Chemistry note: Qualitative method to identify chemical species

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Qualitative analysis on different compounds

Ionic compound: cation (Al³⁺, Ca²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Pb²⁺, Mg²⁺, K⁺, Na⁺, Zn²⁺, Ag⁺, NH₄⁺)

1)       Appearance, e.g. Cu²⁺ is blue-green, Fe³⁺ is brown.

2)       Solubility: Nitrates and hydrogencarbonate are all soluble

Halides are all soluble except silver halides AgX

Sulphates are all soluble except BaSO₄, PbSO₄, CaSO₄ (slightly soluble)

Carbonates and sulphites are all insoluble except Na⁺, K⁺, (NH₄)₂CO₃

3)       Flame test: use a clean Pt wire to dip conc. HCl followed by the sample solid, burn under non-luminous flame (luminous flame has its own colour) to see the characteristic colour. Principle: the compound forms metal chloride under conc. HCl (H₂SO₄ can’t be used as metal chlorides are more volatile than metal sulphates), and the excited metal cation emit photon at visible spectrum to give the characteristic colour.

Na⁺: [persistent] golden yellow, K⁺: lilac, Ca²⁺: brick red, Ba²⁺: green, Cu²⁺: bluish green

4)       Actions on dilute HCl/H₂SO₄ followed by solubility test, etc.

5)       Actions on NaOH: Metal ion [except Na⁺, K⁺] forms hydroxide ppt under NaOH, Al³⁺, Pb²⁺, Zn²⁺ re-dissolve in excess NaOH to give hydroxyl complex.

6)       Actions on NH₃: Metal ion [except Na⁺, K⁺] forms hydroxide ppt under NH₃, Cu²⁺, Ag⁺, Zn²⁺ re-dissolve in excess NH₃ to form amine complex. Note:

I)                     [Cu(NH₃)₄]²⁺ is in deep blue colour, not the blue colour given by Cu²⁺.

II)                   Ag form Ag₂O rather than AgOH by dehydration 2AgOH → Ag₂O + H₂O, but it forms diamminesilver(I) ion under excess NH₃, i.e., [Ag(NH₃)₂]⁺

III)                  Al(OH)₃ and Pb(OH)₂ can be distinguished either by actions in H₂SO₄ or heat until dryness (Al₂O₃ VS PbO), PbO change colour when heated.

7)       Test for ammonium: Warm with NaOH to give pungent small/turn wet red litmus paper blue.

Test for gases (NH₃, CO₂, SO₂, HX)

1)       CO₂: turning lime water milky and become clear when CO₂ is in excess; note that SO₂ has similar effect by Ca(OH)₂ + SO₂ → CaSO₃ + H₂O but CO₂ don’t have reducing power.

2)       SO₂: It act as an reducing agent so it reduces Cr₂O₇^2- (orange) to Cr³⁺

3)       SO₃: equivalent to SO₄^2-

4)       Cl₂: turn wet blue litmus paper red then bleached (white) since Cl₂ + H₂O and HCl + HOCl are in equilibrium. HCl, HOCl are both acidic while OCl^- has bleaching effect.

5)       NH₃: pungent smell, form dense white fumes (NH₄Cl) with HCl(g), there are test each other.

6)       HX: HCl can be tested by NH₃, HBr and HI are in equilibrium with H₂ + X₂ so they gives white fumes and brown liquid (Br₂) or black solid (I₂) as well. They can also be tested by actions on conc. H₂SO₄.

Ionic compound: anion (NO₃^-, CO₃^2-, SO₃^2-, SO₄^2-, OCl^-, S₂O₃^2-, X^-)

1)       Actions of heat: Carbonates decompose upon Bunsen flame: Carbonate → oxide + CO₂

2)       Sulphates are resistant under lab. flames, but iron(II) sulphates decomposes under heating: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃.

3)       Actions on HCl: bleaching effect of OCl^-: OCl^- + H⁺ → Cl₂ + H₂O

4)       Sulphites and thiosulphates gives SO₂ under acid:

SO₃^2- + 2H⁺ → SO₂ + H₂O, S₂O₃^2- + 2H⁺ → SO₂ + H₂O + S, so thiosulphates gives white ppt besides SO₂.

5)       Barium test: sulphates, sulphites and carbonates forms ppt with Ba²⁺, common reagents includes BaCl₂, Ba(NO₃)₂. BaSO₃, BaCO₃ dissolve in HCl but BaSO₄ is resistant in acid. This serves as an unique test for sulphate ions.

6)       Silver nitrate test: Dissolve the sample in HNO₃ followed by AgNO₃, X^- form ppt AgCl (white) / AgBr (pale yellow) / AgI (yellow). They can be further distinguished by:

(I)                   AgCl dissolve in NH₃, AgBr only dissolve in conc. NH₃, AgI is insoluble in NH₃

(II)                 AgCl turns grey in sunlight, AgBr turns yellowish grey, AgI remains yellow

Tests for organic compounds

1)       Saturated hydrocarbons: blue/clear yellow flame under non-luminous flame

2)       Alkene/alkyne: smoky flame under non-luminous flame since higher carbon-hydrogen ratio leads to more CO produced.

Test for addition: add a few drops of Br₂ (in organic solvents)/K₂Cr₂O₇/H⁺, it decolorize rapidly.

3)       Haloalkanes: Boil with ethanolic KOH (KOH with EtOH), acidify with HNO₃ followed by AgNO₃, ppt of AgX is expected.

4)       Alcohol:

(I)                   Esterification: Add acyl chloride (RCOOX) to give colourless/white fumes (HCl) (The smell of ester is probably covered by acid)

(II)                 Esterification: Add carboxylic acid and heat with a few drops of conc. H₂SO₄, a sweet fruity smell is given (ester)

(III)                Lucas test: Add conc. HCl with ZnCl₂, cloudiness appears (NOT ppt; the mixture of liquid in different phase) according to the degree of carbon: primary alcohol (1hr), secondary alcohol (5 min) and tertiary alcohol (1 min).

(IV)              Iodoform test for alcohol that the carbon connected to hydroxyl group also connected to a methyl group. Add iodine with NaOH, yellow ppt (CHI₃) with characteristic smell is given. The mechanism is as follows:

  

(V)                Under K₂Cr₂O₇/H⁺, primary and secondary alcohol oxidizes but tertiary alcohol is resistant to oxidation since CH₃^- is a poor leaving group. Note that KMnO₄ can be used as the oxidizing power is too strong and it may oxidizing the tertiary alcohol.

5)       Carbonyl group (aldehydes/ketones):

(I)                   Test by 2,4-dinitrophenylhydrazine: Aldehydes and ketones react with this reagent to give red~yellow ppt (2,4-dinitrophenylhydrazone). Accurate species can be determined by crystallization of ppt and determine the melting point.

(II)                 Iodoform test (for ketone CH₃COR): yellow ppt under I₂/NaOH

(III)                Tollens’ reagent (silver mirror test): warm with [Ag(NH₃)₂]⁺, the reagent act as oxidizing agent that gives out silver, silver mirror is deposited in the inner surface of the tube

6)       Carboxylic acid

I)                     Esterification with ethanol under conc. H₂SO₄ as catalyst to give sweet, fruity smell

II)                   RCOOH is the only organic compound to react with NaHCO₃. (Most acidic)