Why Starch Is Used as an Indicator in Titrations Involving Thiosulphate

Starch is a crucial component in many titration processes, particularly when dealing with thiosulphate. This article will delve into why starch is used as an indicator in thiosulphate titrations and explain the chemical reactions and visual cues involved.

Introduction

In the realm of analytical chemistry, the use of indicators is fundamental to determining the endpoint of a reaction accurately. One such indicator is starch, which is particularly effective when used in titrations involving thiosulphate. Starch serves as a practical and reliable indicator due to its ability to form a distinctive color change with certain species, notably the tri-iodine ion (I??), making it an indispensable tool for precise analysis.

Chemical Basis of Starch-Indicator Functionality

Starch functions as an indicator in thiosulphate titrations based on its complex formation with iodine, particularly tri-iodine ions (I??).

Iodine and Starch Complex

When iodine (I?), in the presence of starch, forms a blue-black complex. This color change is highly significant as it allows for the easy detection of iodine in a solution.

The formation of this complex is pivotal in the iodometric titration of iodine. Iodine is generated in situ, for example, by reacting potassium iodide with an oxidizing agent. When thiosulphate (S?O?2?) is added to the solution, it reduces the iodine back to iodide ions (I?).

The reaction can be represented as follows:

I2 2S2O32- → 2I- S4O62-

As thiosulphate is added, the concentration of iodine gradually decreases.

The endpoint detection relies on the disappearance of the blue-black color when all the iodine has been reduced to iodide ions. The sudden change from blue-black to colorless provides a clear visual cue that the reaction is complete.

Starch as a Reliable Indicator

Starch is an effective indicator even at low concentrations of iodine due to its high sensitivity to iodine. This property makes it particularly useful in titrations where the concentration of iodine is low. Adding starch to the I?? solution turns it blue; once the I?? ion is completely neutralized, the color disappears. J. Campbell noted the importance of starch in this context, as the faint color of I?? at the endpoint can be difficult to discern without the starch indicator forming a visible complex.

Complex Formation with Starch

The reaction between starch and tri-iodine ions is governed by the following equation:

I?? C?H??O? → C?H??O?I??

This complex forms even at very low concentrations of I??, making the color change highly noticeable and reliable for endpoint detection.

Conclusion

Starch serves as a practical and reliable indicator in thiosulphate titrations because of its ability to form a distinctive color change with iodine, facilitating the detection of the endpoint. This method has been widely recognized for its precision and reliability, making it a preferred choice in many analytical procedures.

References

Further reading on this topic can be found in the original research papers by Campbell, J. and other relevant works in analytical chemistry.