Determination of calcium ions: complexometric/colorimetric titration
Dosage principle
The determination of the Ca2+ concentration is based on the addition of calcon (CAL) to the water to be analyzed. We work at pH 12 by adding a buffer solution. Mg2+ ions at this pH precipitate as Mg(OH)2 hydroxide, and the Ca-CAL complex is formed (1). The solution takes on a red-purple color characteristic of this complex.
The solution obtained is then titrated with ethylene diamine tetraacetic acid (EDTA). At first, Ca-EDTA complexes are formed with free Ca2+ ions (i.e. not complexed with CAL) (2). Then, the ions already complexed with CAL react with EDTA (3). This releases CAL in the solution and gives it a blue color, characteristic of this molecule at pH 12.
(1) Mg2+(aq) + 2HO-(aq) = Mg(OH)2(s) and Ca2+(aq) + CAL (aq) = Ca-CAL(aq)
(2) Ca2+(aq) + EDTA(aq) = Ca-EDTA(aq)
(3) Ca-CAL(aq) + EDTA = Ca-EDTA(aq) + CAL(aq)
(2) Ca2+(aq) + EDTA(aq) = Ca-EDTA(aq)
(3) Ca-CAL(aq) + EDTA = Ca-EDTA(aq) + CAL(aq)
Data:
pKS (Ca(OH)2) = 5.26; pKS (Mg(OH)2) = 9.2; logβ (Ca-CAL) = 5.3; logβ (Ca-EDTA) = 10.7.