Overview
Hydrogen cyanide, HCN, is a monoprotic weak acid that donates one equivalent of H+ to form cyanide ion, CN⁻:
HCN(aq) ⇌ H⁺ + CN⁻ Ka = 10−9.21
Both forms of cyanide are very toxic, so human exposure should be minimized. In answering the following, assume an ionic strength of 0.5 M and a temperature of 25 ºC.
a) Calculate the cKa and pcKa for this reaction
b) A solution of industrial wastewater containing TOTCN = 450 μM is brought into the lab. The laboratory personnel measure the paH to be 7.91 using a pH electrode and meter. What is the HCN(aq) concentration?
c) Human exposure concerns arise when the HCN(aq) concentration is greater than 1.0 μM because it can volatilize and become HCN(g). Instruct the laboratory personnel on what paH they should adjust the solution to so as to keep the lab safe from HCN vapors.