Identifying Anions: Precipitation and pH Indicators

Tests for Chloride (Cl^–)

• Precipitation test – adding silver ions e.g. AgNO₃ will produce a white precipitate (AgCl). Exposing this precipitate to UV light (e.g. sunlight) will cause it turn grey as AgCl decomposes into silver metal.

Tests for Bromide (Br^–)

• Precipitation test – adding silver ions e.g. AgNO₃ will produce a cream-coloured precipitate (AgBr).

Tests for Iodide (I^–)

• Precipitation tests:
• Adding silver ions e.g. AgNO₃ will produce a yellowish precipitate (AgI).
• Adding lead(II) ions e.g. Pb(NO₃)₂ will produce a bright yellow precipitate (PbI₂)

AgCl, AgBr, AgI

PbI₂

Tests for Hydroxide (OH^–)

• Precipitation test – hydroxide ions produce soluble ionic compounds with most cations except:
• Cu²⁺: blue precipitate
• Fe²⁺: green precipitate
• Fe³⁺: brown precipitate

• Indicator test – hydroxide ions cause solution to become alkaline (pH > 7).
• Red litmus paper turns blue
• Universal indicator turns blue

Fe(OH)₂, Fe(OH)₃, Cu(OH)₂.

Source: Collins UK

Tests for Acetate (CH₃COO^–)

• Precipitation tests are not effective for identifying acetate ions as salts containing acetate are usually quite soluble.
• Indicator test – acetate is the conjugate base of acetic acid (weak acid). When dissolved in water, it exists in the following equilibrium:

$$CH_3COO^-(aq) + H_2O(l) \rightleftharpoons CH_3COOH(aq) + OH^-(aq)$$

• Red litmus paper turns blue
• Universal indicator turns blue

• Adding nitric acid shifts acetate ion’s ionisation equilibrium to the right (Le Chatelier’s principle) to form more acetic acid. Production of acetic acid at high concentrations produces a distinct vinegar smell.

Red litmus paper turns blue in basic solution

Tests for Carbonate (CO₃^2–)

• Adding nitric acid to produce carbon dioxide, causing bubbles to form.
• HNO₃ is used instead of HCl & H₂SO₄ because the latter two introduce precipitable anions into the unknown solution which may produce a false positive result.

$$HNO_3(aq) + CO_3^{2-}(aq) \rightarrow NO_3^-(aq) + H_2O(l) + CO_2(g)$$

•  Identity of carbon dioxide can be verified by the limewater test (bubbling the gas in calcium oxide solution). Formation of calcium carbonate turns the solution milky.

 $$Ca(OH)_2(aq) + CO_2(aq) \rightarrow CaCO_3(aq) + H_2O(l)$$

• Indicator test: carbonate is the conjugate base of hydrogen carbonate (weak acid). When dissolved in water, it exists in the following equilibrium:

$$CO_3^{2-}(aq) + H_2O(l) \leftrightharpoons HCO_3^-(Aq) + OH^-(aq)$$ 

• Red litmus paper turns blue
• Universal indicator turns blue

• Precipitation test: carbonate ions produce precipitates with most cations Thus, this is usually an ineffective option for identifying carbonate.

Limewater test for CO₂.

Tests for Sulfate (SO₄^2–)

• Precipitation test: adding acidified barium ions (Ba²⁺) will produce a white precipitate.
• ‘acidified’: a few drops of dilute nitric acid are added prior to precipitation test to remove any carbonate ions that could also form a precipitate with Ba²⁺.

Precipitation of barium sulfate BaSO₄ (s)

• Indicator test: sulfate is the conjugate base of H₂SO₄ (strong acid). Therefore, salts containing sulfate are neutral.
• Red litmus paper remains red
• Universal indicator remains green (pH = 7.0–7.4)

Tests for Phosphate (PO₄^3–)

• Precipitation test: adding an alkaline solution of barium ions (Ba²⁺) will produce a white precipitate.

• Phosphate ions do not precipitate with Ba²⁺ when there are H⁺ present because phosphate ions reacts with H⁺ ions to produce hydrogen phosphate. Ba²⁺ ions do not precipitate with hydrogen phosphate.

$$PO_4^{3-}(aq) + H^+(aq) \leftrightharpoons HPO_4^{2-}(aq)$$ 

• Ba²⁺ will precipitate with phosphate ions when a weak base is added to reduce [H⁺] and shift the equilibrium towards the side with phosphate ions. An example of a weak base is ammonia NH₃.

• Ammonium molybdate phosphate test
• Nitric acid and ammonium molybdate (NH₄)₂MoO₄ will form a yellowcomplex after reacting with phosphate ions (ammonium phosphate molybdate)
• Often used as a qualitative method to monitor environmental phosphate level.
• Multi-step reaction (extension):

• Indicator test: phosphate is the conjugate base of hydrogen phosphate (weak acid). When dissolved in water, it exists in the following equilibrium:

$$PO_4^{3-}(aq) + H_2O(l) \leftrightharpoons HPO_4^{2-}(aq) + OH^-(aq)$$ 

Thus, salts containing phosphate are basic

• litmus paper turns blue
• Universal indicator turns blue

Ammonium phosphate molybdate

1. Carbonate ions are identified through neutralisation with a strong acid (HNO₃) to produce CO₂.
2.  Hydroxide ions are identified by adding Cu²⁺ ions to produce a blue precipitate. This test does not work if phosphate ions are also present.
3. Sulfate ions are identified through addition of Ba(NO₃)₂.
4. Phosphate ions are identified through addition of alkaline Ba(NO₃)₂.
5. Chloride ions are identified through addition of AgNO₃.
6. If no precipitate forms, acetate ions are present.

Test using HNO₃(aq)

Test using Ba(NO₃)₂(aq)

Test using AgNO₃(aq)

Chloride Cl^–

X

X

White precipitate

Bromide Br^–

X

X

Cream precipitate

Iodide I^–

X

X

Yellow precipitate

Sulfate SO₄^2–

X

White precipitate in acidic conditions

White precipitate

Phosphate PO₄^3–

X

White precipitate in alkaline conditions. Dissolves when nitric acid is added.

X

Carbonate CO₃^2–

Bubbling. Formation of CO₂

White precipitate

White precipitate

Hydroxide OH^–

Temperature increase due to neutralisation

X

Brown precipitate(Ag₂O)

Acetate CH₃COO^–

Smell of vinegar. Formation of acetic acid

X

White precipitate

Chloride Cl^–

Bromide Br^–

Iodide I^–

Sulfate SO₄^2–

Phosphate PO₄^3–

Carbonate CO₃^2–

Hydroxide OH^–

Acetate CH₃COO^–

Lead Pb²⁺

White

White

Yellow

White

White

White

White

Sparingly soluble (white)

Silver Ag⁺

White

Cream

Yellow

White

Yellow

Yellow

Brown

Sparingly soluble (white)

Barium Ba²⁺

✓

✓

✓

White

White

white

✓

✓

Calcium Ca²⁺

✓

✓

✓

White

White

White

Sparingly soluble (cloudy white)

✓

Magnesium Mg²⁺

✓

✓

✓

✓

White

White

White

✓

Copper(II)

Cu²⁺

✓

✓

White

✓

Blue-green

White

Blue

✓

Iron(II) Fe²⁺

✓

✓

✓

✓

Brown

White

Green

✓

Iron(III) Fe³⁺

✓

✓

✓

✓

Brown

Orange-red

Brown

✓