Prep. By dissolving five grains of indigo in one fluid drachm of sulphuric acid with the aid of heat, and then diluting with ten fluid ounces of water.

Prop. & Comp. Indigo owes its fine colour to the presence of a peculiar substance, indigotin (C16 H5 No2); it is insoluble in water, but by the action of deoxidating agents it is changed into white indigo, which contains one more atom of hydrogen than indigotin: this is soluble in water, and by exposure to the air becomes reconverted into the blue variety. The solution of sulphate of indigo contains a peculiar compound of the acid and the colouring matter, called sulph-indylic acid, formula (HO, C16 H4 NO, 2 S03). This solution is used as a test for free chlorine in hydrochloric acid and liquor sodae chlorinatas; if free chlorine be present, the colour is destroyed.

Therapeutics. The action of indigo as a therapeutic agent requires further investigation; it has been employed in epilepsy; it colours the urine green or bluish-green. Indigo is occasionally found in the urine in disease.

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Prep. Made by digesting for an hour, with the aid of the heat of a water-bath, five grains of indigo in one fluid drachm of pure sulphuric acid; then pouring the solution into ten fluid ounces of distilled water, and after thoroughly mixing by agitation, allowing any undissolved matter to subside, and decanting off the the clear liquid for use.

Use. This solution, which contains the sulphate of indigo (HO, C16 H4 NO, 2 So3), when in contact with free chlorine or bodies containing chlorine in a feeble state of combination, becomes decolorized, hence its value as a test. It is employed under Chlori Liquor, Sodae Chloratae Liquor, etc.