Polarographic Study of the Rate of the Dissociation Reaction of the Zinc(II)-Nitrilotriacetate Complex in Ammonia-Ammonium Nitrate Buffer Solutions

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The kinetic current due to the dissociation of zinc-nitrilotriacetate complexes has been investigated in buffers with pH values from 8.3 to 9.5 and containing a large excess of calcium ions with respect to the total concentration of nitrilotriacetate. The ionic strength was kept at 0.4. It was confirmed that the electrode reaction at the surface of the dropping mercury electrode proceeds according to the mechanism: ZnX－+2NH3\rightleftharpoonsZn(NH3)22++X3－ (slow) Zn(NH3)22++2e+Hg\rightleftharpoonsZn(Hg)+2NH3 (fast) The rate constants were calculated as 0.850(mol./l.)－2 sec－1 for the dissociation reaction of ZnX－+2NH3=Zn(NH3)22++X3－ and 2.42×105(mol./l.)－1 sec－1 for the association reaction at 25°C. in buffer solutions of pH values from 8.3 to 9.5 with an ionic strength of 0.4 and containg 0.1 M calcium nitrate. The heat of activation and the entropy of activation of the dissociation reaction were calculated as 31.1 kcal. and －25.4 e. u. respectively.
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