Qualitative Analysis of Salts

• Qualitative analysis is a series of chemical tests conducted on a chemical substance to identify or determine cation or anion in it.    

• To identify the cation and anion in the salt, the following tests are usually conducted:

        (a) Check the physical characteristic of the salt

        (b) Test for certain gases

        (b) The effect of the heat on the salt

        (c) Preparation aqueous salt solution to test for the cation and anion in it

        (d) Confirmatory tests for ions.         

• The following physical properties need to be carefully checked first to ease the identification of salts.

        (a) physical state

        (b) colour of the salt

        (c) solubility of the salt in water

1. Physical state of the salt

• This might help can be used as the sign to identify the type of substance that might exist.

• The physical state of substance might be in the form of gas, liquid, solid or aqueous solution.

• Ionic salts usually in the crystal shape while sulphate and oxide metal usually in the shape of powder.

2. Colour of the salt

• The colour of salt can give a good sign of cation, anion or compound that might exist.

• The deduction can be made from the the colour of salt as below:

3. Solubility of salt in water

• This can be a good sign to the type of the salts. For example:

        (a) Salt X insoluble in water and contains sulphate ion.

             => Salt X might be lead (II) sulphate, barium sulphate, or calcium sulphate

        (b) Salt Y soluble in water and contains carbonate ion.

              => Salt Y might be sodium carbonate, potassium carbonate, or ammonium carbonate

• Certain are released when a chemical substance:

        (a) is heated

        (b) reacts with dilute or concentrated acid

        (c) is heated with an alkali

• The type of gas produced will indicate the ion present in the salt, for example if sulphur dioxide is released, it is sulphate.

• The characteristics and confirmatory tests for several gases are shown below:

• Ammonium salts, carbonates, nitrates and some sulphate decompose when heated. They may exhibit a change in colour or release gases.

• When a salt is heated:

        (a) Gas released gives information about type of anion (nonmetal ion) contains (except for ammonium salts)

        (b) Change in colour (residue left) gives information about type of cation (metal ion) contains

1. Effect of heat on carbonates (released carbon dioxide).

2. Effect of heat on nitrates salt

3. Effect oh heat on sulphate salts

• Usually sulphates do not decompose by heat for example sodium sulphate, potassium sulphate and calcium sulphate.

• Only certain sulphates do decomposed by heat. For example:

        (a) 2FeSO₄.7H₂O (s)        Fe₂O₃ (s)   +  SO₂ (g)   +   SO₃ (g)   +   14H₂O (g)

             When iron (II) sulphate is heated strongly, it will released steam fume, slphur dioxide, sulphur trioxide and brown solid of

             iron (III) oxide is left. 

       (b) ZnSO₄ (s)           ZnO (s)   +  SO₃ (g)  

            CuSO₄ (s)            CuO (s)   +  SO₃ (g)  

            Fe₂(SO₄)₃ (s)      Fe₂O₃ (s)   +  3SO₃ (g)  

            When zinc sulphate, copper sulphate and iron (III) sulphate heated strongly, oxide metal and gas sulphur trioxide (acidic) are

            produced.

       (c) When ammonium sulphate is strongly heated, ammonia (alkalic) and acid sulphuric fume are released as below:

               (NH₄)₂SO₄ (s)      2NH₃ (g)   +   H₂SO₄ (g)

4. Effect oh heat on chloride salts

• Usually chlorides do not decompose by heat except ammonium chloride.

        (a) NH₄Cl (s)         NH₃ (g)   +  HCl (g)

             When ammonium chloride heated strongly, ammonia (turns damp red litmus paper into blue) and hidrogen chloride fume are

             released. When the white fume cold, white

             solid of ammonium chloride reproduced. The reaction is reversible. Hidrogen chloride is a thick white fume when touched with

             a drop of concentrated liquid ammonia.

• The presence of anions can be tested based on

        (a) the gas released when

             (i) the salt is heated

             (ii) a dilute acid is added to the salt

        (b) the confirmatory test for specific anions that is carbonates, nitrates, sulphates and chlorides

1. Tests for carbonates

• All carbonates except for potassium carbonate and sodium carbonate release carbon dioxide when heated (can see in Effect of heat on carbonates) .

