Test for Anions (chpt 6)

Law of Conservation of Mass

Total mass of the products of a chemical reaction is the same as the total mass of the reactants

Law of Conservation of Matter

In any chemical reaction, matter is neither created nor destroyed but merely changes from one form to another

Ions

• Chloride

• Sulfate

• Sulfite

• Carbonate

• Hydrogencarbonate (bicarbonate)

• Nitrate

• Nitrite

• Phosphate

• Hydroxide

• Ammonium

• Thiosulphate

• Dichromate (VI)

• Manganate (VII)

• Hypochlorite

• Peroxide

• Cl⁻

• SO₄²⁻

• SO₃²⁻

• CO₃²⁻

• HCO₃⁻

• NO₃⁻

• NO₂⁻

• PO₄³⁻

• OH⁻

• NH₄⁺

• S₂O₃²⁻

• Cr₂O₇²⁻

• MnO₄⁻

• ClO⁻

• O₂²⁻

Test for Chloride

Steps

• Chloride salt dissolved in deionised water, clear solution. Few drops of silver nitrate solution added, turns cloudy, white precipitate

Explanation

• Silver chloride precipitate (+Ag⁺ → AgCl). Insoluble in water

Conformation Test

• Dilute ammonia solution added to cloudy solution. Cloudiness disappears

Explanation

• Silver chloride soluble in dilute ammonia solution, therefore chloride ions present in solution

Test for Sulfate and Sulfite

Steps

• Sulfate and sulfite salts dissolved in separate tubes w deionised water, clear solution. Few drops of barium chloride solution added to both, cloudy precipitate

Explanation

• Insoluble precipitate is barium sulfate or barium sulfite (Ba²⁺)

To distinguish sulfate and sulfite ions

• Dilute hydrochloric acid added to both tubes

• In sulfate, cloudy precipitate remains

• In sulfite, cloudy precipitate disappears

Explanation

• Barium sulfate insoluble in dilute HCl, barium sulfite dissolves as it reacts with dilute HCl and creates BaCl₂, leaving SO₂ + H₂O, which creates clear solution

Test for Carbonate or Hydrogencarbonate

Steps

• Carbonate and hydrogencarbonate salts placed in boiling tubes, one-holed rubber stopper inserted in tubes

• Stoppers removed, dilute HCl added, then replaced. Brisk effervescence observed w gas given off, turns limewater milky

Explanation

• Carbon dioxide released in both reactions, shown by milky limewater (CaCO₃ formed, chalk)

To distinguish between carbonate and hydrogencarbonate

• Magnesium sulfate (MgSO₄) solution added to solutions

• In carbonate, white precipitate formed

• In hydrogencarbonate, no precipitate

Explination

• Carbonate forms insoluble magnesium carbonate (MgCO₃)

• Hydrogencarbonate forms soluble magnesium hydrogencarbonate (Mg(HCO₃)₂)

Test for Nitrate

Brown Ring Test All nitrate salts are soluble in water so it is not possible to detect nitrate presence by precipitation reaction, so brown ring used instead

Steps

• Nitrate salt dissolved in deionised water in test-tube

• Freshly prepared iron (II) sulfate (FeSO₄) added

• Using dropper, concentrated sulfuric acid (H₂SO₄) added down inside of test-tube at a slant (like champagne added to glass). As acid is so dense, settles at bottom of test tube and forms layer underneath solution. Brown ring formed at junction of two layers

Explanation

• Brown ring formed as result of nitrate ions being present. Brown colour is formation of FeSO₄∙NO∙

• Must be concentrated sulfuric acid, not dilute as doesnt work with dilute

Test for Phosphate

Steps

• Phosphate salt dissolved in dionised water in test-tube

• Ammonium molybdate solution added ((NH₄)₂MoO₄)

• With dropper, few drops concentrated nitric acid added

• Test tube placed in warm water. Yellow precipitate formed

Explination

• Yellow precipitate is ammonium phosphomolybdate, therefore phosphate ions present