Introduction to Analytical Chemistry
Analytical chemistry deals with qualitative and quantitative analysis of substances.
Qualitative analysis: A salt consists of two parts known as radicals. The positively charged part of a salt (cation) which has been derived from a base is termed as basic radical and the negatively charged part of salt (anion) which has been derived from an acid is termed as acidic radical. In qualitative inorganic analysis, the given compound is analysed for the basic and acid radicals (i.e., the cations and the anions), that it contains. For example zinc blende is analysed for the Zn²⁺ and S²⁻ ions that it contains.
Test for Different Gases
(1) Colourless gases
It is colourless and odourless gas. It gives white ppt. with lime water which dissolves on passing excess of CO₂.
It is colourless and odourless gas. It burns with a blue flame.
It is colourless and odourless gas. It rekindles a glowing splinter.
It is a colourless gas with a smell of rotten eggs. It turns moist lead acetate paper black.
It is a colourless gas with a suffocating odour of burning sulphur. It turns acidified K₂Cr₂O₇ solution green.
It is a colourless gas with a characteristic ammonical smell. It gives white fumes of NH₄Cl with HCl, with Nessler’s reagents, it gives brown ppt.
It gives deep blue colour with CuSO₄ solution, NH₃ dissolves in water to give NH₄OH, which being basic, turns red litmus blue.
It is colourless gas with a pungent irritating smell. It turns moist blue litmus paper red i.e., it is acidic in nature. It gives white ppt. with AgNO₃ solution. This white ppt. is soluble in NH₄OH.
These vapours are colourless with a vinegar like smell.
(2) Coloured gases
It is a greenish yellow gas with a pungent smell. In small quantity it appears almost colourless. It bleaches a moist litmus paper, Blue litmus paper first turns red and then becomes colourless.
Brown vapours with a pungent smell. It turns moist starch paper yellow.
Violet vapours with a pungent smell. It turns moist starch paper blue.
Brown coloured pungent smelling gas. It turns moist starch KI paper blue. It turns ferrous sulphate solution black.
Systematic Procedure for Qualitative Analysis of Inorganic Salts
It involves the following steps : (1) Preliminary tests (2) Wet tests for acid radicals and (3) Wet tests for basic radicals.
(1) Preliminary tests
(i) Physical examination
It involves the study of colour, smell, density etc.
| Colour | Salt |
|---|---|
| Black | Oxides : MnO₂, FeO, CuO, Co₃O₄, NiO, Sulphides : CuS, Cu₂S, Ag₂S, PbS, HgS, Bi₂S₃ (blackish brown), NiS, CoS, FeS |
| Blue | Hydrated CuSO₄, anhydrous CoSO₄ |
| Orange | K₂Cr₂O₇, some dichromate (K₂Cr₂O₇), Sb₂S₅, ferricyanides |
| Green | Nickel salts, hydrated ferrous salts, potassium permanganate (KMnO₄), some copper (II) salts |
| Brownish yellow | SnS |
| Dark brown | FeCl₃, CuCrO₄, Ag₂O, PbO₂, CdO, Fe₂O₃ (but yellow in aq. solution) |
| Pale brown | MnCO₃ |
| Light pink | Hydrated manganese salts |
| Reddish pink | Hydrated cobalt (II) salts |
| Red | Pb₃O₄, HgI₂ |
| Yellow | AgI, AgBr, PbI₂, CdS, chromates |
(ii) Dry heating
Substance is heated in a dry test tube.
