Cyanide sulfur redox reactions

Sodium nitroprusside undergoes a redox reaction that releases cyanide (6,9). The cyanide that is produced is rapidly converted into thiocyanate in the liver by the enzyme thiosulfate sulfotransferase (rhodanase) and is excreted in the urine (6,9). The rate-limiting step in the conversion of cyanide to thiocyanate is the availability of sulfur donors, especially thiosulfate. Toxic symptoms of thiocyanate begin to appear at plasma thiocyanate concentrations of 50 to 100 mg/mL. The elimination half-life of thiocyanate is 2.7 to 7.0 days when renal function is normal but longer in patients with impaired renal function. 

As in all liquid-redox processes, part of the sulfur is converted to thiosulfate, although the rate of formation is appreciably lower in the essentially neutral Thylox solution than in more alkaline solutions used in other processes. Hydrogen cyanide, which is absorbed in the absorber, reacts readily with the sulfur formed in the thionizer to yield sodium thiocyanate. Because of these side reactions, the active thioarsenate has to be replenished continuously by addition of arsenic oxide and sodium carbonate. 

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