Question

The formula of the purple colour formed in Laissaigne’s test for sulphur using sodium nitroprusside is

• A. NaFe[Fe(CN)6]
• B. Na[Cr(NH3)2(NCS)4]
• C. Na2[Fe(CN)5(NO)]
• D. Na4[Fe(CN)5(NOS)]

Answer 1

Answer: (A)

NaFe[Fe(CN)6]

Answer 2

The correct answer is (D) : Na4[Fe(CN)5(NOS)]
While preparation of Lassaigne's extract, sulphur from the organic compound reacts with sodium to form sodium sulphide.
$Na + s → Na_2S$
Sodium sulphide reacts with sodium nitroprusside to form violet colour compound, which confirms presence of Sulphur.
$Na_2S + Na_2[Fe(CN)_5NO] → Na_4[Fe(CN)_5NOS]$