comm., 1951-2, 16-17, 133. 8 Sharp and White, J. Chem. SOC., 1952, 110. 9 Benson, Sutcliffe and Walkley, J. Amer. Chem. SOC., 1959, 81,4488. 10 Benson and Sutcliffe, Trans. Faraday SOC., 1959,55, 2107. 11 Sharp, J. Chem. SOC., 1957, 2030. 12 Proll, Sutcliffe and Walkley, to be published. 13 Smith and Rogers, J. Amer. Chem. SOC., 1930, 52, 1824. 14 Evans, Huston and Norris, J. Amer. Chern. SOC., 1952,74,4985. 15 Martin and Whitley, J. Chem. SOC., 1958, 1394. 16 Burns and Hume, J-TAmer. Chem. SOC., 1956, 78, 3958.