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.  Â
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The 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.
Calculation
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
