Cyanide in Water
Cyanide is expressed in terms of cyanite ion which is present in water in the form of hydrogen cyanide (HCN).
The effect of pH on cyanide is of great importance. The lower the pH, the greater the cyanide toxicity.
HCN is largely undissociated at pH values of 8 and less. The toxicities vary markedly with pH. At a given concentration, that is innocuous at pH 8 may become detrimental if the pH is lowered to 6 or less.
Hydrocyanic acid, HCN, dissociates in water according to the chemical equation:
HCN (aq) ⇆ H+ (aq) + CN- (aq)
Studies show a change in pH from 7.8 to 7.5 increases cyanide toxicity 10 times. The lower the pH, the greater the proportion of undissociated HCN.
When toxicities are expressed in terms of cyanide ion, it must be realized that most of the cyanide in natural water is in the form of HCN. It is apparent that HCN rather than the cyanide ion is the major toxic component.
The maximum safe total ingestion of cyanide by humans has been estimated at something less than 18 mg per day. Part of which will come from normal environment and industrial exposure.
The toxicity of cyanides towards fish is effected by pH, temperature, dissolved oxygen and concentration of minerals. As noted above, with hydrogen cyanide HCN appears to be the major toxic form rather than CN
The toxicity of cyanide is also increased at elevated temperatures. A rise of 10° produces a 2 to 3 fold increase in the rate of lethal action. Low tensions of dissolved oxygen also increase the toxic action of cyanides.
Certain metals such as nickel may complex with cyanide to reduce lethality, especially at higher pH values, but zinc and cadmium cyanide complexes are exceedingly toxic.
When fish are poisoned by cyanide, the gills become considerably brighter in color than those of normal fish. This is due to the inhibition of cyanide of the oxidase responsible for transferring oxygen from blood to the tissues.
Toward lower organisms, cyanide does not appear to be as toxic as towards fish.