Effect of ionic strength on stability constants. A study of the electronic absorption spectra of the mercuric halides HgX+, HgX2, HgX –3 and HgX 2–4 in water

Abstract

Thermodynamic stability constants log K^T₃ and log K^T₄, at 20 °C, are reported for HgX₂+ X^–⇌ HgX^–₃ and HgX^–₃+ X^–⇌ HgX^2–₄, respectively, in water, where X = I, Br and Cl. The electronic absorption spectra of these reactions are very sensitive to ionic strength (I), and give, by a new approach, log K^T₃= 3.79 ± 0.01, 2.23 ± 0.02 and 0.70 ± 0.03 and log K^T₄= 2.03 ± 0.02, 1.40 ± 0.03 and 0.50 ± 0.05, for X = I, Br and Cl, respectively. Values determined at constant ionic strength, with added NaClO₄, compare well with literature values determined by other techniques. To compute stability constants from digitised absorption spectra does not rquire constant ionic strength conditions, and thus the relations between K and I have been studied and they are discussed in terms of activity coefficients. The results are compared with the extended Debye–Hückel equation, employing appropriate distance of closest approach a parameters. The effect of added perchlorate on the reaction HgX₂⇌ HgX⁺+ X^– is reported, and the activity coefficients of the neutral species HgX₂ and HgXY are found to be independent of concentration and probably essentially unity. The reaction between HgX₂ and added X^– always involves a concentration range in which HgX₂, HgX^–₃ and HgX^2–₄ are in equilibrium. Plots of log K against I^½ indicate when a two-species equilibrium becomes a three-species equilibrium, and the ionic strength so identified tallies with that noted in appropriate sets of spectra. Since the K^T values agree well with existing results this spectroscopic technique has great potential for determining thermodynamic stability constants in non-aqueous solvents, especially when electrochemical methods are not available.