Preliminary and confirmatory tests of cations
Answers
Answer:
Qualitative Analysis of Cations
Preliminary Tests
Some preliminary tests needs to be done before doing the analysis of cations.
(a) Physical Appearance: Colour and Smell
The physical examination of the unknown salt involves the study of colour, smell and density. The test is not much reliable, but certainly helpful in identifying some coloured cations. Characteristic smell helps to identify some ions like ammonium ion.
(b) Charcoal Cavity Test
This test is based on the fact that metallic carbonates when heated in a charcoal cavity decomposes to give corresponding oxides. The oxides appear as coloured incrustation or residue in the cavity. In certain cases, the oxides formed partially undergo reduction to the metallic state producing metallic beads or scales.
(c) Cobalt Nitrate Test (Ash test)
This test is applied to those salts that leave white residue in the charcoal cavity test. This test is based on the fact that metallic carbonates when heated in a charcoal cavity decomposes to give corresponding oxides.
The test is based on the fact that cobalt nitrate decomposes on heating to give cobalt oxide, CoO. This combines with the metallic-oxides present as white residue in the charcoal cavity forming coloured compounds.
(d) Flame Test
Certain salts on reacting with conc. HCl forms their chloride, that are volatile in non-luminous flame. Their vapours impart characteristic colour to the flame. This colour can give reliable information of the presence of certain cations . For proceeding to this test, the paste of the mixture with conc.HCl is introduced into the flame using a platinum wire.
(e) Borax Bead Test
Borax, Na2B4O7.10H2O, on heating gets fused and loses water of crystallisation. It swells up into a fluffy white porous mass which then melts into a colourless liquid that later forms a clear transparent glassy bead consisting of boric anhydride and sodium metaborate.