Cysteine and the Sodium Nitroprusside Test (Red Colour Test for Sulphur)
Chemistry · Biomolecules · NEET
Cysteine is an amino acid that has sulphur (the group -SH) in its side chain. When you do the sodium nitroprusside test on it, you get a red or violet colour. This colour appears only because sulphur is present. Memory hook: "No sulphur, no red." If an amino acid has an S atom, nitroprusside turns red.
The sodium nitroprusside test detects the element sulphur. Cysteine (and also cystine and methionine) contains sulphur, which becomes Na2S on sodium fusion and gives a red/violet colour. Sulphur-free amino acids give no colour.
Your doubts, answered
Why does cysteine give a red colour with sodium nitroprusside?
Because cysteine has a sulphur atom in its -SH (thiol) side chain. In the test, the sulphur is first turned into sulphide ions (S2-). These sulphide ions react with sodium nitroprusside and form a coloured complex that looks red or violet. So the red colour is really a signal that sulphur is present, not a signal for cysteine by name.
Does the sodium nitroprusside test detect the amino acid or the element sulphur?
It detects the element sulphur. This is the most important idea for NEET. The test cannot tell one amino acid from another. It only answers one question: does this compound contain sulphur? Any amino acid with an S atom will give the positive red colour.
Do methionine and cystine also give the nitroprusside test?
Yes. Cysteine, cystine, and methionine all contain sulphur, so all three can give a positive (red) result. This is why NEET can put methionine in the options instead of cysteine and still be correct. Do not pick cysteine just because you memorised its name; pick whichever option has sulphur.
Which common amino acids do NOT give the nitroprusside test?
Amino acids with no sulphur do not respond. Examples are glycine, alanine, valine, leucine, serine, threonine, and phenylalanine. Their side chains have no S atom, so no sulphide forms and no red colour appears.
What is the sodium fusion extract used in the question?
To test for sulphur, the compound is first heated (fused) with sodium metal. This converts any sulphur in the molecule into sodium sulphide (Na2S). The water extract of this is the sodium fusion extract. Adding sodium nitroprusside to it gives the red/violet colour if sulphur was present.
⚠️ The NEET trap ✗ The nitroprusside red colour test is specific for cysteine, so the answer must be cysteine. ✓ The test is specific for SULPHUR, not for cysteine. Any sulphur-containing amino acid (cysteine, cystine, or methionine) gives the red colour. In ReNEET 2026 the correct option was methionine because it was the only sulphur amino acid listed. 🧠 Read the options for the S atom, not for the name. -SH or -S- present = red colour.
Real NEET questions
ReNEET 2026
The amino acid that gives a red-blood colour on treating its sodium fusion extract with sodium nitroprusside is:
A · leucine
B · threonine
C · methionine ✓
D · serine
Solution: The sodium nitroprusside test detects sulphur. In the sodium fusion step, sulphur in the molecule becomes sodium sulphide (Na2S). This sulphide reacts with sodium nitroprusside to give a violet/blood-red colour. Among the four options, only methionine [H2N-CH(COOH)-CH2CH2-S-CH3] contains sulphur, so only it gives the positive red test. Leucine, threonine, and serine have no sulphur, so they stay negative. Note: cysteine would also test positive, but it is not in the options here.
Solved Biomolecules NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What colour is a positive sodium nitroprusside test?
A positive result gives a red or violet (purple) colour. The exact shade is often described as blood-red or violet. Any of these shades counts as positive for sulphur.
Is cysteine the only amino acid that gives this test?
No. Cysteine, cystine, and methionine all contain sulphur and can give the red colour. The test is for the sulphur element, so it works for all sulphur amino acids.
Why is this test important for NEET?
NEET often asks which amino acid gives a red colour with nitroprusside. The trap is thinking it is cysteine-specific. Knowing it detects sulphur lets you answer even when the option is methionine or cystine instead.
What is the functional group in cysteine that makes it react?
Cysteine has a thiol group, written as -SH, on its side chain. This -SH group is the source of the sulphur that the test detects.
Does the nitroprusside test work directly, or is a step needed first?
Usually the compound is first fused with sodium to change the sulphur into sodium sulphide (Na2S). Sodium nitroprusside is then added to this extract to produce the red/violet colour.
