WINKLER'S METHOD FOR THE DETERMINATION OF OXYGEN IN WATER

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.
Interfering substances are the organic matters, nitrites, iron (2+ and 3+), reducing and oxidizing agents. Accuracy of definition is equal 0,5-1 % for concentration of oxygen more than 1 mg/L. The limit of method detection is 0,05 mg/L.

 


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.
Before titration, you need to wait until the manganese hydroxide precipitate settles to the bottom. Thereafter, add 5 ml HCl (2:1), close the flask and mix the solution. Bubbles should not be! Put the solution into a conical flask and to titrate with the sodium thiosulfate solution (0,02 N).
When the solution will become slightly yellow coloring, to add 1 ml solution of starch (0.5%) and continue titration to the point of equivalence.
One millilitre thiosulfate solution (1N) allow to determine 8 mg/L of oxygen.

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.)
Mix the prepared solution and dilute it to 1 liter.