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Measurement of Dissolved Oxygen

 PURPOSE:

To describe the Laboratory standard operating procedure for measurement of Dissolved Oxygen of Water and Waste Water samples.


SCOPE:

Applicable to the Laboratory for analysis of Water and Waste Water samples, where measurement of Dissolved oxygen is required.

 

RESPONSIBILITY: 

Lab chemist

:

Analysis of sample for measurement of Dissolved oxygen.

Technical Manager

:

Review of activity 

Quality Manager

:

Implementation and compliance of SOP.


PROCEDURE: 

Principle:

Oxygen present in sample oxidize the divalent Mn to its high valency which precipitates as a brown hydrate oxide after addition of sodium hydroxide and KI. Upon acidification, manganese reverts to divalent state and liberates iodine from potassium iodideiodidel equivalent to DO content in the sample. The liberated iodine is titrated against 0.025N Na2S2O3 using starch as indicator.

Instrument and Equipment:

BOD bottles 300ml capacity, 25 ml Burette 


Reagents: 

Magnus sulfate: Dissolve 480g MnSO4.4H2O or 364 g MnSO4.H2O in distilled water, filter if necessary and dilute it to 1000ml.This solution should not give colour with starch when added to an acidified solution of KI.

Alkali-iodide-azide reagent: Dissolve 500g NaOH and 150 g KI and dilute it to 1000ml Add 10g NaN3 dissolved in 40ml distilled water. It should not give colour with starch solution when diluted and acidified.

H2SO4, Conc.

Starch indicator: Prepare paste or solution of 0.5g starch powder in distilled water. Pour this solution in 100ml boiling water. Allow to boil for few minutes. Cool and then use.

Sodium thiosulfate solution (0.1N): Dissolve 24.82 g Na2S2O3 5H2O in boiled cooled distilled water and dilute to 1000ml. Preserve by adding 5ml chloroform per liter. Standardize this solution with 0.1N K2Cr2O7 solution.

Sodium thiosulfate solution (0.025N): Dilute appropriate volume of 0.1N Na2S2O3 to prepare 0.025N Na2S2O3.Standardise this solution with K2Cr2O7.

Test Method:

Collect sample in a 300ml BOD bottle.

 Add 2ml MnSO4 followed by 2ml of alkali iodide-azide solution. The tip of the pipette should be below the liquid level while adding this reagent. Stopper immediately.

and mix well by inverting the bottle 3-4 times and let the precipitate to settle. 

After that stage, add 2ml conc H2SO4.Mix well till precipitate goes into solution.

Take 203 ml in a conical flask and titrate against Na2S2O3 using starch as indicator.

When 2ml MnSO4 followed by 2ml alkali-iodide-azide is added to the sample, 4.0 ml of original sample I lost. Thus 203ml taken for titration will corresponds to 200ml of original sample.

            200 X 300/ (300 -   4) = 203 ml

Calculation:

1ml of 0.025 N  Na2S2O3  =  0.2 mg of O2

Since we take 200ml sample volume,   0.2 X 1000

                                                                               200

1ml of 0.025 N Na2S2O3 is equal to 1 mg DO/L


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