Assume a wastewater contains a high soluble Cu II concentration. Determine the pH where the minimum solubility of Cu(OH)2 occurs, assuming a constant total nitrogen (NT) concentration (NH4+ and NH3) of 100 mg/l as N. At this pH, what is the total soluble copper concentration (mg/L)?
Neglect ionic strength corrections and assume the solution temperature is 25°C. The appropriate cumulative equilibrium formation constant expressions for metal complexes with inorganic ligands (OH-1 and NH3) which affect the solubility of the Cu II system are readily available in Lange's Handbook of Chemistry, along with the remaining equilibrium relationships: solubility product (Ksp) for Cu(OH)2, ionization constant (Ka) for the NH4+/ NH3 couple.
Utilize the first four cumulative formation constants (K's) for each ligand (OH-1 and NH3) in your calculations. Calculate the Cu concentrations from pH 6 to 12 with increments of 0.25 pH units with and without the NH3 and plot both results on the same graph.
Recall that for a monoprotic species the distribution coefficient is given by α1 = {1/(1+([H+]/Ka))}, where [NH3] = α1{NT}.