test: covered earlier. There
are no simple reagents that will precipitate sodium compounds.
test: covered earlier. There
are no simple reagents that will precipitate potassium compounds.
Dilute sulphuric acid gives a white precipitate of calcium sulphate if the original solution is fairly concentrated:
Ca2+(aq) + SO42-(aq) " CaSO4(s)
Calcium sulphate solution is permanently hard water; its solubility is about 0.05
mol dm-3 at 25oC.
Ca2+(aq) + CO32-(aq)
Ammonium ethanedioate solution precipitates white calcium ethanedioate from neutral
or alkaline solutions. The precipitate dissolves readily in dilute acid:
Ca2+(aq) + C2O42-(aq) " CaC2O4(s)
ethanedioate (calcium oxalate) is found in rhubarb leaves - it is what makes them
Sodium hydroxide solution precipitates grey-green chromium(III) hydroxide, which
reacts with excess NaOH to give a deep green solution of the chromite ion:
[Cr(H2O)6]3+(aq) + 3OH-(aq)
" Cr(OH)3(s) + 6H2O(l)
Cr(OH)3(s) + 3OH-(aq) " [Cr(OH)6]3-(aq)
solution precipitates grey-green chromium(III) hydroxide, which with excess ammonia
very slowly (see below) forms pinkish solutions of ammines. Ammines
are transition metal complexes with ammonia, NH3. Do not confuse them with
amines RNH2.The initial equation is as the
first one above; then
Cr(OH)3(s) + 4NH3(aq) + 2H2O(l)
" [Cr(NH3)4(H2O)2]3+(aq) + 3OH-(aq)
The exact composition of the ammine will depend on
the amount of ammonia used. However, the reaction is so slow that
several days are needed to see anything at all.
However, the reaction is so slow that several days are needed to see anything at all.
agents convert chromium(III) to yellow chromate(VI) in the presence of alkali. Hydrogen
peroxide is favoured:
2[Cr(H2O)6]3+(aq) + 3H2O2(aq) + 10OH-(aq)
" 2CrO42-(aq) + 14H2O(l)
The test solution is made alkaline with NaOH
solution, then an equal volume of 20 volume hydrogen peroxide solution is
added and the mixture boiled.
20-volume hydrogen peroxide
solution gives 20cm3 of oxygen per cm3 of solution
that is decomposed to oxygen and water.
Sodium hydroxide solution
precipitates beige manganese(II) hydroxide. This is insoluble in excess reagent, and
rapidly darkens owing to oxidation to hydrated manganese(IV) oxide:
[Mn(H2O)6]2+(aq) + 2OH-(aq)
" Mn(OH)2(s) + 6H2O(l)
4Mn(OH)2(s) + 2H2O(l) + O2(aq) " 4MnO2.H2O(s)
Sodium or potassium carbonate solution gives a white precipitate of manganese(II) carbonate:
[Mn(H2O)6]2+(aq) + CO32-(aq) " MnCO3(s) + 6H2O(l)
Lead(IV) oxide in the presence of nitric acid converts manganese(II) into
purple manganate(VII); an alternative oxidising agent is sodium bismuthate(V), NaBiO3:
5PbO2(s) + 2Mn2+(aq)
" 2MnO4-(aq) + 5Pb2+(aq) + 2H2O(l)
5BiO3-(aq) + 14H+(aq) + 2Mn2+(aq)
" 5Bi3+(aq) + 2MnO4-(aq) + 7H2O(l)
A small amount of lead(IV) oxide or of sodium bismuthate(V) is added to the test
solution, 6 or so drops of concentrated nitric acid added, and the mixture boiled.
Filtration will give a purple filtrate if Mn2+ was present.
Aqueous iron(III) ions are not [Fe(H2O)6]3+; this ion is an amethyst (pale purple) colour, and is found only in solids such as aluminium iron(III) sulphate. In solution the hexaaqua ion is readily deprotonated by solvent water; the solution is acidic, and the yellow ion is [Fe(H2O)5OH]2+:
[Fe(H2O)6]3+ + H2O " [Fe(H2O)5OH]2+ + H3O+.
Sodium hydroxide solution
precipitates foxy-red iron(III) hydroxide, insoluble in excess alkali:
[Fe(H2O)6]3+(aq) + 3OH-(aq)
" Fe(OH)3(s) + 6H2O(l)
Ammonia solution reacts in the same way as sodium hydroxide iron does not form
complexes with ammonia.
Potassium hexacyanoferrate(II) precipitates dark blue potassium iron(III) hexacyanoferrate(II), Prussian Blue:
K+(aq) + Fe3+(aq) + [Fe(CN)6]4-(aq) " KFeIII[FeII(CN)6](s)
Prussian Blue is the pigment that is used to print 'blueprints'. It was also used for Prussian army uniforms, and for the locomotives and rolling stock of the Somerset & Dorset Joint Railway, pre-1923.
dirty green iron(II) hydroxide; on standing the surface of the precipitate turns foxy-red
owing to air oxidation to iron(III) hydroxide:
[Fe(H2O)6]2+(aq) + 2OH-(aq)
" Fe(OH)2(s) + 6H2O(l)
Ammonia solution behaves similarly to sodium hydroxide again there are no ammine complexes.
