Cyanide measurement by silver nitrate titration: Comparison of rhodanine and potentiometric end-points

The measurement of cyanide using a silver nitrate titration with p-dimethylaminobenzylidene rhodanine indicator has been used for many years by the gold industry and has been adopted in many operations as the trusted free cyanide determination. However, this work shows that there are several interferences, particularly copper and thiosulfate, where the interference increase with increasing concentration of these species. In comparison, cyanide determination using a silver nitrate titration with potentiometric end-point has no interferences as long as the titration pH is adjusted above 12 to eliminate the interference of zinc. A potential issue with the identification of the potentiometric end-point at low copper and/or thiosulfate concentrations is highlighted.With the gold industry now treating more complex gold ores, this work clearly demonstrates that the rhodanine indicator determination for free cyanide in cyanidation leach solutions containing copper and/or thiosulfate will over-estimate the free cyanide concentration and cannot be compared directly against an online cyanide analyser using a potentiometric determination. As no simple direct correlation exists between the two methods for these complex cyanide solutions, it is recommended that gold operations treating complex gold ores and using online cyanide analysers adopt the potentiometric end-point method in the laboratory which allows for direct comparison.

Research Highlights
► Comparison of cyanide measurement using a silver nitrate titration with p-dimethylaminobenzylidene rhodanine indicator or potentiometric end-point.
► Rhodanine end-point has significant interferences, particularly copper and thiosulfate.
► Potentiometric end-point has no interferences when pH is adjusted above 12.
► Rhodanine determination cannot be compared directly against a potentiometric online analyser.