INORGANIC Qualitative TESTS Anions and Alkalis

Sulphate ion or sulphate(VI) ion SO₄^2-

If the solution also contains the chloride ion, you test with barium ions 1st, filter off any barium sulphate precipitate and then test for chloride ion. This is because silver sulphate is also ~insoluble.

(i) To a solution of the suspected sulfate add dilute hydrochloric and a few drops ofbarium chloride/ nitrate solution.

(ii) Add lead(II) nitrate solution.

(i) A white precipitate of barium sulfate.

(ii) A white precipitate of lead(II) sulphate.

Test (i) is more definitive.

(i) Ba²⁺_(aq) + SO₄^2-_(aq) ==>BaSO_4(s)

Any soluble barium salt + any soluble sulphate forms a white dense barium sulphate precipitate.

(ii) Pb²⁺_(aq) + SO₄^2-_(aq) ==>PbSO_4(s)

Neither white precipitate is soluble in excess hydrochloric acid.

Sulphite ion or sulphate(IV) ion SO₃^2-

Test (iii) is easily unreliable, the sulphite ion is oxidised by air (dissolved oxygen) to give the sulphate ion, so you will lucky to obtain a clear solution after adding excess acid.

(i) Add dilute hydrochloric acid to the suspected sulfite.

(ii) Test any gas evolved with fresh potassium dichromate(VI) paper.

(iii) Add barium chloride or barium nitrate solution.

(i) Acrid choking sulfur dioxide gas formed.

(ii) The dichromate paper turns from orange to green.

(iii) A white ppt. of barium sulphite which dissolves in excess hydrochloric acid to give a clear colourless solution.

(i) Any sulphite salt + hydrochloric acid ==> chloride salt + sulphur dioxide.

(ii) The sulphur dioxide reduces the dichromate(VI) to chromium(III). Note: sulphites do  not give ppt. with acidified barium chloride/nitrate because sulphites dissolve in acids.

(iii) Ba²⁺_(aq) + SO₃^2-_(aq) ==>BaSO_3(s)

BaSO_3(s) + 2HCl_(aq) ==>BaCl_2(aq) + H₂O_(l) + SO_2(aq)

Sulphide ion S^2-

In test (ii) dangerous hydrogen sulphide is formed.

(i) If soluble, add a few drops lead(II) ethanoate solution.

(ii) If solid, add dil. HCl(aq) acid, test smelly gas with damp lead(II) ethanoate paper (old name lead acetate).

(i) Black ppt. of lead sulphide.

(ii) Rotten egg smell of hydrogen sulphide and the H₂S gas turns lead(II) ethanoate paper black.

(i) Pb²⁺_(aq) + S^2-_(aq) => PbS_(s) 

(ii) MS_(s) + 2H⁺_(aq) => M²⁺_(aq) +H₂S_(g) (e.g. M = Pb, Fe, Cu, Ni etc.) Then reaction (i) above occurs on the lead(II) ethanoate paper (old name lead acetate).

Chloride ion

Cl^-

If the solution also contains the sulphate ion, you test with barium ions 1st, filter off any barium sulphate precipitate and then test for chloride ion. This is because silver sulphate is also ~insoluble, so the two precipitates of silver sulfate and silver chloride could not be distinguished

(i) If the chloride is soluble, add dilute nitric acid and silver nitrate solution. The silver nitrate is acidified with dilute nitric acid to prevent the precipitation of other non-halide silver salts.

(ii) If insoluble salt, add conc. sulphuric acid, warm if necessary then test gas as for HCl.

(iii) Add lead(II) nitrate solution. Not a very specific test - test (i) is best.

(i) white precipitate of silver chloride soluble in dilute ammonia.

(ii) You get nasty fumes of hydrogen chloride which turn blue litmus red and give a white precipitate with silver nitrate solution.

(iii) A white ppt. of lead(II) chloride is formed.

(i) Ag⁺_(aq) + Cl^-_(aq) ==> AgCl_(s)

Any soluble silver salt + any soluble chloride  gives a white silver chloride precipitate, that darkens in light.

(ii) Cl^-_(s) + H₂SO_4(l) ==> HSO₄^-_(s)+ HCl_(g) ,

then Ag⁺_(aq) + Cl^-_(aq) ==> AgCl_(s)

(iii) Pb²⁺_(aq) + 2Cl^-_(aq) ==>PbCl_2(s)

Bromide ion

Br^-

(i) If bromide soluble, add dilute nitric acid and silver nitrate solution. The silver nitrate is acidified with dilute nitric acid to prevent the precipitation of other non-halide silver salts.

(ii) If insoluble salt, add conc. sulphuric acid, warm if necessary.

(iii) Add lead(II) nitrate solution. Not a very specific test - test (i) is best.

(i) Cream precipitate of silver bromide, only soluble in concentrated ammonia.

(ii) Orange vapour of bromine and pungent fumes of SO₂, test for sulphur dioxide.

(iii) A white ppt. of lead(II) bromide is formed.

(i) Ag⁺_(aq) + Br^-_(aq) ==> AgBr_(s)

Any soluble silver salt + any soluble bromide gives a cream silver bromide precipitate.

(ii) The bromide ion is oxidised to bromine and the sulphuric acid is reduced to sulphur dioxide.

(iii) Pb²⁺_(aq) + 2Br^-_(aq) ==>PbBr_2(s)

Fluoride Ion

F^-

Fluoride and hydrogen fluoride gas are harmful, irritating and corrosive substances.

(i) If the suspected fluoride is soluble add dilute nitric acid and silver nitrate solution.

(ii) You can warm a solid fluoride with conc. sulphuric acid and hold in the fumes (ONLY!) a glass rod with a drop of water on the end.

