How does starch act as an indictor?

Date: 31.10.2007

All Chemistry lovers would know that starch (a bio-polymer) serves as an indicator in iodometric and iodimetric titrations, which are epitomes of redox titrations.

As a student of Chemistry, I wish to explain the intricate mechanism by which the bio-polymer serves as an indicator in the aforesaid redox titrations.

The bio-polymer starch comprises of 20% water soluble amylose and 80% water insoluble amylopectin. The amylose component of starch possesses helical structure by virtue of which it serves as an indicator in iodometric and iodimetric titrations.

In both the titrations, the system will possess a mixture of molecular iodine(I2) and iodide ions(I-). Molecular iodine combines with iodide ions in a reversible manner, thereby giving rise to a complex called tri-iodide ions(I3-). The equilibrium of the reaction of formation of tri-iodide(I3-) ions is always backwards. So, I3- ions are actually considered as molecular iodine(I2) in a solution of iodide ions(I-).

It is hypothesised that when an aqueous solution of starch is added into a system comprising a mixture of I2 and I- ions, the I3- ions(that will be present all the time) and I2 will enter into the cage formed by the helical structure of starch, where they combine together to form I5- ions. The I5- ions complex with starch through weak Van-der-Waals force. This complex of I5- ions and starch is blue in colour. Thus as soon as starch is added into the system, violet colour is observed. With subsequent addition of the titrant, usually sodium thiosulphate(Na2S2O3), the violet colour gradually disappers and the end-point is complete disappearnce of the violet colour.

A necessary precaution that has to be taken while making use of starch as an indicator is that, it should be added only after the colour of the solution in the conical flask turns straw yellow in colour. If starch is added in prior, then excess of iodine would get into the helical structure of starch and get coagulated. Thus, one would end up getting non-reproducible end-points.