3.1.4 Qualitative Analysis

Tests for ions

(a) qualitative analysis of ions on a test-tube scale; processes and techniques needed to identify the following ions in an unknown compound:

(i) Anions:

• CO₃ ^2–, by reaction with H⁺_(aq) forming CO_2(g) (see 2.1.4 c)

• SO₄ ^2–, by precipitation with Ba²⁺_(aq)

• Cl^–, Br^–, I^– (see 3.1.3 g)

(ii) cations: NH₄⁺, by reaction with warm NaOH_(aq) forming NH₃.

{Sequence of tests required is Carbonate, Sulphate then Halide. (BaCO₃ and Ag₂SO₄ are both insoluble.)}

What does this mean?

Qualitative - means that your analysis isn't going to involve numbers - you won't be measuring anything - that would be Quantitative. So you are simply looking for colour changes and other physical indications of a reaction.

Carbonate ion test.

Carbonates react with dilute acids making Carbon Dioxide which we see as bubbles/effervescence/fizzing etc.

Carbonate + Acid → Salt + Water + Carbon Dioxide

REMEMBER : don't write that you can see Carbon Dioxide because you don't KNOW that by looking.

It doesn't matter whether the carbonate you're testing is solid or (more likely) a solution.

You may be asked to write a full equation - but you should have been able to do that in Year 9.

More likely you'd be asked to write the Ionic Equation.

If the Carbonate is in solution

CO₃^2-_(aq)+ 2H⁺_(aq) → CO_2(g) + H₂O_(l)

It doesn't matter which acid you use because the Chloride ions from HCl or Nitrate ions from HNO₃ would stay in solution.

And it doesn't matter which Carbonate solution you're testing because they would stay in solution too.

If the Carbonate is a solid

CaCO_3(s)+ 2H⁺_(aq) → Ca²⁺_(aq) + CO_2(g) + H₂O_(l)

You have to include the metal ion and the whole Carbonate because there is a change of phase (solid to aqueous)

But it still doesn't matter which acid you use because the Chloride ions from HCl or Nitrate ions from HNO₃ would stay in solution again.

Sulphate ion test.

Sulphates don't usually react to make gases so we use our knowledge that Barium Sulphate is insoluble whereas most other Sulphates (except Calcium Sulphate) are soluble.

So we add a soluble Barium compound - usually Barium Chloride - to a solution and if it forms a white precipitate we have confirmed the presence of Sulphate ions.

eg K₂SO_4(aq) + BaCl_2(aq) → BaSO_4(s) + 2 KCl_(aq)

Or as an ionic equation:

SO₄^2-_(aq) + Ba²⁺_(aq) → BaSO_4(s)

Successive tests.

If you are doing a sequence of tests to discover which anion is in a substance you should obviously split your sample and do the tests separately to avoid confusion.

But there is still a set order to do the tests and a good reason for it that's mentioned on the syllabus and so is examinable.

1. Carbonate
2. Sulphate
3. Halide

Why?

We do the Carbonate test first because the Barium ions in the Sulphate precipitates with Carbonate ions too.

Which would give a false positive for Sulphate ions if we hadn't already done the Carbonate test.

So if we do a Carbonate test and it is positive we don't need to do any more tests for anions.

But if it is negative then it is worth carrying on.

The Halide test comes last.

In case you can't split your sample you really shouldn't test for Sulphate with barium Chloride because that would add Chloride ions.

Barium Nitrate would be a suitably soluble alternative.

The Halide tests are carried out after acidifying with Nitric acid.

This is because solutions can absorb Carbon Dioxide to form Carbonate ions.

CO_2(g) + H₂O_(l) → CO₃^2-_(aq) + 2H⁺_(aq)

And the Silver ions we use to test for Halides also precipitate with Carbonate ions

2Ag⁺_(aq) + CO₃^2-_(aq) → AgCO_3(s)

We couldn't use Hydrochloric acid to acidify due to the extra Chloride ions.

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