Learn the procedure for making a starch solution for iodometric titration.
The compound that changes color when exposed to acidic or basic solutions is called an indicator. Color indicators are commonly used to measure pH and are added to the reaction mixture to identify the titration endpoint/equivalence point.
Starch (C6H10O5)n is a polysaccharide comprising glucose monomers joined in α 1, 4 linkages. The linear polymer amylose is the most basic type of starch, while amylopectin is the branched form. It is a tasteless, soft white powder that is insoluble in cold water, alcohol, and other solvents.
Starch is the most common indicator for titrations containing iodine. Iodine with starch forms a dark blue complex. In iodimetry, the endpoint corresponds to a sudden color change to blue.
How to prepare starch indicator for titration:
- Weigh accurately 0.3 gm of starch in 10.00 ml of water with constant stirring.
- Add enough boiling water to make around 100 ml and boil for a few minutes and then set aside to cool.
- The concentration of the prepared solution is about 0.3% w/w.
Starch pKa in strongest acidic is 11.3 and in strongest basic -3. In redox titrations starch and iodide are often used since they can absorb the I2 that is released: when an oxidizing agent is present, the solution turns blue; when a reducing agent is present, the blue color fades because of triiodide.
References:
- Wikipedia contributors. (2021, December 5). Iodine–starch test. In Wikipedia, The Free Encyclopedia. Available Here:
- Britannica, The Editors of Encyclopaedia. "starch". Encyclopedia Britannica, 14 Mar. 2021, Available Here:
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