A photographic fixer solution contains 0.0500molL−1Ag+ ions complexed with 2.00molL−1 thiosulfate, S2O32−. The following equilibria are relevant at 25∘C:
(1) AgCl(s)↔Ag++Cl−; Ksp(AgCl)=1.8×10−10
(2) Ag++2S2O32−↔[Ag(S2O3)2]3−; β2=1.3×1013
A technician needs to remove dissolved silver by adding NaCl and adjusting the pH with HCl so that AgCl(s) begins to form despite the strong complexation.
Assume activity coefficients are unity and that the only silver-containing aqueous species are Ag+ and the complex ion shown above.
(a) Derive an expression for the free Ag+ concentration in terms of the total formal silver concentration, the thiosulfate concentration, and the formation constant β2.
(b) Using the concentrations given, calculate [Ag+] before any chloride is added.
(c) Determine the chloride-ion concentration required for AgCl(s) to begin precipitating under these conditions.
(d) If concentrated HCl (12.0molL−1) is used both to supply Cl− and to lower the pH, explain qualitatively how the pH change could alter the effectiveness of AgCl precipitation, considering possible protonation of thiosulfate.
(e) Propose a procedural modification to maximize silver removal while minimizing the amount of acid needed, and justify your recommendation based on equilibrium considerations.