IGCSE Chemistry 0620: Testing Rinse Water for Chloride and Sulfate Ions

A small leather-tanning workshop discharges its rinse water into a settling tank before further treatment. Local environmental rules require the workshop to test the rinse water for two ions it commonly contains (chloride ions, from the salt used to cure the hides, and sulfate ions, from the tanning chemicals) before the water can be released for full treatment.

(a) Describe the reagent(s) added and the observation that would confirm sulfate ions are present in a sample of the rinse water. [2]

(b) Describe the reagent(s) added and the observation that would confirm chloride ions are present in a second sample of the rinse water. [2]

(c) For part (b), a technician wants to save time by using dilute hydrochloric acid, instead of dilute nitric acid, to acidify the sample before adding the aqueous silver nitrate. Explain why this would make the chloride test unreliable. [2]

Part (a): Testing for sulfate ions

The standard test for sulfate ions, $\text{SO}_4^{2-}$ , in solution is:

Acidify the sample with dilute nitric acid.
Add aqueous barium nitrate.

$\text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s)$

A white precipitate (barium sulfate) forming confirms that sulfate ions are present.

Part (b): Testing for chloride ions

The standard test for chloride ions, $\text{Cl}^-$ , in solution is:

Acidify the sample with dilute nitric acid.
Add aqueous silver nitrate.

$\text{Ag}^+(aq) + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s)$

A white precipitate (silver chloride) forming confirms that chloride ions are present.

Part (c): Why nitric acid, not hydrochloric acid, must be used

Dilute nitric acid is used to acidify the sample because it does not add any ions that would interfere with the precipitation test that follows.

Hydrochloric acid, $\text{HCl}$ , dissociates in water to give chloride ions:

$\text{HCl}(aq) \rightarrow \text{H}^+(aq) + \text{Cl}^-(aq)$

If hydrochloric acid were used instead of nitric acid, these extra chloride ions would react with the aqueous silver nitrate to form a white precipitate of silver chloride, even if the original rinse water contained no chloride ions at all. The test would then give a positive result regardless of what was actually in the sample, so it would no longer be a reliable way to find out whether the rinse water itself contains chloride ions.

Final answers

(a) Add dilute nitric acid, then aqueous barium nitrate: a white precipitate confirms sulfate ions.
(b) Add dilute nitric acid, then aqueous silver nitrate: a white precipitate confirms chloride ions.
(c) Hydrochloric acid supplies its own chloride ions, so it would cause a white precipitate with silver nitrate whether or not the rinse water contained chloride ions, making the test unreliable.