Chemical parameters - Dissolved Oxygen (DO)

DISSOLVED OXYGEN (DO)

  • Dissolved oxygen content is one of the vital environmental characteristics of water. For the determination of primary production and BOD (Biochemical Oxygen Demand) also the oxygen content is determined. Winkler’s method is followed for the estimation of DO

Principle

  • When the solutions of manganese and alkaline iodide are added to the water sample, the following reactions normally take place.
  • Manganese ion reacts with the hydroxide ion to give Mn(OH)2.
  • Mn(OH)2 reacts with the dissolved oxygen present in the water sample to give Mn(OH)3.
  • In the presence of acid (above pH 1.5). Mn(OH)3 reacts with iodide to liberate iodine. The iodine liberated is titrated against sodium thiosulphate (Na2S2O3) to convert the iodine to iodide.

Reagents

  • Manganous sulphate reagent: 48g of MnSO4 4H2O or 40g of MnSO2,2H2O or 36.5 of MnSO4 H2O is dissolved in water and the volume is made up to 100 ml with distilled water.
  •  Alkaline iodide: 60 g of KI and 30 g of KOH are dissolved separately in a minimum amount of water. Both these solutions are mixed together and this solution is then made upto 100 ml with distilled water
  • Conc. Sulphuric acid.
  • 0.1 N Sodium thiosulphate: 2.482 g of Na2S2O3 is dissolved and made up to 1000 ml with distilled water.
  • Starch solution: 1 g of soluble starch is dispersed in 100 ml of distilled water. The solution is quickly heated to boiling. One drop of phenol or formaldehyde is added to every 50 ml of starch solution to check microbial degradation.

Procedure

  • The water sample for which the dissolved oxygen content is to be determined is slowly siphoned out into a BOD bottle with the help of a rubber tube. When the water sample is allowed to overflow in this BOD bottle, the following reagents are added. Extra care should be taken not to permit the entry of any air bubble inside the BOD bottle. One ml of manganous sulphate solution is added first to the water sample keeping the tip of the pipette just below the surface of the water. In the same way, one ml of alkaline iodide is added and the bottle is carefully stoppered. The bottle is tilted upside down so that Mn(OH)2 reacts with dissolved oxygen present in the sample to give a slightly brown coloured Mn(OH)2. The estimation of dissolved oxygen may be done immediately. Also, the mouth of the BOD bottle should be thoroughly sealed with wax and the bottle should be kept in a water bath (at room temperature) so as to avoid the impact of increasing temperature.
  • At the time of titration, 1 ml of Conc. Sulphuric acid is slowly added to the sample and the bottle is stoppered. The bottle is tilted side wise so that all the precipitates are dissolved and the solution turns brownish due to the liberation of iodine.
  • The thiosulphate solution of the given normality (0.01) is then taken in the burette and is kept ready for titration. 50 ml of sample from the reagent fixed BOD bottle is taken in a conical flask and is titrated against the thiosuphate solution till a hay yellow colour appearance at this report. 1 ml of starch is added to this solution and it turns blue indicating the presence of iodine. The titration is continued till the blue colour disappears. The titration is repeated to get concordant values. The oxygen content of the water samples (mg/l) is calculated using the following formula:

Dissolved oxygen (mg/l) = ( CF x N x E x 0.698 x 1000 x Vt ) /Vs

Correction Factor (CF) = Total volume of the stoppered bottle / Total volume of the stoppered bottle-2

Where, E = Equivalent weight of oxygen (i.e. 8)

    N = Normality of thiosulphate (0.01N)

   0.698 = To convert mg/l into ml/l

   1000 = To convert the value to 1000 ml (or) ml

   Vt = Titre value

   Vs = Volume of sample taken for titration in ml

Last modified: Wednesday, 25 April 2012, 5:37 AM