JEE Chemistry Salt Analysis Notes Set 01

Dilute Sulphuric Acid Group Anions

1. Carbonate gives CO2 (turns lime water milky), sulphite gives SO2 (pungent, turns acidified dichromate green), sulphide gives H2S (turns lead acetate paper black), nitrite gives brown fumes and acetate gives a vinegar smell.
2. Sulphite with zinc and acid is reduced to H2S. Nitrite oxidises iodide in acid, giving iodine and a blue colour with starch.

Concentrated Sulphuric Acid Group Anions

1. Chloride gives HCl (white fumes with ammonia) and, with solid dichromate, deep red fumes of chromyl chloride. Bromide gives reddish brown Br2 and iodide gives violet iodine vapour. Nitrate gives brown NO2 and a brown ring with FeSO4.
2. The layer test with chlorine water and chloroform shows Br2 as orange-brown and I2 as violet. With excess chlorine violet goes colourless (iodate) while bromine fades to pale yellow.

Silver Halides and Other Anion Tests

1. AgCl dissolves in dilute ammonia, AgBr only in concentrated ammonia and AgI in neither. AgCl turns yellow with sodium arsenite, while AgI does not change.
2. Phosphate and arsenate give a yellow precipitate with ammonium molybdate and nitric acid. Sulphate gives a white BaSO4 precipitate insoluble in acids.

Group Separation of Cations

1. Group I (Ag+, Pb2+, Hg2 2+) precipitates as chlorides with dilute HCl. PbCl2 dissolves in hot water. Hg2Cl2 turns black with ammonia and AgCl dissolves in it.
2. Group II sulphides precipitate with H2S in dilute acid because the common ion lowers [S2-]. Sulphides of Hg, Pb, Bi and Cu are insoluble in yellow ammonium sulphide while As, Sb and Sn sulphides dissolve. HgS dissolves only in aqua regia, and CuS dissolves in KCN.
3. Group III (Fe3+, Cr3+, Al3+) is precipitated as hydroxides with NH4OH in presence of NH4Cl, which suppresses [OH-]. Group IV (Zn2+, Mn2+, Co2+, Ni2+) precipitates as sulphides in ammoniacal medium. Group V (Ca2+, Sr2+, Ba2+) is precipitated as carbonates with ammonium carbonate. Mg2+ stays for group VI.

Confirmatory Tests for Cations

1. Fe3+ gives blood red with SCN- and Prussian blue with ferrocyanide; Fe2+ gives Turnbull's blue with ferricyanide. Cu2+ gives deep blue [Cu(NH3)4]2+ and a chocolate brown precipitate with ferrocyanide.
2. Zn(OH)2 is amphoteric and dissolves in excess NaOH and in ammonia. Al3+ gives Thenard's blue with cobalt nitrate on a charcoal block. Ni2+ gives a rose red precipitate with dimethylglyoxime.
3. Hg2+ gives red HgI2 with KI, which dissolves in excess as K2HgI4 (Nessler's reagent in alkali). Bi3+ gives black BiI3, which dissolves in excess KI to give orange complex and is hydrolysed to BiOCl or BiOI.
4. Flame colours: Ca brick red, Sr crimson, Ba apple green, K lilac. Borax bead: Cu blue, Cr green, Mn violet, Co blue, Ni brown, Fe yellow to brown.