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

The relationship between pH and acute toxicity of free cyanide (i.e. HCN + CN − ) and dissolved sulfide (i.e. H 2 S + HS − + S 2− ) to the fathead minnow (Pimephales promelas) at 20 °C was determined for pH values ranging from about 6.8 to 9.3 and 6.5 to 8.7, respectively. The 96-h median lethal concentrations (LC50s) 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 LC50s increasing and the HCN LC50s decreasing. Except for some increase with rise of pH from about 6.5 to 7.1, the 96-h LC50 concentrations of molecular H 2 S decreased linearly and by approximately fourfold as the pH increased from 7.1 to 8.7. The 96-h LC50 concentrations of dissolved sulfide in corresponding equally toxic solutions increased logarithmically as pH increased from 6.5 to 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 H 2 S. Change in toxicity of the molecular forms with pH was evaluated by two 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's was evaluated. Increased apparent toxicity of molecular HCN or H 2 S with elevated test pH is believed to result from the CN − and HS − anions penetrating the gill epithelium, though less readily than do the molecular forms, and enhancing the toxicities of these solutions as the pH increases. Key words: toxicology, hydrogen ion concentration, bioassays, lethal limits, gill-permeability theories