Potassium hexacyanoferrate(III) precipitates dark blue potassium iron(II)
hexacyanoferrate(III), Turnbulls Blue:
K+(aq) + Fe2+(aq) + [Fe(CN)6]3-(aq) " KFeII[FeIII(CN)6](s)
Although Turnbull's Blue and Prussian Blue don't have quite the same colour, they are in fact the same substance.
Potassium manganate(VII) oxidises iron(II) to iron(III) in acidic solution. The
purple manganate colour is lost and the resulting solution of iron(III) is yellow:
MnO4-(aq) + 5Fe2+(aq) + 8H+(aq)
" Mn2+(aq) + 5Fe3+(aq) + 4H2O(aq)
dichromate(VI) oxidises iron(II) to iron(III) in acidic solution. The orange dichromate
colour is lost and the resulting solution is green owing to presence of chromium(III)
which masks the iron(III) colour:
Cr2O72-(aq) + 6Fe2+(aq) + 14H+(aq) " 2Cr3+(aq) + 6Fe3+(aq) + 7H2O(l)
solution gives a pale blue precipitate usually described as copper(II) hydroxide:
[Cu(H2O)6]2+(aq) + 2OH-(aq) " Cu(OH)2(s) + 6H2O(l)
In fact this precipitate is the basic salt
thus from copper(II) sulphate solution the product is basic copper sulphate:
2[Cu(H2O)6]2+(aq) + 2OH-(aq) + SO42-(aq) " Cu(OH)2.CuSO4(s) + 12H2O(l)
Basic copper sulphate or basic copper carbonate is responsible for the green patina -
verdigris - found on weathered copper roofs. The sulphate is present in industrial
To get true copper(II)
hydroxide, the solution of the copper(II) salt must be added to excess sodium hydroxide;
the precipitate is then blue rather than turquoise. In both cases CAUTIOUS warming of the mixture causes the precipitate to lose water
and turn black as CuO(s) is formed.
Ammonia solution initially gives a blue precipitate as for sodium hydroxide. Further addition of ammonia gives deep blue soluble cuprammine complexes whose composition depends on the amount of ammonia present. The reaction is usually represented:
Cu(OH)2(s) + 4NH3(aq) + 2H2O(l) " [Cu(NH3)4(H2O)2]2+(aq) +2OH-(aq)
The cuprammine solution has the bizarre property of being able to dissolve cellulose - bits of filter paper, for e example. The resulting viscous substance can be extruded into a bath of dilute sulphuric acid to give viscose fibre, which is a form of artificial silk.
or a saturated solution of sodium chloride gives a bright green
solution containing CuCl42- ions; dilution with water causes the
solution to turn pale blue as the [Cu(H2O)6]2+ ion
[Cu(H2O)6]2+(aq) + 4Cl-(aq)
" CuCl42-(aq) + 6H2O(l)
Sodium carbonate solution
precipitates greenish-blue basic carbonates of indefinite composition.
Potassium iodide solution precipitates iodine and copper(I) iodide; this reaction
is used volumetrically for the estimation of copper, the liberated iodine being titrated
with sodium thiosulphate solution:
2[Cu(H2O)6]2+(aq) + 4I-(aq)
" 2CuI(s) + I2(aq) + 6H2O(l)
The mixture turns a sludgy brown colour; addition of sodium thiosulphate solution leaves a creamy-pink precipitate of copper(I) iodide.
Sodium hydroxide solution precipitates white zinc(II) hydroxide, easily soluble in
excess sodium hydroxide to give sodium zincate(II), because the hydroxide is amphoteric:
[Zn(H2O)6]2+(aq) + 2OH-(aq)
" Zn(OH)2(s) + 6H2O(l)
Zn(OH)2(s) + 2OH-(aq)
Ammonia solution initially precipitates zinc(II) hydroxide, as with sodium
hydroxide. Excess ammonia causes the precipitate to disappear owing to the formation of
ammine complexes a different reason from that in 10.1:
Zn(OH)2(s) + 4NH3(aq)
" [Zn(NH3)4]2+(aq) + 2OH-(aq)
Sodium carbonate solution precipitates a white basic carbonate:
5Zn2+(aq) + 2CO32-(aq) + 6OH-(aq)
This latter substance is what you get when you buy
zinc carbonate which is why on heating it gives off water vapour as well as carbon
Aluminium Al[H2O] 3+
Sodium hydroxide solution
precipitates white gelatinous aluminium hydroxide. This reacts with excess NaOH to give a
colourless solution of sodium aluminate:
[Al(H2O)6] 3+(aq) + 3OH-(aq) " Al(OH)3(s) + 6H2O(l)
Al(OH)3(s) + 3OH-(aq)
Ammonia solution precipitates white aluminium hydroxide, as with NaOH; however it
does not react further with excess ammonia.