• All carbonates also produce  carbon dioxide when a  dilute acid is added. For example:

        MgCO₃ (aq)  +   2HCl (aq)             MgCl₂ (aq)   +   H₂O (l)   +   CO₂

        Na₂CO₃ (aq)  +   2HCl (aq)            2NaCl (aq)   +   H₂O (l)   +   CO₂

• Test for carbonates will released carbon dioxide that turns limewater chalky.

2. Tests for nitrates

• All nitrates except for potassium nitrate and sodium nitrate release brown gas nitrogen dioxide and oxygen when heated (can see in Effect of heat on nitrates) .

• Test on nitrates solution (Brown Ring Test)

     About 2cm³ nitrate solution is mixed with dilute sulphuric acid and iron (II) sulphate solution in a test tube. Then a few drops 

        of concentrated sulphuric acid are added slowly and carefully down rhe side of the test tube. (Do not shake the mixture!) A 

        brown ring is formed between the layers of concentrated acid and nitrate mixture. The brown ring composed of a complex ion,

        confirm the presence of nitrate.

3. Tests for chlorides

• The effect of heat on nitrates can be seen in Effect of heat on nitrates.

• About 2cm³ chloride solution is mixed with dilute sulphuric acid and followed by silver nitrate solution in a test tube.

• A white precipitate, silver chloride forms, confirming the presence of a chloride.

                              Ag⁺ (aq)   +   Cl^-(aq)        AgCl(s)

4. Tests for sulphates

• The effect of heat on nitrates can be seen in Effect of heat on sulphates.

• About 2cm³ sulphate solution is mixed with dilute hydrochloric acid or nitric acid and followed by barium chloride or barium nitrate  solution in a test tube.

• A white precipitate, barium sulphate forms, confirming the presence of a sulphate.

                          Ba²⁺ (aq)   +   SO₄^2-(aq)       BaSO₄(s)

• The following tests are conducted to identify cations in solution:

        (a) Test with aqueous sodium hydroxide, NaOH (aq)

        (b) Test with aqueous ammonia, NH3 (aq)

        (c) Confirmatory tests with specific reagents

• The cations identified include:

        (a) Sodium ion, Na⁺

        (b) Calcium ion, Ca⁺

        (c) Iron (II) ion, Fe²⁺

        (d) Iron (III) ion, Fe³⁺

        (e) Copper (II) ion, Cu²⁺

        (f) Lead (II) ion, Pb²⁺

        (g) Zinc (II) ion, Zn²⁺

        (c) Ammonium ion, NH₄⁺

• The reagents used to identify specific ions are given in table below:

Confirmatory tests for Fe²⁺, Fe³⁺, Pb²⁺ and NH₄⁺

• Special tests are conducted to identify Fe²⁺ and Fe³⁺, other than their test in sodium hydroxide, NaOH and aqueous ammonia, NH₃. They are test with potassium hexacyanoferrate (II) [K₃Fe(CN)₆], potassium hexacyanoferrate (III), [K₄Fe(CN)₆] and potassium thiocyanate, KSCN as given below.

                                        

• The presence of ion lead (II), Pb²⁺ ion are confirmed by testing them with potassium iodide (KI), potassium chloride (KCl) or   potassium sulphate (K₂SO₄) solutions. All three form precipitates as follows:

        Pb²⁺   +   2I^-                 PbI₂ (s) (yellow precipitate)

        Pb²⁺   +   2Cl^-               PbCl₂ (s) (white precipitate)

        Pb²⁺   +   SO₄^2-            PbSO₄ (s) (white precipitate)

        (b) When heated, PbI₂ and PbCl₂ dissolves in hot water. Upon cooling PbI₂ forms golden-yellow needle-like crystals,        

             while PbI₂ forms white crystals.

• NH₄⁺  salts form a brown precipitate when Nessler's Reagent is added to it.