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– Collected sample was very murky
– The first end point colour was of a yellow colour, perhaps not light enough
– Addition of starch indicator produced a grey-black solution; perhaps did not add enough indicator, however this did not pose as a problem
– There was one occasion where I suspected to have added too much sodium thiosulphate, however the second trial indicated that I did not go past the end point too much
To calculate the dissolved oxygen content (mg/dm³ or ppm), we must first determine the number of mols of thiosulphate from the mean titrated volume of both samples. In this case, we will use sample A.
The Winkler method indicates that the relationship between the amount of oxygen and thiosulphate is a four to one ratio. Therefore, we will divide the number of mols of thiosulphate by four to get the number of mols of oxygen.
Then we convert this to milligrams per dm³ first by multiplying by 20 to arrive with the number of mols present in 1dm³ (because the pond water sample used was 50cm³). Then we multiply that number by 32 (molecular mass of oxygen) to attain the number of grams of oxygen. Then finally, the number is multiplied by 1000 to be converted into milligrams.
The biological oxygen demand (BOD) would therefore be the difference between the DO of sample A and sample B.