A new generation of cyanide ion-selective membranes for flow injection application: Part III. A simple approach to the determination of toxic metal–cyanide complexes without preliminary separation

Abstract

A new flow injection approach to total weak acid-dissociable (WAD) metal–cyanide complexes is proposed, which eliminates the need of a separation step (such as gas diffusion or pervaporation) prior to the detection. The cornerstone of the new methodology is based on the highly selective flow-injection potentiometric detection (FIPD) system that makes use of thin-layer electroplated silver chalcogenide ion-selective membranes of non-trivial composition and surface morphology: Ag 2 + δ Se 1 − x Te x and Ag 2 + δ Se. An inherent feature of the FIP-detectors is their specific response to the sum of simple CN − + Zn(CN) 4 2− + Cd(CN) 4 2−. For total WAD cyanide determination, ligand exchange (LE) and a newly developed electrochemical pre-treatment procedure for release of the bound cyanide were used. The LE pre-treatment ensures complete recovery only when the sample does not contain Hg(CN) 4 2−. This limitation is overcome by implementing electrochemical pre-treatment which liberates completely the bound WAD cyanide through cathodic reduction of the complexed metal ions. A complete recovery of toxic WAD cyanide is achieved in the concentration range from 156 μg L −1 up to 13 mg L −1. A three-step protocol for individual and group WAD cyanide speciation is proposed for the first time. The speciation protocol comprises three successive measurements: (i) of non-treated, (ii) LE-exchange pre-treated; (iii) electrochemically pre-treated sample. In the presence of all WAD complexes this procedure provides complete recovery of the total bound cyanide along with its quantitative differentiation into the following groups: (1) Hg(CN) 4 2−; (2) CN − + Cd(CN) 4 2− + Zn(CN) 4 2−; (3) Cu(CN) 4 3− + Ni(CN) 4 2− + Ag(CN) 2 −. The presence of a 100-fold excess in total of the following ions: CO 3 2−, SCN −, NH 4 +, SO 4 2− and Cl − does not interferes. Thus the proposed approach offers a step ahead to meeting the ever increasing demand for cyanide-species-specific methods. The equipment simplicity makes the procedure a good candidate for implementing in portable devices for in-field cyanide monitoring.