Dilute hydrochloric acid or
other soluble chlorides precipitate white lead(II) chloride from moderately concentrated
solutions of lead(II) salts.
Pb2+(aq) + 2Cl-(aq) " PbCl2(s)
dissolves in an excess of concentrated hydrochloric acid to give yellow solutions
containing various species including PbCl3- and PbCl42-.
Sodium hydroxide solution gives a white precipitate of lead(II) hydroxide which is
soluble in excess NaOH to give colourless sodium plumbate(II)
Pb2+(aq) + 2OH-(aq)
Ammonia solution precipitates white lead(II) hydroxide; this does not dissolve in
excess ammonia, because no complexes are formed and ammonia is not a strong enough base to
bring out the amphoteric properties of lead(II) hydroxide.
sulphuric acid or other soluble sulphates give a white precipitate of lead(II) sulphate:
Pb2+(aq) + SO42-(aq)
Potassium chromate(VI) solution gives a bright yellow precipitate of lead(II) chromate(VI):
Pb2+(aq) + CrO42-(aq)
Potassium iodide solution gives a bright yellow precipitate of lead(II) iodide.
This will dissolve in boiling water to give a colourless solution the colour comes
from interactions in the crystal lattice rather than from coloured ions:
Pb2+(aq) + 2I-(aq) " PbI2(s)
Heat: all ammonium salts
decompose on heating; ammonium nitrate may explode. Ammonium chloride and sulphate give
products that recombine on cooling, so that the salts apparently sublime.
NH4Cl(s) " NH3(g) + HCl(g)
NH4NO3(s) " N2O(g) + 2H2O(g)
+ SO3(g) + H2O(g)
Ammonium dichromate(VI) decomposes spectacularly on ignition in a reaction that is
oxidation of the cation by the anion; the initially orange solid gives a fluffy green
product of much larger volume:
" N2(g) + Cr2O3(s) + 4H2O(g)
Alkalis (sodium hydroxide, calcium hydroxide) liberate ammonia from ammonium salts
on warming with the solution, or even from a mixture of the solids. This is because OH-
is a stronger base than ammonia, so removes a hydrogen ion from the ammonium ion:
NH4+(aq) + OH-(aq)
" NH3(g) + H2O(l)
The test solution is warmed with sodium hydroxide solution and the vapours tested with moist red litmus paper. It is important to test the vapours immediately heating begins, since the ammonia is lost very quickly and by the time the solution boils it may well have all gone.
The (+2) state is the most stable for simple cobalt salts; they are coloured pink or blue. The pink colour of the hydrated ion [Co(H2O)6]2+ changes to blue on heating, on dehydration, or in the presence of concentrated acids.
Sodium hydroxide solution precipitates a blue basic salt which on warming with excess sodium hydroxide forms solid pink cobalt(II) hydroxide. With a solution of cobalt(II) nitrate the equations are:
Basic salts, usually hydroxides or carbonates, contain not only the expected hydroxide or carbonate ion but also the anion from the original salt. The precipitate from cobalt(II) nitrate solution is therefore different from that which would be given from cobalt(II) chloride. The latter would be Co(OH)Cl.
If the pink precipitate is boiled for some time (or is warmed with hydrogen peroxide solution) it is converted to brownish-black cobalt(III) hydroxide:
4Co(OH)2 (s) + 2H2O (l) + O2 (aq) " 4Co(OH)3 (s)
Ammonia solution gives a blue basic precipitate as with sodium hydroxide; excess ammonia converts this to a brown solution of a cobaltammine which turns red on exposure to air giving an ammine of cobalt(III). The overall reaction is:
Co(OH)NO3 (s) + 28NH3(aq) + 6H2O(l) + O2(aq) " 4[Co(NH3)6](OH)3(aq) + 4NH4NO3(aq)
Sodium hydroxide solution gives a green gelatinous precipitate of nickel(II) hydroxide, insoluble in excess of the reagent:
[Ni(H2O)6]2+ (aq) + 2OH (aq) " Ni(OH)2 (s) + 6H2O(l)
Ammonia solution gives a green precipitate of a basic salt which dissolves readily in excess ammonia solution to form a blue-violet solution of complex nickel ammines. With nickel(II) chloride the reactions are:
[Ni(H2O)6]2+(aq) + Cl (aq) + NH3(aq) " Ni(OH)Cl(s) + NH4+(aq) + 5H2O(l)
Ni(OH)Cl(s) + 5NH3(aq) + H2O(l) " [Ni(NH3)4](OH)2(aq) + NH4+(aq) + Cl (aq)
+ 7NH3(aq) + H2O(l)
+ NH4+(aq) + Cl (aq)
© JRG Beavon 2007