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

Abstract

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)₄^2–, Cu(CN)₃^2–, Cd(CN)₄^2–, Hg(CN)₄^2–, Hg(CN)₂ and Ag(CN)₂^– complexes and low recovery from Ni(CN)₄^2–. No recovery was obtained from the species that are considered as non-free cyanide producing, viz., Fe(CN)₆^4–, Fe(CN)₆^3– and Co(CN)₆^3–. 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.