Determination is based on the oxidation of Mn(OH)2 by water oxygen. The reaction product is Mn(OH)4. Environmental pH in the sample should be alkaline. Then add acid and KI to the sample. The formed iodine is titrated sodium thiosulfate.
PROCEDURE FOR THE DETERMINATION
Fill the glass-stoppered flask (100-200 mL) by the sample of water. The flask must be filled to the brim. (The flask must be necessarily calibrated before.) Add to the flask 1 ml of manganese sulfate or chloride, 1 ml of alkaline solution of potassium iodide. Close a vessel a stopper. The flask should not contain air bubbles after closing. Turn a flask some times for mixing. After mixing the solution is preserved for one day.
PREPARATION THE REAGENTS
1. Solution of salt of manganese (II)
Dissolve a manganese sulfate or chloride salt in water. Concentration must be equal to 0,5 M. The solution must be tested before using. Potassium iodide not must produce iodine in this solution, diluted in 200 time. (The solution should be acidic.) The yellow color of the solution demonstrates a insufficient purity of manganese salt.
2. Alkaline solution of potassium iodide
Dissolve 150 g KI in 100 mL of distilled water. (The solution not must produce iodine in acidic environment.) Dissolve 500 g NaOH in 500 mL of distilled water. (Water should be boiled to remove CO2.)