Chemistry: Testing for ions

Test for ammonium ions NH₄⁺_(aq) in solution

Add NaOH_(aq)

Gently heat

Test fumes with damp red litmus paper

If present paper turns red to blue

NH₄⁺_(aq) + OH^-_(aq) → NH_3(aq) + H₂O_(aq)

Heat to (g)

Then ammonia reacts with water on damp litmus to reform ammonium and hydroxide ions, which turns litmus blue.

NH_3(g) + H₂O_(l) → NH₄⁺_(aq) + OH^-_(aq)

Test for halide ions

Add nitric acid, HNO_3(aq) to remove impurities such as CO_3^2- which would also form a white precipitate. H⁺ react to make CO₂ and H₂O.

Add silver nitrate solution, AgNO_3(aq)

_{If present insoluble silver halide precipitates are formed, AgX (s)}

_{AgCl - White, soluble in dilute} NH_3(aq).

_{AgBr - Cream, insoluble in dilute} NH_{3(aq), soluble in concentrated.}

_{AgI - Yellow, insoluble in dilute and concentrated} NH_3(aq).

Test for CO₃^2-_(aq)

Add dilute acid e.g HCl_(aq)

If present efference viewed and CO₂ produced

CO₃^2-_(aq) + 2H⁺ → CO_2(g) + H₂O_(l)

Bubbled gas through limewater which turns cloudy

CO_2(g) + Ca(OH)_2(aq) → CaCO_3(s) + H₂O_(l)

Test for SO₄^2-

Add HCl, to remove impurities such as CO_3^2- which would also form a white precipitate. H⁺ react to make CO₂ and H₂O.

Do not add H₂SO₄ as this would add SO₄^2- and give a positive result

Add BaCl₂ If present white precipitate forms, BaSO_4(s)

Testing for unsaturation

Bromine water

If it contains a C=C then it will open up and Br₂ will be added across the double bond by electrophilic addition, making a colourless solution.

Orange/Brown to colourless

Test for alcohol oxidation products

Fehling’s solution is an alkaline solution containing copper(II) ions which act as the oxidising agent.

When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Cu²⁺ ions are reduced to Cu⁺ ions.

Colour change blue solution to red Cu₂O precipitate.

Ketones cannot be oxidised and therefore give a negative test.

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Tollens' reagent is an aqueous alkaline solution of silver nitrate in excess ammonia solution.

When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Ag⁺ ions are reduced to Ag atoms, forming a silver mirror.