Anion analysis |
Chloride Cl- |
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AgCl, PbCl2, Hg2Cl2 and CuCl are insoluble in water. Concentrated sulphuric acid liberates steamy acidic fumes of HCl from solid chlorides: NaCl(s) + H2SO4(l) " NaHSO4(s) + HCl(g) Silver nitrate solution added to a solution of a chloride that has been acidified (test with blue litmus paper) with dilute nitric acid gives a white precipitate of silver chloride. The precipitate is readily soluble in dilute ammonia or in sodium thiosulphate solution: Ag+(aq) + Cl-(aq) " AgCl(s) AgCl(s) + 2NH3(aq) " [Ag(NH3)2]+(aq) + Cl-(aq) AgCl(s) + 2S2O32-(aq) " [Ag(S2O3)2]3-(aq) + Cl-(aq) Acidification with nitric acid is necessary to eliminate carbonate or sulphite, both of which interfere with the test by giving spurious precipitates. Concentrated solutions of sulphates can give a precipitate of silver sulphate in this test; its appearance is wholly different from AgCl. The latter is truly white; the sulphate is a pearly white, rather like pearlescent nail varnish. |
Inorganic analysis introduction
Preliminary tests
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Ions |
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chloride |
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Bromide Br- |
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AgBr, PbBr2, Hg2Br2 and CuBr are insoluble in water. Concentrated sulphuric acid gives a mixture of hydrogen bromide, bromine and sulphur dioxide with solid bromides; the HBr produced is oxidised by sulphuric acid. The mixture evolves steamy brownish acidic fumes: NaBr(s) + H2SO4(l) " NaHSO4(s) + HBr(g) 2HBr + H2SO4 " Br2 + SO2 + 2H2O Silver nitrate solution added to a solution of a bromide that has been acidified (test with blue litmus paper) with dilute nitric acid gives a cream precipitate of silver bromide. The precipitate is readily soluble in concentrated ammonia: Ag+(aq) + Br-(aq) " AgBr(s) AgBr(s) + 2NH3(aq) " [Ag(NH3)2]+(aq) + Br-(aq) Acidification with nitric acid is necessary to eliminate carbonate or sulphite, both of which interfere with the test by giving spurious precipitates. Oxidising agents oxidise bromide to bromine, which is yellow or orange in aqueous solution. Bromine can be extracted from the solution by shaking with an immiscible organic solvent, for example hexane, the organic layer then turning orange. A suitable oxidising agent is sodium chlorate(I); this is added to the test solution, followed by a little dilute hydrochloric acid and a few cm3 of hexane: OCl-(aq) + 2H+(aq) + 2Br-(aq) " Br2(aq) + Cl-(aq) + H2O(l) |
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Iodide I- |
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AgI, PbI2, Hg2I2 and CuI are insoluble in water. Concentrated sulphuric acid gives a mixture of hydrogen iodide, iodine, hydrogen sulphide, sulphur and sulphur dioxide when added to solid iodides; the HI produced is oxidised by sulphuric acid. The mixture evolves purple acidic fumes, turns to a brown slurry, and is a mess: NaI(s) + H2SO4(l) " NaHSO4(s) + HI(g) 2HI + H2SO4 " I2 + SO2 + 2H2O 6HI + H2SO4 " 3I2 + S + 4H2O 8HI + H2SO4 " 4I2 + H2S + 4H2O There are no state symbols in these equations because the mixture is such a mess, and is more sulphuric acid than water. Silver nitrate solution added to a solution of an iodide that has been acidified (test with blue litmus paper) with dilute nitric acid gives a yellow precipitate of silver iodide. The precipitate is insoluble even in concentrated ammonia: Ag+(aq) + I-(aq) " AgI(s) Acidification with nitric acid is necessary to eliminate carbonate or sulphite, both of which interfere with the test by giving spurious precipitates. Oxidising agents oxidise iodide to iodine, which is yellow or orange in aqueous solution. Iodine can be extracted from the solution by shaking with an immiscible organic solvent, for example hexane, the organic layer then turning purple. A suitable oxidising agent is sodium chlorate(I); this is added to the test solution, followed by a