Can anyone explain why my starch indicator is not giving blue colour after adding it to the volume of sample titrated to a straw colour with Sodium Thiosulphate in the BOD test by Winkler's method . Even changing the pH of the sample does not bring the blue colour. This is the case when I am doing BOD with sample or with a blank. However, when I do standardization of my Sodium Thiosulphate solution using Potassium Dichromate solution, Potassium Iodide and HCl, blue colour does appear upon addition of starch indicator to the straw coloured pre-titrated solution but I have to ensure that the pH remains acidic, or the blue colour does not appear. Can anyone tell me why?
Thank you so much for the answer. However, I had tried with fresh starch too...It didn't work. Moreover, I had done standardization of Sodium Thiosulphate using the same starch where it had shown the blue colour.
ok - if you will perform analysis of blank sample - you get the colour change?
Not blank exactly. I am getting the colour change when I standardize my Sodium Thiosulphate solution with Sodium Dichromate solution, Potassium Iodide and HCl. However, I am not geting any blue colour, neither with blank nor with my sample.
Thank you so much but please tell me you mean improper dilution of what exactly ? Because I am getting no colour even with the blanks.
Arghyadeep Gupta
Hi! Did you ever figure out the problem because I'm struggling with the same issue?
Help will be greatly appreciated!
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