"Add starch just before titration end"WHY?

well, it would help if we knew what experiment you were doing. there are literally thousands of titrations where starch would be added near the end.

I guess he wants to know why, unlike most indicators, starch is added near the end of the titration. The fact that this is true for thousands of titrations doesn't really matter.

 

Anyway there are several reasons.

Starch is a reducing agent (especially if it hydrolyses to glucose) so you don't want to add it too soon or it may react with the oxidant and upset the result.

More importantly when the starch reacts with a large excess of iodine it tends to "trap" some of the iodine so, even after enough thiosulphate has been addded, the mixture stays blue for a while and you tend to add too much thiosulphate. This spoils the accuracy of the titration.

IIRC, we used starch as an indicator. The reaction gave a purple color...

 

This was a while ago, and I can't recall the exact reaction, sorry.

Determination of the activation energy for the Iodination of Propanone

by finging the rate constant at different temperature

I guess he wants to know why, unlike most indicators, starch is added near the end of the titration. The fact that this is true for thousands of titrations doesn't really matter.

 

Anyway there are several reasons.

Starch is a reducing agent (especially if it hydrolyses to glucose) so you don't want to add it too soon or it may react with the oxidant and upset the result.

More importantly when the starch reacts with a large excess of iodine it tends to "trap" some of the iodine so, even after enough thiosulphate has been addded, the mixture stays blue for a while and you tend to add too much thiosulphate. This spoils the accuracy of the titration.

 

thanks