Water Analysis : To Estimate Dissolved Oxygen Content of Wastewater

INTRODUCTION
Dissolved oxygen levels in natural and wastewater depends on physical chemical and biochemical activities in the water body. Analysis for dissolved oxygen is the key test in water pollution and waste treatment process control. A dissolved oxygen level of 0-8 ppm is normal and supports aquatic life.
METHODS FOR ESTIMATION:
1. lodometric method
2. Membrane electrode method
3. Azide modification method
4. Permanganate method
5. Alum flocculation method
6. CuSO4 sulfuric acid modification method
AZIDE MODIFICATION METHOD PRINCIPLE:
lodometric test is most precise and reliable titrimetric procedure for dissolved oxygen analysis. It is based on addition of divalent manganese solution followed by addition of strong alkali azides. An equivalent amount of dispersed divalent manganous hydroxide precipitates to hydroxide of higher valency state (Mn+3). If no O2 is present, white precipitates of Mn(OH)2 are formed. But in the presence of O2,, divalent (Mn+2) is oxidized to Mn+4 i.e., MnO2. In the presence of iodine ions in acidic solution, oxidized manganese reverts to divalent state (Mn+2) with liberation of I2 equivalent to original dissolved oxygen content. The liberated I2 is then titrated with standard solution of thiosulfate.
REAGENTS:
·   MnSO4 solution: Dissolve 480gm MnS04.4H2O or 400 gm MnSO4.2H2O or 364 gm MnSO4.H2O in distilled water and make volume up to 1 liter.
·   Alkali iodide azide reagent: dissolve 700gm KOH, 150 gm KI and 10 gm NaN3 in 1000 ml of distilled water.
·   Concentrated sulfuric acid
·   Starch solution: 2% starch solution.
·   Standard sodium thiosulfate titrant: Dissolve 6.205 gm Na2S2O3.5H2O in distilled water and 1.5 ml 6N NaOH or 0.4 gm solid NaOH and make total volume to 1000 ml.
PROCEDURE:

·   To the sample collected in 250/300 ml bottle add 1 ml MnSO4 solution followed by 1 ml alkali azide reagent.
·   Stopper the bottle to exclude air bubbles and mix by inverting the bottle.
·   Allow the precipitates to settle sufficiently.
·   Add 2 ml concentrated H2SO4 and shake the bottle so that the precipitates dissolve.
·   Take 200 ml from it and add 1-2 drops of starch.
·   Titrate with 0.025 M Na2S2O3 solution until the blue color disappears.

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