Determination of the permanganate index by colorimetric titration
Determination of a sample:
Sample the water, NON-filtered (100 mL of sampled water and add 100 mL of ultra-pure water).
Alkalinize the test sample with the monosodium carbonate solution to reach an alkalinity of about 10 méq.L-1.
This result is generally obtained by adding 2 mL of monosodium carbonate solution to the 200 mL of the test sample.
Bring to a boil on the hot plate.
Add 20 mL of KMNO4 solution at 0.0025 mol/L.
Return to the boil, and keep boiling for 10 minutes.
Cool for 5 minutes under cold running water.
Acidify with 5 mL of the sulfuric acid solution at ½.
Add 20 mL of the Mohr salt solution: there is discoloration of the solution.
Titrate with KMNO4 at 0.0025 mol/L until the pink color appears (persistent for 1 min).
Note the volume of KMNO4 solution consumed (V1) (in mL).
Carry out the blank test with 100 mL of ultra-pure water, and proceed as for the sample (add 100 mL of ultra-pure water to keep the same volume). Let Vblank be the volume (mL) of KMNO4 solution used for this blank test.
The permanganate index, which is the oxygen (in mg.L-1) released by the potassium permanganate, is:
PI = 100 (V1 – V0 – Vblank) / V
V0: volume (mL) of KMNO4 solution consumed during the titration of the Mohr salt solution.
Vblank: volume (mL) of KMNO4 solution used for the blank test.
V1: volume (mL) of KMNO4 solution consumed during the titration of the sample
V: volume (mL) of the test sample (generally 100 mL of the water sampled).