AS Level Qualitative Analysis (Inorganic Chemistry Experiments)

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AS level Qualitative Analysis (inorganic chemistry experiments)

Qualitaitive analysis concerns the detection and identification of ions present in substances or in mixtures. These can solids or solutions. Simple tests with suitable reagents are carried out in test tubes to identify the ions in the mixture.

A systematic approach to salt analysis may involve the following steps;

(a)  The initial examination of the substance.

(b)  Heating the solid

(c)  Adding concentrated sulphuric acid and heating

(d)  Making a stock solution of the substance.

(e)  Detecting the ions by carrying out prescribed tests.

A.  Initial examination or observation of the sample.

The physical appearance of the sample can give an indication of its nature and identity. Its colour, smell and for solids whether crystalline or powdered can be used to draw tentative initial conclusions about the sample. Some guidelines are given below.

(a)  Solid samples

Colour of sample / Probable ions
Brown / Fe3+
Pink / Mn2+
yellow / PbO, ZnO(yellow hot & white cold), CrO42-
Light green / Cu2+, Fe2+
Dark green / Cr3+
Blue / Cu2+
Black / Cu2+(in CuO), Mn4+ (in MnO2)
Purple / Cr3+, MnO4-
Orange / Cr2O72-, Pb2+ (in PbO)
White / compound of s-block element or no transition metal cation

(b)  Aqueous solutions.

Colour of solution / Inference
Brown- yellow / I2, Fe3+
Orange / Br2, Cr2O72-
yellow / CrO42-
Green / Cr3+, Fe2+, Cu2+
Purple / MnO4-, Cr3+
Pink / MnO4-(dilute), Mn2+
Blue / Cu2+
colourless / No transition metal cations.

B.  Heating the solid

The solid is at first gently heated and then more strongly heated. Observations may be made as the solid is heated. These include;

(i)  Whether the solid melts then decomposes or simply decomposes without melting.

(ii)  Colour changes during heating and final colour of residue when cold.

(iii)  Evolution of gas or vapour. This should be tested and a summary of test that gives positive result given.

The table below gives some observations and inferences;

Observation / Inference
Solid appears to melt / Hydrated salt (had water of crystallisation).
White sublimate / NH4+ may be present
Charring, solid becomes black / May be an organic substance, contain CH3COO- or C2O42- ions.
Colourless gas which turns limewater milky / CO32- or HCO3- present
SO2 evolved, turns dichromate green / SO32- present or HSO32-
O2, rekindles glowing splint / Group 1 metal nitrate or oxide
Brown gas, NO2 / Heavy metal nitrate
Residue is yellow when hot and white when cold / Zn2+
Yellow residue / Pb2+
Solid cracks and decrepitates / Pb(NO3)2
NO2 and O2 / Pb(NO3)2, Group 2 nitrates.

C.  Warming with concentrated sufuric acid.

This is used mainly to test for the presence of halide ions in solid samples.

The sample is moistened with two or three drops of the acid and gently warmed.

Observation / Deduction or inference
White fumes / Cl- present, gas evolved is HCl
White fumes followed by reddish brown vapour / Br- present, HBr and Br2 evolved
Very little white fumes, violet vapour and black solid / I-  present, HI, H2S and I2 produced.
Brown fumes which become denser with a few Cu turnings. / NO3-

D.  Addition of dilute HCl to solid or solution.

1.  This usually results in the evolution of gases from samples with the following anions;

Gas produced / Ions present
CO2 / CO32- or HCO3-
SO2 / SO32- or HSO3- or S2O32-
H2S / S2-
NO2 / NO2-
O2 / O22-
H2 / Reactive metal (put in acid)
Cl2 / MnO4-

2.  A white precipitate can be formed after the addition of dilute HCl if the solution contains Pb2+ or Ag+ ions. These are precipitated as their respective chlorides which are white in colour.

3.  A yellow ppt is formed from solutions with S2O32- ions. The ppt is sulphur.

4.  A colour change from yellow to orange occurs with solutions that have chromate VI ions, (CrO42- to Cr2O72-).

E.  Adding dilute sulphuric acid to solution or solid.

1.  Gases can be liberated as given under dilute HCl.

2.  Precipitates can be formed with solutions that contain Pb2+, Ba2+, Ca2+, and Ag+. (ppts will be sulphates). A pale yellow ppt of sulphur can be formed from solutions with S2O32-.

