Some Effects of Complex Formation in Electrolyte Solutions

Part I: The increased rate of hydrolysis of histidine methyl ester in the presence of copper (II) and nickel (II) ions has been investigated. Potentiometric studies have indicated that in the pH range in which ester hydrolysis proceeds, several ester-metal ion chelates may exist, depending on the relative concentrations of ester and metal ion. Formation constants for the chelates CuE2+ 2, CuE2+, CuE+, NiE2+ 2 and NiE 2+, have been measured, where E represents the ester. Kinetic studies have shown that in all cases hydrolysis proceeds via a simple bimolecular reaction between hydroxyl ion and ester, the latter being either free in solution or bound to metal ion. In the case of the 2:1 chelates the reaction involves two consecutive competitive reactions ME2+ 2 + OH- → MEA+ MEA+ OH- → MA2 , where A- represents the anion of histidine, the hydrolysis product. A comparison of the second order rate constants for the alkaline hydrolysis of the ester in the absence and presence of metal ions, has shown that metal ion catalysis is due to chelate formation which increases the reactivity of the ester towards hydroxyl ions. The overall catalytic effect may arise in three ways: 1. Introduction of positive charge into the ester molecule 2. A statistical effect. 3. An electron withdrawal effect. The introduction of positive charge into the vicinity of the ester molecule increases the chance of effective collisons. A statistical effect is introduced by the fact that more than one ester molecule may complex with the positively charged matal ion. In view of the consistent relative difference in the rates of hydrolysis of similarly charged copper and nickel chelates, an electron withdrawal, or inductive, effect which depends on the degree of interaction between metal ion and ester is also considered to contribute towards catalysis. Part II: The conductivity of lanthanum cobalticyanide in water and in 10% and 20% (W/W) dioxan water mixtures has been measured. Negative deviations from the conductivities predicted by the Onsager equation have been attributed to ion pair formation and dissociation constants have been evaluated. Calculation of "a" the distance of closest approach of ions has shown that the Bjerrum theory of ion association adequately describes the behaviour of this salt in the range of dielectric constants studied, which is in accord with previous results for lanthanum ferricyanide. The ferrieyanides of lanthanum, neodymium and gadolinium, have been prepared and conductivity measurements have been used to evaluate dissociation constants for these salts in water. The close similarity in the dissociation constants confirm the prediction that one would not expect to find large differences for a series of rare earth salts in the same solvent