and then acidified with Partition Chromatography: Partition chromatography is based on continuous differential partitioning of components of a mixture between stationary and mobile phases. Paper chromatography is a type of partition chromatography. In paper chromatography, a special quality paper known as chromatography paper is used. Chromatography paper contains water trapped in it, which acts as the stationary phase.
A strip of chromatography paper spotted at the base with the solution of the mixture is suspended in a suitable solvent or a mixture of solvents (Fig. . ). This solvent acts as the mobile phase.
The solvent rises up the paper by capillary action and flows over the spot. The paper selectively retains different components according to their differing partition in the two phases. The paper strip so developed is known as a chromatogram. The spots of the separated coloured compounds are visible at different heights from the position of initial Fig.
. Paper chromatography. Chromatography paper in two different shapes. concentrated sulphuric acid.
The formation of Prussian blue colour confirms the presence of nitrogen. Sodium cyanide first reacts with iron(II) sulphate and forms sodium hexacyanidoferrate(II). On heating with concentrated sulphuric acid some iron(II) ions are oxidised to iron(III) ions which react with sodium hexacyanidoferrate(II) to produce iron(III) hexacyanidoferrate(II) (ferriferrocyanide) which is Prussian blue in colour. 6CN – + Fe + → [Fe(CN) ] – [Fe(CN) ] – + 4Fe + Fe [Fe(CN) ] .xH O Prussian blue (B) Test for Sulphur (a) The sodium fusion extract is acidified with acetic acid and lead acetate is added to it.
A black precipitate of lead sulphide indicates the presence of sulphur. S – + Pb + → PbS Black (b) On treating sodium fusion extract with sodium nitroprusside, appearance of a violet colour further indicates the presence of sulphur. S – + [Fe(CN) NO] – → [Fe(CN) NOS] – Violet In case, nitrogen and sulphur both are present in an organic compound, sodium thiocyanate is formed. It gives blood red colour and no Prussian blue since there are no free cyanide ions.