Colorimetric determination of clonidine

2-(2,6-Dichloroanilino)-2-imidazoline hydrochloride (clonidine, Catapres) reacts with sodium nitroprusside in presence of sodium hydroxide, and on treatment with saturated boric acid it gives a violet colour measurable at 570 nm. The effect of pH, time, concentration of sodium nitroprusside and the type of acid used was studied. This colour is used to develop a colorimetric method for the determination of clonidine in its pharmaceutical preparations. The method can be adopted by quantities ranging from 200 to 1500 microgram with 100% +/- 1.68 accuracy.