Titration against sodium thiosulphate

Mohit Pandey · 08-30-2007

In my book it written that-concentration of iodine can be measured at different intervals of time by Titration against sodium thiosulphate.
please explain it in simple terms.

eric l · 08-30-2007

Quote:

Originally Posted by Mohit Pandey

In my book it written that-concentration of iodine can be measured at different intervals of time by Titration against sodium thiosulphate.
please explain it in simple terms.

Iodine is an oxidizing agent, and it is NOT an absolute standard for making titrant solutions. One of the reasons is that there may be some sublimation between the weighing and the preparation of the solution, an other reason is that it may loose some activity in time, due to influence of light and other parameters.

Sodium thiosulphate is a reducing agent. It is considered an absolute standard, which means that if you weigh an amount, you know exactly how many moles of product you have, you won't loose an amount between weighing and making the solution, and there is no indefinite amount of impurities (like absorbed water) to disturb the measurement.

An oxidizing agent reacts with a reducing agent in a fixed ration of weights. A titration is adding one to the other in a controlled way, so that you can see when there is no more excess A, and you start to have excess B. With iodine, the colour of iodine acts as indicator : the mix becomes light yellow as soon as there is excess iodine.

You can titrate thiosulphate with an iodine solution, and use this to verifiy the concentration of the iodine solution. If you have to keep your iodine solution over a longer period, it is advisble to check the activity on a regular basis (e.g; every two weeks). This checking of the activity is equally important if the iodine solution has to serve other purposes than chemical analysis.

I hope that book of yours gives a detailed method on how to proceed.