A method is reported for determining the species formed in solution by ligand replacement reactions. Reactions in dilute aqueous solution were studied in which the Cl^- on HgCl_2(aq) was replaced by hydroxide ion, ethylenediamine, glycinate ion, and methylamine. The species formed by the replacement reactions were established through computer analysis of pH titration data. Various reaction sequences were proposed for each system and log K values were calculated for each sequence. The selection of the correct reaction sequence for each system was based on the premise that only when the correct sequence is proposed will consistent log K values be obtained as a function of both n (average number of ligands bound per metal ion) and total metal concentration. Omitting ionic charges the reactions were found to be of the general type HgCl_2 + Y = HgClY + Cl (1) HgClY + Y = HgY_2 + Cl (2) where Y is one of the four ligands used in the study. Log K values valid at 25° C and zero ionic strength were determined. The reactions were carried out at ionic strength values of about 10^-3, and activity coefficients were used to correct the calculated concentration quotients to thermodynamic equilibrium constants. Values of ΔH° were determined calorimetrically at 25° C for each of the replacement reactions using a thermometric titration procedure. Values of ΔS° for the reactions were calculated from the corresponding ΔG° and ΔH° values. The thermodynamic data obtained indicate that the ethylenediamine and glycinate ion act as bidentate ligands in the HgClY species (equation (1)).



College and Department

Physical and Mathematical Sciences; Chemistry and Biochemistry



Date Submitted


Document Type





Heat of solution



Included in

Chemistry Commons