Question

In doing Biochemical oxygen demand (B.O.D) what does sodium thiosulphate do and why is starch use as indicator?

Answer 1

Biological Oxygen in Demand (BOD) is the amount of the oxygen dissolved in the water used or required by the aerobic organisms in the water for the biological degradation of the organic material in the water. The BOD test is carried out to determine the presence of amount of the Oxygen in the sample, whether it is low or high. Sample is added with oxygen saturated water and kept for 5 days for incubation during which intial and final Dissolved Oxygen is amount is calculated to find the result.

During the procedure, elemental iodine is liberated which is equal to the dissolved oxygen. In the titration step, sodium thiosulphate is slowly added to neutralise all the iodine where color changed to straw yellow color. This determines the amount of iodine present in the sample by analysing the amount of Sodium Thiosulphate used. Furthermore, because each iodine molecule is the result of the reaction of the single oxygen atom hence amount of thiosulphate added will tell us how much oxygen was dissolved in the sample.

Strach is used for the tittarion of the iodine as it forms the complex with the iodine and changes the color to blue. During titration with sodium thiosulphate when pale straw yellow color appeared, starch solution is added which turns the solution color to blue and makes it more distinguishable. Titration is continued until clear solution is appeared. So to conclude that, starch indicator is used to determine the endpoint of the titration reaction is dissolve oxygen analysis.