Soil and Water Chemistry (1+2)

Determination of Dissolved oxygen in water (Winkler’s Method)

Introduction

Principle:
MnSO₄ reacts with the alkali to form white precipitate of Manganese hydroxide which in the precipitate of O₂ get oxidized to a brown color higher hydroxide, which on acidification liberate iodine equivalent to that of O₂ fixed. The iodine is titrated against thiosulphate using starch as indicator.

Reagents

• Manganese sulphate solution: Dissolve 480 g of MnSO₄.4H₂0 or 400 g of MnSO₄.2H₂0 or 363 g of MnSO₄.H₂0 in 1000 ml distilled water.
• Alkaline iodide solution: Dissolve 500 g NaOH [or 700 g KOH] and 135 g NaI [or 150 g KI] in 1000 ml distilled water (or dissolve 100 g KOH and 50 g KI in 200 ml distilled water).
• Concentrated sulphuric acid.
• Starch solution: Dissolved 2 g laboratory grade soluble starch and 0.2 g salicylic acid in 100 ml hot distilled water.
• Sodium thiosulphate solution (0.025 N): Dissolve 6.203 g of Na₂S₂O₂.5H₂O in 1000 ml distilled water.

Procedure:

• Water sample is collected from surface in 125 ml Dissolved Oxygen bottle avoiding formation of bubbles.
• Add 1 ml alkaline iodide and 1 ml MnSO₄ to that Dissolved Oxygen bottle.
• Mix well and then precipitate formed is allowed to settle for few minutes.
• Add 1 ml of conc. H₂SO₄ and shake well to dissolve the precipitate.
• Then from DO bottle take 50 ml solution in conical flask.
• Titrate against std. Na₂S₂O₃ using starch as indicator.
• The end point is blue to colorless and titre value is measured.

Calculation:

A x N x V x 1000 x 22.4

DO (mg/l) = ---------------------------------

B (A-L) x 0.4 x 0.698

Where,
A= Volume of the DO bottle
N= Normality of Na₂S₂O₃
V= Titrate value
B= Volume of sample taken
L= Volume of reagent used