| Observation | Result |
|---|---|
| (a) A gas or vapour is evolved. | Compounds with water of crystallisation |
| Vapour, evolved, test with litmus paper. | Ammonium salts, acid salts, and hydroxides. (usually accompanied by change of colour) |
| The vapour is alkaline. | Ammonium salts. |
| The vapour is acidic. | Readily decomposable salts of strong acids. |
| Oxygen is evolved | Nitrates, chlorates and certain oxides. |
| Dinitrogen oxide | Ammonium nitrate or nitrate mixed with an ammonium salt. |
| Dark-brown or reddish fumes (oxides of nitrogen), acidic in reaction. | Nitrates and nitrites of heavy metals. |
| CO₂ is evolved, lime water becomes turbid. | Carbonates or hydrogen carbonates. |
| NH₃ is evolved which turns red litmus blue. | Ammonium salts. |
| SO₂ is evolved, which turns acidified K₂Cr₂O₇ green, decolourises fuschin colour. | Sulphates and thiosulphates. |
| H₂S is evolved, turns lead acetate paper black, or cadmium acetate yellow. | Hydrates, sulphides or sulphides in the presence of water. |
| Cl₂ is evolved, yellowish green gas, bleaches litmus paper, turns KI – starch blue, poisonous. | Unstable chlorides e.g., copper chlorides in the presence of oxidising agents. |
| Br₂ is evolved (reddish brown, turns fluorescent paper red). | Bromides in the presence of oxidising agents. |
| I₂ is evolved, violet vapours condensing to black crystals | Free iodine and certain iodides |
| (b) A sublimate is formed | Ammonium and mercury salts. |
| White sublimate | As₂O₃, Sb₂O₃ |
| Grey sublimate | Hg |
| Steel grey, garlic odour | As |
| Yellow sublimate | As₂S₃, HgI₂ (Red) |
Action of heat on different compounds
Many inorganic salts decomposes on heating, liberating characteristic gases. A few such reactions are tabulated as follows,
| Reaction | Observation |
|---|---|
| 2HgO → 2Hg + O₂ (Red) | Silvery deposit |
| 2Pb₃O₄ → 6PbO + O₂ (Red) | Yellow |
| PbO₂ → PbO + ½O₂ | Brown |
| CuCO₃ → CuO + CO₂ | Green |
| ZnCO₃ → ZnO + CO₂ (White cold, Yellow hot) | White |
| CuSO₄·5H₂O → CuSO₄ + 5H₂O | Blue to White |
| CuSO₄ → CuO + SO₃ | |
| 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ | |
| 2AgNO₃ → 2Ag + O₂ + 2NO₂ (450°C) | |
| Zn(NO₃)₂ → ZnO + 2NO₂ + ½O₂ | White, Brown |
| 2Ag₂CO₃ → 4Ag + 2CO₂ + O₂ | |
| 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ | |
| Cr₂O₃ + 3C + 2N₂ → 2Cr + 3CO (Green, Orange) | |
| 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ | |
| NH₄HCO₃ → NH₃ + H₂O + CO₂ | |
| CaCO₃ → CaO + CO₂ | |
| NaNO₂ → NaNO₃ + ½O₂ | |
| MgCO₃ → MgO + CO₂ | |
| NH₃ + hot Red → N₂ + 3H₂ | |
| Mg(NO₃)₂ → MgO + 2NO₂ + ½O₂ | |
| Ca(NO₃)₂ → CaO + 2NO₂ + ½O₂ | |
| Al₂(SO₄)₃ → Al₂O₃ + 3SO₃ (hot Red) | |
| CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O (Plaster of Paris) | |
| AlCl₃·6H₂O → Al₂O₃ + 6HCl + 9H₂O | |
| BeSO₄ → BeO + SO₂ + ½O₂ | |
| AgNO₃ → AgNO₂ + ½O₂ (350°C) | |
| MgSO₄ → MgO + SO₂ + ½O₂ | |
| ZnSO₄ → ZnO + SO₂ + ½O₂ | |
| Sn(COO)₂ → SnO + CO + CO₂ | |
| CaC₂O₄ → CaCO₃ + CO | |
| NH₄NO₂ → N₂ + 2H₂O | |
| NH₄NO₃ → N₂O + 2H₂O | |
| 2KClO₃ → 2KCl + 3O₂ | |
| 2FeCl₃ → 2FeCl₂ + Cl₂ | |
| Li₂CO₃ → Li₂O + CO₂ | |
| Fe(COO)₂ → FeO + CO + CO₂ | |
| 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂ | |
