Spectrophotometric determination of uncomplexed cyanide and thiocyanate in wastewater with p-phenylenediamine

Abstract

According to the Aldridge method (Aldridge, W.N. Analyst (London) 1944, 69, 262-265) cyanide and thiocyanate are converted to cyanogen bromide by the addition of bromine water to a neutral or slightly acidic sample: (1) CN/sup -/ + Br/sub 2/ ..-->.. CNBr + Br/sup -/; (2) CNS/sup -/ + 4Br/sub 2/ + 4H/sub 2/O ..-->.. CNBr + 6HBr + Br/sup -/ + H/sub 2/SO/sub 4/. After the excess bromine was removed with arsenious acid, a solution of benzidine (the coupling agent) in pyridine is added to give an intense orange-red color whose intensity is proportional to the sum of the cyanide and thiocyanate concentrations. Differentiation between the cyanide and the thiocyanate is made by making two determinations. One determination measures the total of CN/sup -/ plus SCN/sup -/ and the other measures the thiocyanate concentration alone, after the sample has been purged with air to remove the cyanides. The cyanide is determined by difference. Purging with air at room temperature and subsequent bromine treatment was adequate for removing small amounts of sulfide. However, for large amounts of sulfide it was necessary to acidify, purge, and remove the sulfide before the bromine treatment, in the procedure for determining thiocyanate alone. To determinemore » total CN/sup -/ plus SCN/sup -/, ammonium polysulfide was added to convert the uncomplexed cyanides to thiocyanate before acidification and purging. (NH/sub 4/)/sub 2/S/sub x/ + CN/sup -/ ..-->.. SCN/sup -/ + (NH/sub 4/)/sub 2/S/sub x-1/. Further modificationsx were made to reduce phenol and dihydroxylbenzene content by extraction with butyl acetate, and the coupling agent was changed to p-phenylenediamine.« less