little dilute hydrochloric acid and a few cm3 of hexane: OCl-(aq) + 2H+(aq) + 2Ir-(aq) " I2(aq) + Cl-(aq) + H2O(l) Lead ethanoate or lead nitrate solutions give a bright yellow precipitate of lead(II) iodide with iodides: Pb2+(aq) + 2I-(aq) " PbI2(s) The colour of lead(II) iodide comes from interactions in the lattice; dissolving the salt in boiling water gives a colourless solution which deposits glittering yellow plates on cooling. Solutions of copper(II) salts give a brown mixture containing iodine and copper(I) iodide when added to solutions of iodides. Addition of sodium thiosulphate solution decolourises the iodine and leaves pinkish-cream copper(I) iodide as a precipitate. 2Cu2+(aq) + 4I-(aq) " 2CuI(s) + I2(aq) 2S2O32-(aq) + I2(aq) " 2I-(aq) + S4O62-(aq) This
reaction is the basis for the volumetric estimation of copper; the
iodine liberated from a known amount of a copper(II) solution is
titrated with standard sodium thiosulphate solution. |
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Sulphite SO32- |
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Sulphurous acid is considerably stronger than carbonic acid, so sulphites do not give the effervescence that is characteristic of carbonates when dilute acid is added. Dilute hydrochloric acid on warming with a sulphite evolves sulphur dioxide; this turns acidified potassium dichromate(VI) solution (or paper) green: SO32-(aq) + 2H+(aq) " H2O(l) + SO2(g) Barium chloride solution gives a white precipitate of barium sulphite; addition of dilute hydrochloric acid causes the precipitate to dissolve without effervescence: SO32-(aq) + Ba2+(aq) " BaSO3(s) |
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Sulphate SO42- |
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BaSO4, SrSO4 and PbSO4 are insoluble; CaSO4 is sparingly soluble. Barium chloride solution added to the test solution acidified with dilute hydrochloric acid gives a white precipitate of barium sulphate: Ba2+(aq) + SO42-(aq) " BaSO4(s) HSO4- does the same thing with barium ions; however the original test solution would then be very acidic, so that should be tested for. The addition of HCl destroys any carbonate or sulphite ions present so prevents the spurious positive result due to the precipitation of these barium salts. Barium nitrate solution can be used instead of barium chloride. 18.2 Lead ethanoate solution gives a precipitate of white lead sulphate: Pb2+(aq) + SO42-(aq) " PbSO4(s) |
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Nitrate NO3- |
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Since all nitrates are water soluble, there is no precipitation reaction for this ion. Solid nitrates decompose on heating; those of group 1 (except Li) give the nitrite and oxygen; 2NaNO3(s) " 2NaNO2(s) + O2(g) All others give the metal oxide, nitrogen dioxide, and oxygen. A brown gas is emitted that re-lights a glowing splint: 2Pb(NO3)2(s) " 2PbO(s) + O2(g) + 2NO2(g)
3NO3-(aq) + 8Al(s) + 18H2O(l) + 21 OH-(aq) " 8[Al(OH)6]3-(aq) + 3NH3(g) Not an equation to be remembered! |
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Carbonate CO32- |
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Only the alkali metal and ammonium carbonates are water soluble. Some carbonates (e.g. zinc, copper(II)) are basic carbonates and contain a proportion of the hydroxide in their structure. Heating decomposes all but the alkali and alkaline earth metal carbonates (at Bunsen temperatures) giving the oxide and carbon dioxide: CuCO3(s) " CuO(s) + CO2(g) Dilute hydrochloric acid gives vigorous effervescence with carbonates, evolving carbon dioxide: CO32-(aq or s) + 2H+(aq) " H2O(l) + CO2(g)
Bicarbonates also give this effervescence. The reaction of carbonates
with acid is exothermic; bicarbonates react endothermically. |
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Hydrogen carbonate (bicarbonate) HCO3- |
