Chemical Test for Starch

Aim

To identify the chemical characters of the given sample.

Principle

Amylose forms a colloidal dispersion in hot water whereas amylopectin is completely insoluble. The structure of amylose consists of long polymer chains of glucose units connected by an alpha acetal linkage. Starch – Amylose shows a very small portion of an amylose chain. All of the
monomer units are alpha -D-glucose, and all the alpha acetal links connect C #1 of one glucose and to C #4 of the next glucose. As a result of the bond angles in the αacetal linkage, amylose actually forms a spiral much like a coiled spring. See the graphic below, which show four views
in turning from a side to an end view.

Chemical Test for Starch or Iodine

Amylose in starch is responsible for the formation of deep blue colour in the presence of iodine. The iodine molecule slips inside of the amylose coil. Iodine – KI Reagent: Iodine is not very soluble in water; therefore the iodine reagent is made by dissolving iodine in water in the
presence of potassium iodide. This makes a linear triiodide ion complex with is soluble that slips into the coil of the starch causing an intense blue-black colour.

Conclusion

From the above morphological characters and chemical tests, the given crude drug is identified as starch.

References

1.Kokate CK. Practical Pharmacognosy, 4 edition, VallabhPrakashan. Delhi; 1994: 98