Iodometric Titration to determine caffeine content

Which recorded volume of sodium thiosulphate do i need to use to determine the caffeine content?
Short overview of the experiment: I am titrating iodine with caffeine against thiosulphate to determine the amount of caffeine. Therefore, I add iodine with hydrochlorid acid and the caffeine into a conical flask. Then I titrate it against the sodium thiosulphate until the mixture pale brown. Then I add a few drops of starch until it turns dark blue. Afterwards i titrate it again until it turns colourless.

But now i do not know if I have to use the whole amount of sodium thiosulphate to calculate the amount of caffeine, or the amount until it turns pale brown, or the amount after the starch is added.

In my opinion it would make sense to use the recorded volume after the starch is added.

Reply 1

charco

Study Forum Helper

Original post by saskia050505

The whole volume of thiosulfate ions.

The reason for adding the indicator just before the end point is that if you add it too early it can form a dark blue-black complex with the iodine that does not release the iodine to react with the thiosulfate.