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Only the alkali metal and ammonium bicarbonates are obtainable as solids; group 2 bicarbonates exist only in solution. Calcium chloride solution on addition to a bicarbonate solution gives no precipitate since calcium bicarbonate is soluble; this distinguishes it from carbonate, which does give a precipitate. On heating the calcium chloride/bicarbonate mixture a white precipitate appears since the bicarbonate decomposes to carbonate: Ca2+(aq) + 2HCO3-(aq) " CaCO3(s) + CO2(g) + H2O(l) |
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Ethanoate CH3COO- |
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Ethanoates on heating with dilute hydrochloric acid give ethanoic acid, recognisable by its vinegary smell. Neutral iron(III) chloride solution added to neutral solutions of ethanoate ion give a deep red colouration owing to formation of iron(III) ethanoate. |
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Ethanedioate C2O42- |
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Concentrated sulphuric acid added to a solid ethanedioate salt gives a mixture of carbon monoxide and carbon dioxide from dehydration of the ethanedioic acid formed: HOOC-COOH " H2O + CO2 + CO Potassium manganate(VII) solution acidified with dilute sulphuric acid added to a solution of an ethanedioate causes the purple colour to disappear: 2MnO4-(aq) + 5C2O42-(aq) + 16H+(aq) " 2Mn2+(aq) + 10CO2(g) + 8H2O(l) Calcium chloride solution added to a solution of an ethanedioate gives a white precipitate of calcium ethanedioate: Ca2+(aq) + C2O42-(aq) " CaC2O4(s) The precipitate dissolves readily in dilute hydrochloric acid. |
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Chromate(VI) CrO42- and dichromate(VI) Cr2O72- |
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These ions are related through the equilibrium Cr2O72-(aq) + 2OH-(aq) D 2CrO42-(aq) + H2O(l) In alkaline solution the yellow chromate(VI) dominates, in acidic solution orange dichromate(VI). All dichromate(VI) salts are soluble; addition of dichromate(VI) ions to solutions of ions of metals which have insoluble chromate(VI) salts leads to the precipitation of chromates. This means that the only dichromates that can exist are those of group 1 metals, ammonium, magnesium, calcium and strontium. Barium chloride solution added to a chromate(VI) or dichromate(VI) solution precipitates bright yellow barium chromate(VI): Ba2+(aq) + CrO42-(aq) " BaCrO4(s) The addition of a heavy metal ion to potassium dichromate solution precipitates the chromate and therefore moves the equilibrium to the right hand side. If the chromate is sparingly soluble (e.g. strontium) the supernatant liquid will remain yellow. Very insoluble chromates, such as lead, remove all the colour from the supernatant liquid. Dichromate(VI) ion solution in sulphuric acid is an oxidising agent; oxidation is shown by the solution turning from orange to green (Cr(III)). The following can be oxidised: (a) iron(II) to iron(III): Cr2O72-(aq) + 14H+(aq) + 6Fe2+(aq) " 2Cr3+(aq) + 7H2O(l) + 6Fe3+(aq) (b) iodide to iodine (the solution turns murky greenish-brown): Cr2O72-(aq) + 14H+(aq) + 6I-(aq) " 2Cr3+(aq) + 7H2O(aq) + 3I2(aq) (c) sulphite to sulphate: Cr2O72-(aq) + 8H+(aq) + 3SO32-(aq) " 2Cr3+(aq) + 4H2O(l) + 3SO42-(aq) (d) nitrite to nitrate: Cr2O72-(aq) + 8H+(aq) + 3NO2-(aq) " 2Cr3+(aq) + 4H2O(l) + 3NO3-(aq) (e) hydrogen peroxide reacts with acidified dichromate(VI) solutions to give a blue compound that rapidly turns green and evolves oxygen. The blue compound can be extracted into an organic solvent such as butan-1-ol. The blue compound is CrO5, which contains a peroxy structure. It is covalent, and is stable in organic solvents though not in water. Cr2O72-(aq) + 8H+(aq) + 3H2O2 (aq) " 2Cr3+(aq) + 7H2O(l) + 3 O2(g) Alcohols
are oxidised by acidified potassium dichromate(VI) solution. Primary alcohols
give aldehydes and then acids, secondary alcohols give ketones. Ethanol can be used to test for
dichromate(VI) ions, therefore, the solution turning green and the apple smell of ethanal
being evident. |
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© JRG Beavon 2007