12.7 - Tests for Anions

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 - Acidify with dilute nitric acid

• Carbonate ions also form an insoluble white solid 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 aqueous barium nitrate

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.

The test for nitrate ions

• The nitrate ion, NO₃^-, is a negatively charged ion (an anion).
• Because nitrate salts are soluble, nitrate ions are identified by reducing them to a gas rather than by precipitation.

Carrying out the test

1. Add aqueous sodium hydroxide to the test solution.
2. Add aluminium foil and warm the mixture gently.
3. The aluminium reduces the nitrate ion, producing ammonia gas (NH₃).
4. Test the gas with damp red litmus paper.

Reading the result

• Ammonia dissolves in the water on the damp paper to form an alkaline solution, turning the red litmus blue.
• The paper must be damp - dry paper won't change colour because the gas needs moisture to form hydroxide ions.