1992-06-05_REVISION - M1988112 (2)

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C~anidr Chcmis[rr and Trenrmrm • Sulphide which rains access to the absorption solution may cause positive or negative interference with colourimetric procedures at very low concentrations. • Thiocyunate is a major imerference to the total cyanide distillation mekhod. usually resulting in high cyanide values: however, decomposition is not predictable and the mechanism i.c uncertain. • Bisulphate appears to be a primary negative interference with the potential For .come de;rec of cyanide destruction. Residual bisulphate is an interference. • Sulphur species interference may be minimized by the Following treatments: - sulphide precipitation from the sample - distillation of smallest possible sample - sulphide precipitation from the absorption solution - increased levels of chloramine-T in the colourimetric procedure 2.5 CALCULATION OF A MASS BALANCE It is possible to check the validity of a series of cyanide analyses by completing a mass balance. comparing the various cyanide and metals levels obtained From the analysis of the same solution. The mass calculations are prepared assuming that iron is bound to six cyanide molecules (i.e. Fe(CN)„), copper is bound to three cyanide molecules (i.e. Cu(CN),). nickel is bound to four cyanide molecules (i.e. Ni(CN},), and zinc cyanide is completely dissociated as free cyanide and the Tree metal. These are the four mt+in metals in cyunidation solutions, although other metals could be added to pie calculations (i.e. lead, silver or mercury) if present in elevated concentrations. Bayed upon the number of cyanide molecules bound w each mead, the weieht ratio of each metal ui cvunide is as follows: • Iron l}.~f~ me Fefms CN • Copper 0.82 mg Cu/m,~ CN • Nickel 0.5h mg Ni/m~_ CN As an example assume a total. wad and Iree cyanide ^nalysi.c ul the same solution produced concentrations of Z00 mg/I, 17i mg/I and 13i rog(I. respectively. It should .Smith rm[1 A9ndder .~?