Study of equilibria in cyanide systems by gas-diffusion measurement of hydrogen cyanide

Speciation of hydrogen cyanide in different metal-cyanide systems at the μg ml−1 level was established by the selective measurement of hydrogen cyanide (HCN) diffused through a microporous Teflon membrane under thermodynamic equilibrium conditions. The test solution was continuously aspirated through the donor side of the gas-diffusion unit until equilibrium was achieved with the stagnant buffer solution on the receptor side. The final assay of HCN was carried out spectrophotometrically (chloramine-T/barbiturate/isonicotinate) in the flow injection mode. The theoretical relation between the enrichment factor and the pH of the donor and receptor solutions was derived and experimentally verified. As confirmed by calculation the speciation of HCN can be established for simple soluble cyanides and cyanide complexes with log formation constants as high as 20. For more stable cyanide complexes the results are satisfactory; but for the highly stable hexacyanoferrate(III) this method is not feasible.