Identify the test mentioned

The question refers to the formation of a purple coloured complex on adding sodium nitroprusside to the sodium carbonate extract of a salt.

The complex given is: $\mathrm{Na}_4[\mathrm{Fe}(\mathrm{CN})_5\mathrm{NOS}]$

Recall the salt analysis test

In qualitative inorganic analysis, sodium nitroprusside is a confirmatory reagent for sulphide ion $\left(\mathrm{S}^{2-}\right)$.

When sulphide ion is present, it reacts with sodium nitroprusside to give a violet/purple complex.

Match with the options

• A: Sulphide ion $\rightarrow$ gives purple colour with sodium nitroprusside ✅
• B: Sulphite ion $\rightarrow$ does not give this characteristic purple nitroprusside complex ❌
• C: Sulphate ion $\rightarrow$ does not respond in this way ❌
• D: Sodium ion $\rightarrow$ unrelated to this test ❌

Conclusion

Therefore, the presence of the purple complex $\mathrm{Na}_4[\mathrm{Fe}(\mathrm{CN})_5\mathrm{NOS}]$ indicates the presence of: $\boxed{\text{Sulphide ion}}$

Comparison with stored answer

Stored correct answer: A

Derived answer: A

Hence, they agree.