Relative Toxicity of Free Cyanide and Dissolved Sulfide Forms to the Fathead Minnow (Pimephales promelas)

Abstract

The relationship between pH and acute toxicity of free cyanide (i.e., HCN + CN-) and dissolved sulfide (i.e., H2S + HS- + S2-) to the fathead minnow (P. promelas) at 20.degree. C was determined for pH values ranging about 6.8-9.3 and 6.5-8.7, respectively. The 96 h LC50 of free cyanide and molecular HCN were little different and fairly constant within the pH range 6.8-8.3. Beyond this, to pH 9.3 the values diverged markedly, with the free cyanide LC50 increasing and the HCN LC50 decreasing. Except for some increase with rise of pH from about 6.5-7.1, the 96 h LC50 concentrations of molecular H2S decreased linearly and by approximately 4-fold as the pH increased from 7.1-8.7. The 96 h LC50 concentrations of dissolved sulfide in corresponding equally toxic solutions increased logarithmically as pH increased from 6.5-8.7. Acute toxicities to fathead minnows of free cyanide and dissolved sulfide do not depend entirely on the concentration of ambient molecular HCN or H2S. Change in toxicity of the molecular forms with pH was evaluated by 2 gill-permeability theories. The likelihood of certain chemical changes occurring at the gill surface or the possible penetration of the gill by both molecular and anionic forms present in the test solutions at different pH was evaluated. Increased apparent toxicity of molecular HCN or H2S with elevated test pH is believed to result from the CN- and HS- anions penetrating the gill epithelium, although less readily than do the molecular forms, and enhancing the toxicities of these solutions as the pH increases.