3.  Solutions with CrO42- change colur from yellow to orange due to formation of dichromate ions, Cr2O72-.

F.  Dilute nitric acid has the same effects as dilute HCl except that no precipitates are formed as all nitrates are soluble.

G.  Adding solutions of K2CrO4 or K2Cr2O7 to test solutions.

1.  A pale yellow ppt can be formed with solutions that have Ba2+ and Pb2+ ions. These ppts are BaCrO4 and PbCrO4 respectively. A brick red ppt of Ag2CrO4 is produced with solutions that have Ag+ ions.

2.  A colour change from yellow to orange occurs with solutions that have H+ ions and the reverse occurs with solutions that have OH- ions.

3.  A green colour appears with solutions that have reducing agents. (Cr2O72- reduced to Cr3+).

H.  Adding Ag+(aq) to test solution.

This usually results in precipitates being formed.

1.  White ppt. This can be AgCl formed with Cl- ions in test solution (ppt is AgCl which is soluble in dilute NH3 solution) or it can be Ag2CO3 formed with carbonate ions in solution. (ppt is soluble in HNO3)

2.  Cream ppt. Ppt is AgBr which is soluble in concentrated NH3 and insoluble in dilute NH3.

3.  Pale yellow ppt. Ppt is AgI which is insoluble in both dilute and conc NH3.

4.  Grey- brown ppt. Ppt is Ag2O which shows presence of OH- ions.

5.  Black ppt. Ppt is Ag2S or metallic Ag formed with reducing agents.

I.  Adding aqueous KI to test solution.

1.  The colour of the solution changes to red-brown or yellow if the test solution has an oxidant and I- ions will be oxidised forming I2.

2.  Precipitates can be formed if the solution has Ag+, Pb2+ and Cu2+ ions.

Colour of ppt / inference
Pale yelllow / Ag+ ions (ppt is AgI –insol in NH3 and HNO3)
Bright yellow / Pb2+ (ppt isPbI2 – sol in excess and boiling)
Muddy brown / Cu2+ (grey ppt is CuI settles on bottom of brown solution of I2.
Black ppt / Oxidising agent. (ppt is free I2)

J.  Adding aqueous lead nitrate to test solution.

This usually results in precipitation of insoluble Lead salts.

Colour of precipitate / inference
White / Cl- (ppt PbCl2 which dissolves on boiling and recrystallise on cooling.
Br- (ppt PbBr2).
SO42- (ppt PbSO4, does not dissolve on boiling).
Yellow / CrO42- (ppt PbCrO4).
yellow / I-, ppt is PbI2 which dissolves on boiling and recrystallise on cooling.

K.  Adding aqueous sodium carbonate to test solution.

1.  CO2 is evolved from acidic solutions with H+, Al3+ and Fe3+ ions.

2.  The following precipitates can be formed.

Colour of ppt / inference
white / Mg2+, Al3+, Zn2+, Ba2+, Ca2+, Pb2+ (ppt is carbonate)
Buff / Mn2+, ppt is MnCO3 which turns brown in air
green / Fe2+, (ppt is FeCO3 ,turns brown in air).
blue / Cu2+, (ppt is CuCO3, turns black on heating/ boiling).
Red-brown / Fe3+, ppt is Fe(OH)3

