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).

Blank test:

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 V_blank be the volume (mL) of KMNO₄ solution used for this blank test.

Results:

The permanganate index, which is the oxygen (in mg.L^-1) released by the potassium permanganate, is:

PI = 100 (V₁ – V₀ – V_blank) / V

where
V₀: volume (mL) of KMNO₄ solution consumed during the titration of the Mohr salt solution.
V_blank: 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).