On-line gas-diffusion separation and fluorimetric detection for the determination of acid dissociable cyanide

A flow injection system with gas diffusion separation and spectrofluorimetric detection is described for the determination of acid dissociable cyanide in waters. Cyanide diffuses through a microporous PTFE membrane from an acidic donor stream into a sodium hydroxide acceptor stream. The cyanide transferred reacts with o-phthalaldehyde and glycine to form a highly fluorescent isoindole derivative. Complete recovery of cyanide was found for Zn(CN)42–, Cu(CN)32–, Cd(CN)42–, Hg(CN)42–, Hg(CN)2 and Ag(CN)2– complexes and low recovery from Ni(CN)42–. No recovery was obtained from the species that are considered as non-free cyanide producing, viz., Fe(CN)64–, Fe(CN)63– and Co(CN)63–. The sampling frequency was 10 h–1 and the detection limit was 0.5 µg l–1. The method tolerates a 40-fold excess of sulfide. The results of cyanide determination in water samples obtained with the proposed method are consistent with those obtained with APHA Method 4500 CN– I for weak acid dissociable cyanide.