Dissolved oxygen

ESTIMATION OF DISSOLVED OXYGEN

(Winkler Method)

         The amount of oxygen present or dissolved in water is called the dissolved oxygen (DO). The measurement of DO indicates the purity of water and is important aerobic treatment of sewage and industrial water wastes. Further DO determination is the basis for BOD test which is used to evaluate strength of waste waters and the rate of biochemical oxidation. Water samples have to be collected in BOD bottles and the bottles are to be filled without entrapment of any air. Sample collected in the field and brought to the lab is likely to undergo a change in Do value because of changes in temperature and also occurrence of biological reactions with time. Therefore, to obtain correct DO value, the sample must be fixed immediately after collection. Fixing is done by adding 2 ml of manganous sulphate solution and alkali –iodide azide to the sample in the BOD bottles. The fixed sample is preserved by keeling at 40C in dark.

Aim

         To determine the dissolved oxygen content of the given water sample.

Principle

         The “Winkler” method based on the fact that when manganous sulphate is added to the sample containing alkaline potassium iodide, manganous hydroxide is formed, which is oxidized by the dissolved oxygen of the sample to basic manganic oxide. On addition of sulphuric acid, the basic manganic oxide liberates loading equivalent to that of dissolve oxygen originally present in the sample. The liberated iodine is treated with standard solution of sodium thiosulphate using starch as an indicator.

MnSO4 + 2KOH       →    Mn(OH)2 + K2SO4

2Mn(OH)2 + O2                →    2MnO (OH)2

(Dissolved oxygen)              (Basic manganic oxide)

Mn(OH)2 + 2 H2 SO4 →    Mn(SO4)2 + 3 H2O

(Manganic sulphate)

Mn(SO4)2 + 2KI       →    MnSO4 + K2SO4 +I2

2Na2S2O3 + I2           →    Na2S4O6 + 2 NaI

Materials Required

i)                    Manganous sulphate solution: Dissolve 48g of MnSO4.4H2O in 100 mL of distilled water, filter the solution if it is not clear.

ii)                  Alkali-iodide azide reagent: Dissolve 175 g KOH (or 124 g NaOH) and 37.5 g KI (or 33.7g NaI) in distilled water and dilute to 250 mL. Dissolve 2.5 g sodium azide in 10 mL distilled water separately. Pour the azide solution to the alkali iodide solution and mix well.

iii)                Concentrated sulphuric acid

iv)                Starch indicator : Dissolve 1.0 g starch in 10 mL of water. Stir it with a glass rod to make it as a thin paste. Pour this paste in about 100 mL boiling distilled water and boil for two minutes and cool. And

v)                  0.1N Sodium thiosulphate solution: Dissolve 24.82 g sodium thiosulphate (Na2S2O3.5H2O) in boiled and cooled distilled water and make up to 1000 mL in a volumetric flask. Standardize it against potassium dichromate solution (0.1N).

Procedure

·         Pipette out 100mL of sample into a stoppered bottle and add 2 mL of manganous sulphate solution followed by 2 mL of alkali odine-azide solution.

·         Stopper the bottle without entrapment of air and mix by inverting the bottle atleast about 10 minutes.

·         Then add 2 mL of concentrated sulphuric acid by the sides of the bottle to dissolve the precipitate formed.

·         Titrate the liberated iodine with the standard thisulphate in the burette.

·         Add 1 mL starch solution when the colour of the solution turns straw yellow.

·         Continue the titration by adding the thisulphate solution in drops till the first disappearance of the blue colour.

Calculation

1000 mL of 1 N thiosulphate   = 8g of oxygen

Dissolved oxygen (mgL-1)    = (V2x N x 8 x 1000)/V1

Where,

V1    -       Volume of sample in mL

V2    -       Titre value (Thiosulphate) in mL

N     -       Normality of thiosulphate