Chemical methods of qualitative analysis

Qualitative analysis of a sample is generally carried out in one of two stages or methods. In the dry method, the sample under test is examined without first dissolving it — this is normally used as a quick preliminary test before more detailed work. In the wet method, the sample is first dissolved in a suitable solvent and the resulting solution is then analysed to determine what the sample is made of; wet methods usually give more definite and reliable results than dry preliminary tests. Semi-micro qualitative analysis is carried out with a standard kit of small-scale apparatus — test tubes, beakers, an evaporating dish, a crucible, a spot plate, a watch glass, wire gauze, a water bath, a burner, a blow pipe, a pair of tongs and a centrifuge among them — chosen because they let small quantities of sample and reagent be handled precisely. For organic compounds, qualitative analysis mainly checks for a small set of elements that make up the great majority of organic molecules: carbon, hydrogen, oxygen, nitrogen, sulphur, halogens and phosphorus; the identification of an organic compound typically also involves detecting its functional group and measuring physical constants such as its melting …