4.8.3.3,4.8.3.4,4.8.3.5 - Tests for carbonate, halide and sulfate ions

Carbonate and gas evidence

Carbonate ions react with dilute acid to release carbon dioxide, so the carbonate test combines a reaction with a gas identification.

An anion is a negatively charged ion. Carbonate is $\ce{CO3^2-}$. Add dilute acid to a small sample of the unknown solid or solution in a test tube. If a carbonate is present, effervescence may be seen as carbon dioxide is formed. Pass the gas through fresh limewater using suitable delivery apparatus. Limewater turning milky/cloudy confirms carbon dioxide and supports the presence of carbonate in the stated sample.

For example, calcium carbonate reacts with hydrochloric acid:

The gas then forms an insoluble calcium carbonate precipitate in limewater. Effervescence alone is not sufficient: acids can release other gases from other substances. The observation “limewater becomes cloudy” is the specific gas evidence.

Use small quantities and eye protection. A fitted bung and delivery tube can direct gas into the limewater, but the gas must have an open route out; do not seal a gas-producing reaction in a closed container. Check connections if no gas reaches the reagent. Record both bubbling in the sample and the limewater observation rather than writing “carbonate” as though it were directly visible.

Acidified silver nitrate

Halide ions are distinguished by the colours of precipitates formed with silver nitrate after acidification with dilute nitric acid.

The halide ions here are chloride $\ce{Cl^-}$, bromide $\ce{Br^-}$ and iodide $\ce{I^-}$. Take a fresh small portion of the sample solution. Add dilute nitric acid, then silver nitrate solution. Silver ions combine with halide ions to form insoluble silver halides.

A full example equation is:

The solid silver chloride is the white precipitate; sodium nitrate stays in solution. The test identifies the halide anion, so it does not show that sodium is the cation in an unknown. Sodium chloride is simply the example salt used in the equation.

Nitric acid removes carbonate interference: otherwise carbonate ions could also form an insoluble silver compound and confuse the result. Add the acid before silver nitrate so this interference is removed first. Do not replace nitric acid with hydrochloric acid: hydrochloric acid adds chloride ions and can itself produce a misleading white silver chloride precipitate. Nitric acid introduces no halide from the candidate set.

Observe the precipitate against a suitable light background. Cream and pale yellow can be difficult to distinguish, so compare with known samples and repeat with clean apparatus when necessary. Handle acid and silver nitrate carefully with eye protection and follow the disposal instructions.

Acidified barium chloride

Sulfate ions produce a white barium sulfate precipitate when barium chloride solution is added in the presence of dilute hydrochloric acid.