Identifying Anions using Precipitation and pH Indicators

 

This is part of the HSC Chemistry course under the topic Analysis of Inorganic Compounds. 

HSC Chemistry Syllabus

Conduct qualitative investigations – using flame tests, precipitation and complexation reactions as appropriate – to test for the presence in aqueous solution of the following

  • anions: chloride (Cl–), bromide (Br–), iodide (I–), hydroxide (OH–), acetate (CH3COO–), carbonate (CO32–), sulfate (SO42–), phosphate (PO43–)

Identifying Anions Using Precipitation and pH Indicators

This video explores a range of qualitative tests that can be conducted in a school laboratory to identify anions including:

  • Halides: chloride, bromide and iodide
  • Acetate
  • Phosphate
  • Sulfate
  • Hydroxide
  • Carbonate

 

 

Common Tests for Identifying Anions

Chloride (Cl)

  • Precipitation test – adding silver ions e.g. AgNO3 will produce a white precipitate (AgCl).

 

Bromide (Br)

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

 

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

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

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:

 

 

    • 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

 

 

 

Carbonate (CO32–)

  • 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.

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

    •  Identify 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.

Sulfate (SO42–)

  • 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)

 

 

 

Phosphate (PO43–)

  • 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

 

 

 

Sequence of Tests for Identifying Anions

  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 SO42–

X

White precipitate in acidic conditions

White precipitate

Phosphate PO43–

X

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

X

Carbonate CO32–

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

 

Cross-solubility table between cations & anions outlined by the HSC syllabus

 

Chloride Cl

Bromide Br

Iodide I

Sulfate SO42–

Phosphate PO43–

Carbonate CO32–

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

 

Note: all nitrates, sodium, potassium and ammonia containing ionic compounds are aqueous.