5.5 - Tests for Anions

Qualitative analysis

• Qualitative analysis identifies which substances or ions are present in a sample.
• Quantitative analysis determines how much of a substance is present.
• A chemical test must give a result unique to that ion so it can be identified without ambiguity.

Precipitation reactions

• A precipitate is an insoluble solid that forms when two solutions are mixed.
• In a precipitation test, a reagent is added that forms an insoluble solid with the ion being tested.
• If that ion is present, a precipitate forms; its distinct colour or appearance identifies the ion.
• Both negative ions (anions) and positive ions (cations) can be identified using precipitation tests.

The test for carbonate ions

• A carbonate contains the carbonate ion, CO₃^2-, a negatively charged ion (an anion).
• Carbonates react with dilute acids to produce a salt, water, and carbon dioxide gas.

Carrying out the test

1. Add dilute acid (usually dilute hydrochloric acid) to the sample.
2. If carbonate ions are present, the mixture fizzes (effervesces) as carbon dioxide gas bubbles off.
3. Bubble the gas through limewater (calcium hydroxide solution).

Reading the result

• Carbon dioxide reacts with calcium hydroxide to form insoluble calcium carbonate, turning the limewater cloudy (milky).
• Fizzing and the limewater turning cloudy white is the positive result: the sample contains carbonate ions.

The test for halide ions

Chloride (Cl^-), bromide (Br^-), and iodide (I^-) are the halide ions - the negatively charged ions (anions) of Group 7.

Carrying out the test

1. Add dilute nitric acid - This reacts away any carbonate ions in the sample that would otherwise form a pale precipitate and give a false positive.
   • Hydrochloric acid must not be used, as it contains chloride ions that would react with the silver nitrate.
2. Add silver nitrate solution - Silver ions react with halide ions to form insoluble silver halides.

Results

• Chloride (Cl^-) - Forms a white precipitate (silver chloride, AgCl).
• Bromide (Br^-) - Forms a cream precipitate (silver bromide, AgBr).
• Iodide (I^-) - Forms a yellow precipitate (silver iodide, AgI).

The test for sulfate ions

The sulfate ion, SO₄^2-, is the negatively charged ion (the anion) in a sulfate.

Step 1 - Add dilute hydrochloric acid

• Carbonate and sulfite ions also form insoluble white solids with barium ions; the acid reacts them away first to prevent a false positive.
• Sulfuric acid must not be used, as it adds sulfate ions and guarantees a false positive.

Step 2 - Add barium chloride solution

Barium ions react with sulfate ions to form a white precipitate of insoluble barium sulfate.

A white precipitate is the positive result: the sample contains sulfate ions.