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Key value for chemical safety assessment

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The abiotic dissociation of HCN depends on temperature and pH. Typical pH values of natural waters range from pH 6 to 8. The pKa value of the dissociation equilibrium of HCN  is 9.36 at 20°C. The HCN fraction will be 92% at pH 8.5 and > 99% at pH < 7 (Leduc et al,1982). In a model hard water, 98% of free cyanide was HCN at pH 7.7 (Murgatroyd et al, 1998).

Hydrolysis, to formic acid and ammonia, is normally a very slow reaction and it does not play a role under environmental conditions. The reaction is accelerated by strongly acidic or alkaline conditions.