Nitroprusside Reaction

Nitroprusside is a complex anion that contain a centered octahedral iron, surrounded by five cianide ligands which firmly bond and one ligand of linier nitric oxide.
Sodium nitroprusside test is performed to test for the presence of proteins 'cysteine' in the test solution.

Cysteine is found in most high-protein foods, including:
•Animal sources: pork, poultry, eggs, dairy
•Plant sources: red peppers, garlic, onions, broccoli, brussels sprout, oats, granola, wheat germ, sprouted lentils.

Beside having the –COOH group, -NH2 group and R group, cysteine also has a free –SH group (sulfhydryl group ). If it reacted with sodium nitroprusside in an excess ammonia resulting a red-coloured complex.

Reaction:

[Fe3+(CN)5NO]2- + NH3 + RSH --> NH4+ + [Fe2+ (CN)5NOSR]2-