be detected by putting the plate under ultraviolet light. Another detection technique is to place the plate in a covered jar containing a few crystals of iodine. Spots of compounds, which adsorb iodine, will show up as brown spots. Sometimes an appropriate reagent may also be sprayed on the plate.
For example, amino acids may be detected by spraying the plate with ninhydrin solution (Fig. .12b). Fig. .
(b) Developed chromatogram. Fig. . (a) Thin layer chromatography.
Chromatogram being developed. spot on the chromatogram. The spots of the separated colourless compounds may be observed either under ultraviolet light or by the use of an appropriate spray reagent as discussed under thin layer chromatography. .
Qualitative Analysis of Organic Compounds The elements present in organic compounds are carbon and hydrogen. In addition to these, they may also contain oxygen, nitrogen, sulphur, halogens and phosphorus. . .
Detection of Carbon and Hydrogen Carbon and hydrogen are detected by heating the compound with copper(II) oxide. Carbon present in the compound is oxidised to carbon dioxide (tested with lime-water, which develops turbidity) and hydrogen to water (tested with anhydrous copper sulphate, which turns blue). C + 2CuO 2Cu + CO 2H + CuO Cu + H O CO + Ca(OH) CaCO ↓ + H O 5H O + CuSO CuSO .5H O White Blue . .
Detection of Other Elements Nitrogen, sulphur, halogens and phosphorus present in an organic compound are detected by “Lassaigne’s test” . The elements present in the compound are converted from covalent form into the ionic form by fusing the compound with sodium metal. Following reactions take place: Na + C + N NaCN 2Na + S Na S Na + X Na X (X = Cl, Br or I) C, N, S and X come from organic compound. Cyanide, sulphide and halide of sodium so formed on sodium fusion are extracted from the fused mass by boiling it with distilled water.
This extract is known as sodium fusion extract. (A) Test for Nitrogen The sodium fusion extract is boiled with iron(II) sulphate