| HgCl₂ + Mg → Hg + MgCl₂ | |
| NH₄Cl → NH₃ + HCl | |
| 2LiNO₃ → Li₂O + 2NO₂ + ½O₂ | |
| Hg(NO₃)₂ → Hg + 2NO₂ + O₂ | |
| 2CuCl₂ → 2CuCl + Cl₂ | |
| Co(NO₃)₂ → CoO + 2NO₂ + ½O₂ | |
| 2K₂Cr₂O₇ → 2K₂CrO₄ + Cr₂O₃ + ½O₂ | |
| MgNH₄PO₄ → Mg₂P₂O₇ + 2NH₃ + H₂O | |
| ZnNH₄PO₄ → Zn₂P₂O₇ + 2NH₃ + H₂O | |
| K₄[Fe(CN)₆] → 4KCN + (CN)₂ + Fe + N₂ | |
| ZnCl₂·6H₂O → ZnCl₂ + 6H₂O | |
| ZnCl₂·2H₂O → ZnOCl₂ + 2H₂O | |
| FeCl₃·6H₂O → Fe₂O₃ + 6HCl + 9H₂O | |
| ZnSO₄ → ZnO + SO₂ + ½O₂ (800°C, Glassy bead) | |
| Na₂B₄O₇·10H₂O → Na₂B₄O₇ → 2NaBO₂ + B₂O₃ (hot Red) | |
| ZnSO₄·7H₂O → ZnSO₄·6H₂O → ZnSO₄·H₂O → ZnSO₄ (450-700°C) → ZnO + SO₂ + ½O₂ (100-160°C) |
(iii) Flame test
Characteristic flame colour : Certain metals and their salts impart specific colours to Bunsen burner flame.
- (a) Pb imparts pale greenish colour to the flame.
- (b) Cu and Cu salts impart blue or green colour to the flame.
- (c) Borates also impart green colour to the flame.
- (d) Ba and its salts impart apple green colour to the flame.
- (e) Sr imparts crimson red colour to the flame.
- (f) Ca imparts brick red colour to the flame.
- (g) Na imparts yellow colour to the flame.
- (h) K imparts pink-violet (Lilac) colour to the flame.
- (i) Li imparts crimson-red, Rb imparts violet and Cs imparts violet colours to the flame.
- (j) Livid-blue flame is given by As, Sb and Bi.
(iv) Borax bead test
The transparent glassy bead (NaBO₂ + B₂O₃) when heated with inorganic salt and the colour produced gives some idea of cation present in it.
| Colour of bead in oxidising flame | Colour of bead in reducing flame | Basic radical present |
|---|---|---|
| Greenish when hot, blue in cold. | Red and opaque | Cu |
| Dark green in hot and cold | Same | Cr |
| Deep – blue | Deep blue | Co |
| Yellow when hot | Green | Fe |
| Violet in hot and cold | Colourless | Mn |
| Brown in cold | Grey or black or opaque | Ni |
Microcosmic salt bead test
Microcosmic salt, Na(NH₄)HPO₄·4H₂O is also used to identify certain cations just like borax. When microcosmic salt is heated in a loop of platinum wire, a colourless transparent bead of sodium metaphosphate is formed.
Now NaPO₃ reacts with metallic oxides to give coloured orthophosphates.
(v) Charcoal cavity test
| (a) Compound fused in cavity directly | Nature and colour of bead | Cation |
|---|---|---|
| Yellow, brittle bead | Bi³⁺ | |
| Yellow, soft bead which marks on paper | Pb²⁺ | |
| White, brittle | Sb³⁺ | |
| White yellow when hot | ZnO | |
| White garlic odour | As₂O₃ | |
| Brown | CdO | |
| Grey metallic particles attracted by magnet | Fe, Ni, Co | |
| Maleable beads | Ag and Sn (White), Cu (Red flakes) |
(b) Compound mixed with Na₂CO₃ Crystalline Salts, NaCl, KCl; Decrepitates
Substance; Oxidising agents like NO₃⁻, NO₂⁻ deflagrate
Substance infusible, perform test (a)
(vi) Cobalt Nitrate test
| Colour | Composition | Result |
|---|---|---|
| Blue residue | CoO·Al₂O₃ | Al |
| Green residue | CoO·ZnO | ZnO |
| Pink dirty residue | CoO·MgO | MgO |
| Blue residue | NaCoPO₄ | PO₄³⁻ in absence of Al. |
(2) Wet tests for acid radicals
Salt or mixture is treated with dil. H₂SO₄ and also with conc. H₂SO₄ separately and by observing the types of gases evolved. Confirmatory tests of anions are performed.
