Question

The thiosulfate tirant must be standardized. What does this mean and how is it accomplished? What is a primary standard?

Answer 1

Primary standard: In analytical chemistry, a primary standard is a substance which is of high purity and relatively very stable. For the two reasons, the substance can be dissolved in a suitable solvent to prepare a solution of a fixed and known concentration and the solution is directly used in titration to determine the strength of solutions with unknown strength. If you make a solution of primary standard then the concentration of the solution is constant over time and you can directly use it in titrimetric analysis.

Other characteristics of a primary standard are:

1. It should not be hydrated

2. Should have relatively higher molecular weight

3. Should be highly soluble

Thiosulfate is not a primary standard and if you make a thiosulfate soulution with a fixed concentration then the concentration is not constant over time. So, after some time, when you are going to use it, the strength of the solution does not remain same. So, it is always necessary to find out the strength of the thiosulfate solution by titration with a primary standard just before you use it. The process of determining the exact strength of thiosulfate solution by titrating it with a solution of known concentration of primary standard solution is known as standardisation.

Thiosulfate solution is standardised by using a KI solution (primary standard) in presence of starch as indicator.

The end point is detected by the discharge of the color of the solution until it becomes yellowish from blue.