3.31 AS Inorganic Chemistry - Inorganic analysis
Chemical analysis is used to determine either the identity or the quantity of a species in a sample. This section deals with the qualitative analysis, i.e. the identity of ions in aqueous solutions.
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Cations - positive ions
Most chemical tests for ions take place in aqueous solution, relying on the formation of insoluble salts from a combination of the ion being sought with a suitable counter-ion.
However, the ions formed by group 1 metals do not make precipitates with any common counter-ions, so another form of analysis must be carried out.
The group 1 metal ions (and some group 2 metal ions) give characteristic colours to a bunsen flame when a small sample of a crystalline salt is placed at the edge of the flame..
- 1 Dip a piece of nichrome wire into some concentrated hydrochloric acid. This serves two purposes - it cleans the wire of extraneous ions and leaves a little HCl on the wire which helps to form some volatile chloride of the new salt.
- 2 Dip the wire into the salt sample to be tested
- 3 Hold the salt into the very edge of a transparent hot bunsen flame.
- 4 Observe the colour of the flame. It may only last for a second or so.
|Calcium ions||Ca2+||brick red|
|Barium ions||Ba2+||apple green|
Analysis by precipitation
Most metal ions (apart from group I) give precipitates with hydroxide ions. The colour of the precipitate and its behavious towards excess base help to identify the metal ion.
There are three possibilities on addition of dilute sodium hydroxide solution to an unknown metal ion.
- 1 No precipitate
- 2 White precipitate
- 3 Coloured precipitate
If 1 - no precipitate, then a flame test must be carried out.
If 2 - white precipitate, this could indicate either Mg2+, Zn2+, Al3+, Pb2+
On addition of excess NaOH solution the precipitate formed by Al3+ redissolves.
Al(OH)3(s) + OH-(aq) [Al(OH)4]-(aq)
Addition of concentrated ammonia solution dissolves the precipitate formed by Zn2+.
Zn(OH)2(s) + NH3(aq) [Zn(NH3)4]2+(aq) + 2OH-
Neither Mg(OH)2 nor Pb(OH)2 are affected by excess NaOH or NH3(aq). These two ions can be differentiated by the addition of potassium iodide solution. This gives a yellow precipitate with lead ions:
Pb2+(aq) + I-(aq) PbI2(s)
If 3 - coloured precipitate:
Blue precipitate - indicative of copper(II) ions:
Cu2+(aq) + 2OH-(aq) Cu(OH)2(s)
Green gelatinous precipitate - indicative of iron(II) ions - precipitate turns brown at the surface due to aerial oxidation to iron(III):
Fe2+(aq) + 2OH-(aq) Fe(OH)2(s)
Brown gelatinous precipitate - indicative of iron(III) ions:
Fe3+(aq) + 3OH-(aq) Fe(OH)3(s)
Blue precipitate from a red solution - indicative of cobalt(II) ions - turns grey at surface due to aerial oxidation to cobalt(III):
Co2+(aq) + 2OH-(aq) Co(OH)2(s)
Specific cation reactions
Copper(II) ions in aqueous solution react with ammonia solution forming a light blue precipitate of copper(II) hydroxide.
[Cu(H2O)6]2+(aq) + 4NH3(aq) [Cu(H2O)4(OH)2](s) + 2NH4+
This precipitate redissolves forming a deep blue solution of the tetramminecopper(II) complex ion on addition of excess ammonia solution.
[Cu(H2O)4(OH)2](s) + 4NH3(aq) [Cu(NH3)4(H2O)2]2+(aq) + 2OH-
Anions- negative ions
The negative ions that we are interested in are:
chloride, bromide , iodide, hydroxide, carbonate, sulfate, sulfite, nitrate.
A straightforward scheme allows all anions to be analysed:
Addition of barium chloride (or barium nitrate)
Barium chloride solution contains soluble barium ions, Ba2+(aq). These can interact with sulfate, sulfite and carbonate ions in solution to form precipitates:
Ba2+(aq) + SO42-(aq) BaSO4(s)
Initially, there are two possibilities
- 1 White precipitate
- 2 No precipitate
If 1 - white precipitate, add dilute hydrochloric acid. There are three possibilities:
- A The precipitate redissolves with effervescence
- B The precipitate redissolves without effervescence (odour of sulfur dioxide)
- C There is no change
If A - the anion is a carbonate
BaCO3(s) + 2HCl(aq) BaCl2(aq) + CO2(g) + H2O(l)
If B - the anion is a sulfite (although sulfur(IV) oxide is formed, it is too soluble for effervescence)
BaSO3(s) + 2HCl(aq) BaCl2(aq) + SO2(g) + H2O(l)
If C - the anion is a sulfate
BaSO4(s) + 2HCl(aq) no reaction
If 2 - take a fresh sample and add silver nitrate solution in dilute nitric acid.
Addition of silver nitrate/dilute nitric acid
Silver nitrate solution contains soluble silver ions, Ag+(aq). These can interact and form precipitates with sulfates, sulfites, carbonates and halide ions. For this reason the barium chloride test must be carried out first to eliminate the possibility of sulfate, sulfite and carbonate.
Initially, there are two possibilities:
- A - Precipitation
- B - No precipitation
If A - examine the colour of the precipitate.
Yellow precipitate = iodide
Ag+(aq) + I-(aq) AgI(s)
Cream coloured precipitate = bromide
Ag+(aq) + Br-(aq) AgBr(s)
White precipitate = chloride
Ag+(aq) + Cl-(aq) AgCl(s)
Note that all three precipitates darken slowly on exposure to light.
If B - no precipitate, take a fresh sample of the unknown and add sodium hydroxide and aluminium powder.
Addition of sodium hydroxide/aluminium powder
Warm the mixture and test for any gas given off using damp indicator paper. There are two possibilities:
- 1 - Indicator paper turns blue = nitrate ions
- 2 - Indicator paper remains unchanged = phosphate ions
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