Table : 21.7 Observations with Dilute H₂SO₄
| Observations | Acid Radical | Confirmatory test |
|---|---|---|
| Brisk effervescence with evolution of colourless and odourless gas. | CO₃²⁻ (carbonate) | Gas turns lime water milky but milkyness disappears on passing gas in excess, Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂↑, Ca(OH)₂ + CO₂ → CaCO₃↓ (milky) + H₂O, CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ (soluble) |
| Brown fumes | NO₂⁻ (Nitrite) | Add KI and starch solution blue colour, 2NaNO₂ + H₂SO₄ → Na₂SO₄ + 2HNO₂; HNO₂ → NO (colourless); 2NO + O₂ (air) → 2NO₂ (brown), 2KI + H₂SO₄ + 2HNO₂ → K₂SO₄ + 2H₂O + 2NO + I₂, I₂ + starch → blue colour |
| Smell of rotten eggs (H₂S smell) on heating | S²⁻ (sulphide) | Gas turn lead acetate paper black, Sodium carbonate extract (SE)* + sodium nitroprusside – purple colour, Na₂S + H₂SO₄ → 2NaHSO₄ + H₂S↑, (CH₃COO)₂Pb + H₂S → PbS↓ (black) + 2CH₃COOH, Na₂S + Na₂[Fe(CN)₅NO] → Na₄[Fe(CN)₅NOS] (purple) |
| Colourless gas with pungent smell of burning sulphur | SO₃²⁻ (sulphite) | Gas turns acidified K₂Cr₂O₇ solution green [different from CO₃²⁻] since gas also turns lime water milky, Na₂SO₃ + H₂SO₄ → Na₂SO₄ + H₂O + SO₂↑, Cr₂O₇²⁻ + 3SO₂ + 2H⁺ → 2Cr³⁺ (green) + 3SO₄²⁻ + H₂O, Ca(OH)₂ + SO₂ → CaSO₃↓ (milky) + H₂O |
| Solution gives smell of vinegar | CH₃COO⁻ (acetate) | Aq. Solution + neutral FeCl₃ → blood red colour, 3CH₃COONa + FeCl₃ → Fe(CH₃COO)₃ (red) + 3NaCl (neutral) |
| White or yellowish white turbidity on warming | S₂O₃²⁻ (thiosulphate) | Aq. Solution + AgNO₃ → white ppt. changing to black (viii) on warming, Na₂S₂O₃ + 2AgNO₃ → Ag₂S₂O₃↓ (white) + 2NaNO₃, Ag₂S₂O₃ + H₂O → Ag₂S↓ (black) + H₂SO₄ |
Table : 21.8 Observation with concentrated H₂SO₄
| Observation | Acid Radical | Confiramatory Test |
|---|---|---|
| Colourless pungent gas giving white fumes with aq. NH₄OH | Cl⁻ (chloride) | Add MnO₂ in the same test tube and heat–pale green Cl₂ gas (i), S.E.+ AgNO₃ + HNO₃ solution –white ppt. soluble in aq. NH₃ (ii), Chromyl chloride test (iii) |
| Reddish brown fumes | Br⁻ (bromide) | Add MnO₂ and heat –yellowish brown Br₂ gas (iv), S.E.+ AgNO₃ + HNO₃ solution –pale yellow ppt. partially soluble aq. NH₃ (v), Layer test (vi) |
| Violet pungent vapours turning starch paper blue. | I⁻ (iodide) | S.E.+ AgNO₃ + HNO₃ → yellow ppt. insoluble in aq. NH₃ (vii), Layer test (vi) |
| Brown pungent fumes intensified by the addition of Cu- turnigs. | NO₃⁻ (nitrate) | Ring test (viii) |
| Colourless gases turning lime water milky and burning with blue flame. | C₂O₄²⁻ (oxalate) | Acidified KMnO₄ solution is decolorised (ix), S.E.+ CH₃COOH + CaCl₂ white ppt. decolorising acidified KMnO₄ solution (x) |
Reactions
Chloride
(iii) Chromyl-chloride test
Chloride + conc. H₂SO₄ (solid) + K₂Cr₂O₇ heat → reddish brown vapours of chromyl-chloride (CrO₂Cl₂). Pass these vapours into NaOH, when yellow Na₂CrO₄ solution is formed. On adding CH₃COOH and (CH₃COO)₂Pb, yellow ppt. of lead chromate (PbCrO₄) is formed.
