Search: Kaliumdichromat

Potassium dichromate oxidizes many organic and certain inorganic substances to various extents in an acidic solution. Since the level of oxidation depends upon the kinds of substances, the concentration of potassium dichromate, the pH of the solution, and the temperature and reaction time, the procedure described below must be followed precisely. The volume of potassium dichromate required in the analysis is determined potentiometrically. In an acidic solution, the dichromate ions are reduced to chromium(III) ions:

Cr₂O₇^2- + 6 e^- + 14 H₃O⁺ → 2 Cr³⁺ + 21 H₂O

Dichromate ions in excess of those required are determined through titration with an ammonium iron(II) sulfate solution:

Cr₂O₇^2- + 6 Fe²⁺ + 14 H₃O⁺ →2 Cr³⁺ + 6 Fe³⁺ + 21 H₂O 

Potassium dichromate is added to the acidified sample as an oxidizing agent along with silver sulfate as a catalyst; mercury sulfate is also added to prevent the formation of elemental chlorine from chlorides. After oxidation of the organic substances in the sample (the dichromate ion is reduced to the chromium(III) ion in an acidic solution), the chromate required to achieve this is determined through reverse titration of the excess potassium dichromate with iron(II) solution (adjusted) against ferroin as an indicator [1].

Cr₂O₇^2- + 6 e^- + 14 H₃O⁺ → 2 Cr³⁺ + 21 H₂O

Cr₂O₇^2- + 6 Fe²⁺ + 14 H₃O⁺ → 2 Cr³⁺ + 6 Fe³⁺ + 21 H₂O