Solubility Rules Chart

Negative Ions (Anions) / + / Positive Ions
(Cations) / = / Solubility of Compounds in water / Example
any anion / + / Alkali Ions
(Li+, Na+, K+, Rb+, Cs+, Fr+) / = / soluble / Sodium fluoride, NaF, is soluble
any anion / + / hydrogen ion
[H+ (aq)] / = / soluble / hydrogen chloride, HCl, is soluble
any anion / + / ammonium ion
(NH4) / = / soluble / ammonium chloride, NH4Cl, is soluble
nitrate
NO3- / + / any cation / = / soluble / potassium nitrate, KNO3, is soluble
acetate
(CH3COO-) / + / any cation
(except Ag) / = / soluble / sodium acetate, CH3COONa, is soluble
Chloride (Cl-),
Bromide (Br-), Iodide (I-) / + / Ag+, Pb2+, Hg2+, Cu+, Tl+ / = / low solubility (insoluble) / silver chloride, AgCl, forms a white precipitate
+ / any other cation / = / soluble / potassium bromide, KBr, is soluble
Sulfate
(SO42-) / + / Ca2+, Sr2+, Ba2+, Ag2+, Pb2+, Ra2+, Hg2+ / = / low solubility (insoluble) / barium sulfate, BaSO4, forms a white precipitate
+ / any other cation / = / soluble / copper sulfate, CuSO4, is soluble
sulfide
(S2-) / + / alkali ions (Li+, Na+, K+, Rb+, Cs+, Fr+),
alkali earth metals (Be2+, Mg2+, Ca2+, Sr2+, Ba2+, Ra2+),
and H+(aq), and NH4+ / = / soluble / magnesium sulfide, MgS, is soluble
+ / any other cation / = / low solubility (insoluble) / zinc sulfide, ZnS, is insoluble
hydroxide
OH- / + / alkali ions (Li+, Na+, K+, Rb+, Cs+, Fr+),
Sr2+, Ba2+, Ra2+, Tl+,
and H+(aq), and NH4 / = / soluble / strontium hydroxide, Sr(OH)2, is soluble
+ / any other cation / = / low solubility (insoluble) / silver hydroxide, AgOH, is insoluble (forms a precipitate)
Phosphate (PO43-),
Carbonate (CO32-),
Sulfite (SO32-) / + / alkali ions (Li+, Na+, K+, Rb+, Cs+, Fr+),
and H+(aq), and NH4 / = / soluble / ammonium phosphate, (NH4)3PO4, is soluble
+ / any other cation / = / low solubility (insoluble) / magnesium carbonate, MgCO3, is insoluble
Chromate
CrO42- / + / alkali ions (Li+, Na+, K+, Rb+, Cs+, Fr+),
Ca2+, Sr2+, and NH4+, / = / soluble / sodium chromate, Na2CrO4, is soluble
+ / any other cation / = / low solubility (insoluble)

General Solubility Trends:

·  All compounds of the ammonium ion (NH4+), and of the Alkali metal (Group IA) cations, are soluble.

·  All nitrates and actetates are soluble.

·  All chlorides, bromides, and iodides are soluble EXCEPT those of silver, lead, and mercury(I).

·  All sulfates are soluble EXCEPT those of silver, lead, mercury(I), barium, strontium, and calcium.

·  All carbonates, sulfites, and phosphates are insoluble EXCEPT those of ammonium and Alkali metal (Group IA) cations.

·  All hydroxides are insoluble EXCEPT those of ammonium, barium, and alkali metal (Group IA) cations.

·  All sulfides are insoluble EXCEPT those of ammonium, Alkali metal (Group I) cations, and Alkali earth metal (Group II) cations.

·  All oxides are insoluble EXCEPT those of calcium, barium, and Alkali metal (Group I) cations; these soluble ones actually react with the water to form hydroxides.

Key reactions associated with the anions prescribed in the AS syllabus.

1.  Carbonate (CO32-) and hydrogen carbonate (HCO32-).

These evolve carbon dioxide in the presence of acids.

CO32-(aq) + 2H+(aq) → H2O(l) + CO2(g).

HCO3-(aq) + H+(aq) →H2O(l) + CO2(g).

The gas gives a white precipitate, CaCO3, when bubbled through lime water. The ppt dissolves in excess CO2.

CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l).

In excess,

CaCO3(s) +CO2(g) +H2O(l) → Ca(HCO3)2(aq).

2.  Sulfate(VI) ions, SO42- and Sulphite ions,SO32-

When dilute HCl is added to sulphite ions and the mixture warmed, sulphur dioxide is given out.

SO32- + 2H+ →H2O + SO2.

Sulfur dioxide is acidic and has a pungent smell. It decolourise a dilute solution of acidified potassium mangate(VII) and turns an acidified solution of potassium dichromate (VI) from orange to green/blue.

A white precipitate of barium sulphate or sulphite is produced when a solution of barium nitrate or chloride is added to a solution with sulphate or sulphite ions.

Barium sulphite dissolves in excess dilute HCl, but barium sulphate does not dissolve.