Bromide
(vi) Layer Test
S.E. + Cl₂ water + CHCl₃ shake → yellowish orange colour in CHCl₃ layer (CS₂ or CCl₄ can be taken instead of CHCl₃); Br₂ soluble orange yellow in CHCl₃, 2NaBr + Cl₂ → 2NaCl + Br₂ (orange yellow in CHCl₃)
In case of I⁻, violet colour of I₂ in CHCl₃ layer, 2NaI + Cl₂ → 2NaCl + I₂ (violet)
Iodide
Nitrate
(viii) Ring test
To water extract (all NO₃⁻ are water soluble) add freshly prepared FeSO₄ solution and then conc. H₂SO₄ carefully by the side of the test-tube. A dark brown ring of [Fe(H₂O)₅NO]²⁺ SO₄²⁻ at the interface between the two liquids is formed.
Oxalate
Specific test in solution
(i) Sulphate : S.E. add dil. (to decompose CO₃²⁻ until reaction ceases). Add BaCl₂ solution. White ppt. insoluble in conc. HNO₃, BaCl₂ + Na₂SO₄ → BaSO₄↓ (white ppt.) + 2NaCl
(ii) Borate : Ignite the mixture containing borate, conc. H₂SO₄. And ethanol in a china-dish with a burning splinter –green edged flame of ethyl borate.
In presence of Cu²⁺, perform this test in a test tube since Cu²⁺ salts are not volatile.
(iii) Phosphate : S.E. + HNO₃ + ammonium molybdate solution. Heat, yellow crystalline ppt. confirms PO₄³⁻
Arsenic also gives this test. Hence presence of phosphate should also be checked after group II.
(iv) Fluoride : Sand + salt (F⁻) + conc. H₂SO₄; heat and bring a water wetted rod in contact with vapours at the mouth of the test tube. A white deposit on the rod shows the presence to F⁻
(3) Wet tests for basic radicals
Analysis of Basic Radicals
| Group | Basic radicals | Group reagent | Ppt. as | Explanation |
|---|---|---|---|---|
| I | Ag⁺, Hg₂²⁺, Pb²⁺ (I) | dil HCl | Chloride (AgCl, Hg₂Cl₂, PbCl₂) | Ksp values of chlorides are low, hence precipitated. Others have higher Ksp values hence not precipitated. |
| II | Pb²⁺, Cd²⁺, Cu²⁺, Hg²⁺ (II), Bi³⁺, As³⁺, Sb³⁺, Sn²⁺ | H₂S gas in presence of dil. HCl | Sulphides (CuS, As₂S₃ etc.) | Ksp values of sulphides are low hence precipitated by low [S²⁻] ion. HCl (with common H⁺ ion) decreases ionization of H₂S which gives low [S²⁻]. Hence II group is precipitated. Others with higher Ksp values not precipitated. |
| III | Al³⁺, Cr³⁺, Fe³⁺ | NH₄OH in presence of NH₄Cl | Hydroxide, Al(OH)₃ etc. | Ksp values of Al(OH)₃ etc. are low. NH₄Cl (with common NH₄⁺ ion) decreases ionization of NH₄OH giving low [OH⁻]. Hence group III is precipitated. |
JEE Main Weightage
Typically 2-3 questions from Qualitative Analysis. Focus on gas tests, radical identification, and confirmatory tests.