Determination of Cl- ions by colorimetric titration: Mohr’s method
Dosing principle
The determination of Cl- concentration is based on the addition of potassium chromate (K2CrO4) to the water to be analyzed. The Cl- ions are then titrated with a silver nitrate solution AgNO3. A white solid of AgCl is formed (1) before equivalence is reached. At the equivalence point, the solution takes on an orange color characteristic of the Ag2CrO4 precipitate (2).
(1) Ag+(aq) + Cl-(aq) = AgCl(s)
(2) Ag+(aq) + CrO42-(aq) = Ag2CrO4(s)
(2) Ag+(aq) + CrO42-(aq) = Ag2CrO4(s)
Data:
pKs(AgCl) = 9.8; pKs(Ag2CrO4)= 12.4; s(AgCl) = 1.8x10-5 mol/L; s(Ag2CrO4) = 4.6x10-5 mol/L