Thermodynamic analysis of free cyanide determination by silver nitrate titration in copper bearing solutions

The present study is a thermodynamic approach to explain the titration of free cyanide by silver nitrate in cyanide solutions containing copper. When copper is present in cyanide solutions, the visual determination of the endpoint of the titration leads to an overestimation of the free cyanide concentration that can be up to 56% when the indicator is potassium iodide, or up to 121% for rhodanine. Four alternatives were evaluated to detect the endpoint of the titration: visual detection with potassium iodide, visual detection with rhodanine, potentiometric endpoint detection with silver electrode and derivative curve of the potentiometric titration. The last two methods showed the best results (the smallest overestimation, reaching no more than 8%). A comparison between the titration graphs and the equilibrium composition diagrams allows the understanding of the overestimation, and permits the qualitative and quantitative determination of all the copper cyanide species that are being titrated with silver nitrate.