Anion tests: carbonate, sulfate and halide

Chemical analysis • Identification of ions

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Observation distinguishing a sulfate precipitate from a carbonate precipitate with barium

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A barium precipitate that remains after acidification indicates sulfate; a precipitate that disappears on acidification indicates carbonate.

Key concepts

What you'll likely be quizzed about

Carbonate test - reaction and observation

Carbonate ions react with dilute acids to produce carbon dioxide gas and a salt. Addition of any dilute acid causes effervescence as CO2 forms. The evolved gas turns limewater cloudy because CO2 reacts with calcium hydroxide to form insoluble calcium carbonate. Limitations include the presence of acids that may react with other species and slow reactions with insoluble carbonates. Confirmatory testing requires collecting the gas or directing it through limewater to avoid misinterpretation of bubbling caused by other reactions.

Sulfate test - procedure and interpretation

Sulfate ions produce an insoluble white precipitate of barium sulfate when acidified sample is treated with barium chloride. Acidification with dilute hydrochloric acid removes carbonate ions that would otherwise produce a barium carbonate precipitate, preventing false positives. Observation of a white precipitate that is insoluble in acid indicates sulfate. Limiting factors include very low sulfate concentration that produces only faint precipitate and sample turbidity that obscures colour. Always acidify before adding barium reagent to distinguish sulfate from carbonate interference.

Halide tests - sequence, colours and solubility

Halide ions (chloride, bromide, iodide) form silver halide precipitates on addition of silver nitrate after acidification with nitric acid. Typical colours: chloride produces a white precipitate, bromide produces a cream precipitate, iodide produces a yellow precipitate. Nitric acid removes carbonate or hydroxide that would otherwise form interfering precipitates. Solubility in ammonia distinguishes halides: silver chloride dissolves in dilute ammonia, silver bromide dissolves in concentrated ammonia, and silver iodide remains insoluble. Correct identification follows: acidify → add AgNO3 → observe colour → test solubility with ammonia.

Common interferences and procedural limits

Carbonate, sulfate and halide tests depend on selective precipitation and gas evolution; interfering ions and incorrect reagent order produce false results. Carbonates cause CO2 evolution and creation of barium carbonate unless acidified first. Hydroxides and other precipitating species can mimic silver halide colours if nitric acid is omitted. Low ion concentrations can give weak or slow reactions that require careful observation and sometimes repeat testing with fresh reagents. Analytical confidence increases through proper acidification, use of correct reagents (HCl or HNO3 as specified), and follow-up solubility tests for confirmation.

Key notes

Important points to keep in mind

Add dilute acid to test for carbonate; effervescence indicates CO2 formation.

Confirm carbon dioxide by bubbling gas through limewater to produce cloudiness.

Acidify samples before adding barium chloride to test for sulfate to avoid false positives from carbonates.

A white, acid-insoluble precipitate with barium indicates sulfate.

Acidify with nitric acid before silver nitrate to avoid introducing chloride contamination.

Silver chloride is white, silver bromide is cream, silver iodide is yellow.

Test halide precipitate solubility with dilute ammonia then concentrated ammonia to distinguish halides.

Low concentrations and sample impurities reduce test reliability; repeat tests or use fresh reagents if results are unclear.

Observe precipitate colour in good lighting and compare with known standards for accurate identification.

Order of reagents and correct acids prevents false positives from carbonate or chloride contaminants.