Determination of Cl- ions by colorimetric titration: Charpentier-Volhardt’s method
Dosing principle
The determination of Cl- concentration is based on the addition of excess AgNO3 to the water to be analyzed in order to precipitate all the Cl- ions as AgCl (1). A characteristic white solid is formed. A few drops of concentrated nitric acid and iron (III) nitrate Fe(NO3)3 are then added, which acts as a color indicator. The excess Ag+ ions are then titrated with a solution of sodium thiocyanate KSCN. A new white solid of AgSCN is formed (2). The equivalence point is determined when the solution takes on a bright red color characteristic of the Fe(SCN)2+ complex (3).
(1) Ag+ + Cl- = AgCl
(2) Ag+ + SCN- = AgSCN
(3) Fe3+ + SCN- = Fe(SCN)2+
(2) Ag+ + SCN- = AgSCN
(3) Fe3+ + SCN- = Fe(SCN)2+
Data:
pKs(AgCl) = 9.8; pKs(AgSCN) = 12