(i) There is NO precipitate!

(ii) Look foretching effects on the surface of the glass rod.

(i) Silver fluoride, AgF, is moderately soluble so this test proves little except that it isn't chloride, bromide and iodide!

(ii) Hydrogen fluoride gas is produced by displacement

F^- + H₂SO₄ ==> HSO₄^- + HFwhich reacts with the glass silica to form silicic acid, silicon oxyfluoride, silicon fluoride. The chemistry is messy and complexBUT the glass rod is clearly etched.

Iodide ion

I^- 

(i) If iodide soluble, add dilute nitric acid and silver nitrate solution. The silver nitrate is acidified with dilute nitric acid to prevent the precipitation of other non-halide silver salts.

(ii) If insoluble salt can heat with conc. sulphuric acid, (ii) get purple fumes of iodine and very smelly hydrogen sulphide.

(iii) If iodide soluble, add lead(II) nitrate solution.

(i) Yellow precipitate of silver iodide insoluble in concentrated ammonia.

(ii) purple vapour and rotten egg smell!

(iii) Yellow precipitate of lead(II) iodide. Not too definitive -Test (i) best.

(i) Ag⁺_(aq) + I^-_(aq) ==> AgI_(s) , any soluble silver salt + any soluble iodide  ==> silver iodide precipitate,

(ii) iodide ion is oxidised to iodine and the sulphuric acid is reduced to 'rotten eggs' smelly hydrogen sulphide,

(iii) insoluble lead(II) iodide formed

Pb²⁺_(aq) + 2I^-_(aq) ==> PbI_2(s)

Nitrate ion or nitrate(V) ion NO₃^-(i) Boil the suspected nitrate with sodium hydroxide solution and fine aluminium powder (Devarda's Alloy) or aluminium foil.

(ii) Add iron(ii) sulphate solution and then conc. sulphuric acid (the 'brown ring' test)

(iii) Strongly heating nitrates of M²⁺ salts.

(i) the fumes contain ammonia, which turns red litmus blue, seeammonia test details

(ii) Where the liquids meet a brown ring forms

(iii) Nasty brown gas (beware!) ofnitrogen (IV) oxide (nitrogen dioxide)

(i) The aluminium powder is a powerful reducing agent and converts the nitrate ion, NO₃^-, into ammonia gas, NH₃

(ii) NO complex of iron(II) formed

(iii) a general thermal decomposition equation for this reaction is

2M(NO₃)_2(s) ==> 2MO_(s) + 4NO_2(g) + O_2(g)

where M = Pb, Zn, Mg, Cu etc.

Nitrite ion or nitrate(III) ion NO₂^-No simple test to clearly i.d. it, (i) in acid solution it decomposes to give colourless NO gas which rapidly oxidises to nasty brown fumes of NO₂, (ii) it decolourises (purple ==> colourless) acidified potassium manganate(VII), (iii) it liberates iodine from acidified potassium iodide solution, (iv) forms ammonia with hot Al powder-foil/NaOH_(aq) (see nitrate test) and gives 'brown ring' test Ammonium ion

NH₄⁺

If no smell at first, add COLD sodium hydroxide solution to the suspected ammonium salt and test any gas with red litmus.Smelly ammonia released! and red litmus turns blue.Ammonia gas is evolved:

 NH₄⁺_(aq) + OH^-_(aq) ==> NH_3(g) + H₂O_(l)

Acids  Hydrogen ion i.e. H⁺ or H₃O⁺ ion (note: to completely identify acids you need to test for the anion e.g. chloride for HCl etc.)(i) Litmus or universal indicator or pH meter. 

(ii) Add a little sodium hydrogencarbonate powder.

(i) Litmus turns red, variety of colours with univ. ind. strong - red, weak - yellow /orange, depending on strength of acid.

(ii) Fizzing with any carbonate -test for CO₂ as above.

(i) A pH meter reading gives a value of less than 7, the lower the pH number the stronger the acid, the higher the H⁺ concentration,

(ii) HCO₃^-_(aq) + H⁺_(aq) ==> H₂O_(l)+ CO_2(g)

Alkali:   Hydroxide ion i.e. a soluble base (alkali) which forms the OH^- ion in water(note: to completely identify alkalis you need to test for the cation e.g. sodium for NaOH etc.)(i) Litmus or universal indicator or pH meter.

(ii) Add a little of an ammonium salt.

(i) It turns litmusblue, variety of colours univ. ind. dark green - violet for weak - strong.

(ii) If strongly alkaline ammonia should be released, see ammonia testfor rest of details

(i) A pH meter gives a value of more than 7, the higher the pH number the stronger the alkali, the higher the OH^- concentration, (ii) ammonia gas is evolved:

(ii) Ammonia released from the salt.

NH₄⁺_(aq) + OH^-_(aq) ==> NH_3(g) + H₂O_(l)

Chromate(VI) ion

CrO₄^2- (yellow)

These tests are not very definitive, but collectively they are a good 'pointer'!

(i) Add dilute sulphuric acid.

(ii) Add barium chloride/nitrate solution.

(iii) Add lead(II) nitrate solution.

(i) The yellow solution turns orange as the dichromate(VI) ion is formed.

(ii) A yellow precipitate of barium chromate(VI) is formed.

(iii) A yellow precipitate of lead(II) chromate(VI) is formed. 'lead chromate'

(i) CrO₄^2-_(aq) + 2H⁺_(aq) ==> Cr₂O₇^2-_(aq)

(ii) Ba²⁺_(aq) + CrO₄^2-_(aq) ==> BaCrO_4(s)

(iii) Pb²⁺_(aq) + CrO₄^2-_(aq) ==> PbCrO_4(s)