REACTIONS AND IDENTIFICATION 383 SPECIAL REACTIONS FOR CYSTINE. 1. Removal of sulphur as sodium sulphide. Dissolve about o-i g. of cystine in a few ml. of 10% NaOH solution. Add a few drops of lead acetate solution and boil for i minute. The solution darkens owing to the formation of lead sulphide. 2. Nitroprusside test. Dissolve about o-i g. of cystine in a few ml. of dilute ammonia and then add a few drops of potassium cyanide solution. This reduces cystine to cysteine. S-CH 8 CH(NH 2 )COOH + 2H = zHS CH 2 CH(NH 1 )COOH I (Cysteine) S-CH 1 CH(NH 1 )COOH (Cystine) Now add a few drops of a freshly prepared solution of sodium nitroprusside. A purple coloration is obtained (test for — SH Group). (B) AMINO-AROMATIC CARBOXYLIC ACIDS. Anthra-nilic acid. Physical Properties. Colourless solid when pure, usually pale brown. Sparingly soluble in cold water, soluble in hot water: soluble also in cold mineral acids and caustic alkalis. Dissolves readily in cold alcohol, and solution possesses a faint blue fluorescence. REACTIONS.
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