Determine of Dissolved Oxygen in Water Sample by Titration

The Winkler method with Azide modification Oxygen present in the sample oxidizes the divalent manganous to its higher valency which precipitates as a brown hydrates oxide after the addition of NaOH and KI.

Upon acidification, manganese reverts to a divalent state and liberates Iodine from KI equivalent to D.O. content in the sample.

The liberated iodine is titrated against a standard (N/40) solution of Sodium thiosulphate using starch as an indicator and Palatability decreases affect the mucous membrane.     

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Procedure of Dissolved Oxygen

Collect the sample in a BOD bottle and add 2 ml MnSO4+ 2 ml Alkali iodide-azide+close stopper mix well + allow the ppt to settle.

Now add 2 ml concentrated H2SO4 + mix well till ppt dissolves.

Take 203 ml (Correspond to 200 ml) sample in a conical flask+titrate against Sodium thiosulphate (0.025 N) till pale yellow color add starch indicator + titrate till blue to colorless.


1 ml of 0.025N Na2S2O3 = 0.2 mg of O2
D.O. in mg/l = (0.2 x 1000) x ml of thiosulphate / 200


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