Determination of cyanide levels in Hygas wastewater

The CN/sup -//S/sup =/ reaction in Hygas raw quench water is rapid, causing significant sample degradation. If all of the S/sup =/ is not precipitated from the solution immediately, true CN/sup -/ levels are lost. Time becomes a critical factor in the handling of samples. As fast as practical, pH should be raised and the S/sup =/ precipitated to ensure a successful preservation. The S/sup =/ precipitate may remain in the sample if immediate filtration is not practical. Excesses of the precipitating solution may be used to ensure complete removal of S/sup =/ without requiring a HAc/PbAc paper check for free S/sup =/ on each sample. Because of the severe buffering of the Hygas water samples by dissolved CO/sub 2/ and NH/sub 3/, adjusting the sample pH is difficult. It is recommended that solid NaOH pellets be used in place of NaOH solution to avoid unnecessary sample dilution. Care should be taken to ensure that the sample pH is indeed 12 to prevent losses of HCN by volatilization.