Question

NH₃ the most harmful form of ammonia to fish, is toxic to several species at cencentrations as low as 0.2 mg/L. If water discharging into a small river from an agricultural processing system has a total ammonia concentration of 10 mg/L, what maximum pH can be allowed in order to keep the NH₃ concentration below 0.2 mg/L ? ( to be conservative assume a zone of no mixing in the receiving stream, which means no dilution of the discharge.)

Answer 1

concentration = 0.2 mg/L = 0.2 x 10^-3 g/L

molar mass of NH3 = 17 g/mol

concentration in mol/L (C) = 0.2 x 10^-3 / 17 = 1.176 x 10^-5 M

Kb value of NH3 = 1.8 x 10^-5

[OH-] = sqrt (Kb x C)

          = sqrt (1.8 x 10^-5 x 1.176 x 10^-5)

          = 1.46 x 10^-5 M

pOH = log [OH-] = -log (1.46 x 10^-5)

         = 4.84

pH = 14 - 4.84

      = 9.16

pH = 9.16