Question

Silver nitrate is used in the limit test of ________.

• A. lead
• B. sulphate
• C. chloride
• D. iron

Hint:

Associate these key reagents with their corresponding limit tests: - Silver Nitrate (\(\text{AgNO}_3\)) \(\longrightarrow\) Limit Test for Chloride (forms white \(\text{AgCl}\) turbidity). - Barium Chloride (\(\text{BaCl}_2\)) \(\longrightarrow\) Limit Test for Sulfate (forms white \(\text{BaSO}_4\) turbidity). - Thioglycolic Acid \(\longrightarrow\) Limit Test for Iron (forms a purple complex).

Answer 1

The Correct Option is C

Concept: In pharmaceutical quality control and analytical chemistry, limit tests are semi-quantitative experiments designed to detect and restrict small traces of inorganic impurities present in chemical substances. These tests rely on simple precipitation or colorimetric reactions. Comparing the visual turbidity or color of a sample container against a prepared standard container determines if the impurity level falls within permissible safety guidelines. Let us review the chemical reagents used in the limit tests for each option:
• Option (A) Limit Test for Lead: This test relies on reacting lead impurities with dithizone (diphenylthiocarbazone) in an alkaline medium. This reaction forms a lead-dithizone complex that is dull red, which is then extracted using chloroform for colorimetric comparison.
• Option (B) Limit Test for Sulphate: This test is based on precipitating sulfate ions using barium chloride (\(\text{BaCl}_2\)) in an acidic medium containing a small amount of hydrochloric acid. This reaction produces an insoluble precipitate of barium sulfate (\(\text{BaSO}_4\)), creating a visible white turbidity.
• Option (C) Limit Test for Chloride: This test is based on the chemical reaction between soluble chloride impurities and silver nitrate (\(\text{AgNO}_3\)) in the presence of dilute nitric acid (\(\text{HNO}_3\)). The reaction can be expressed as: \[ \text{Cl}^- (\text{aq}) + \text{AgNO}_3 (\text{aq}) \xrightarrow{\text{dil. } \text{HNO}_3} \text{AgCl} (\text{s}) \downarrow + \;\text{NO}_3^- (\text{aq}) \] The formation of insoluble silver chloride (\(\text{AgCl}\)) creates a white opalescence or turbidity in the solution. Nitric acid is added to make the precipitate insoluble and to prevent interference from other acid-soluble ions like carbonates or phosphates.
• Option (D) Limit Test for Iron: This test relies on reacting iron with thioglycolic acid in a medium buffered with citric acid and made alkaline with ammonia. This reaction produces a soluble ferrous thioglycolate complex that displays a deep reddish-purple color. Conclusion: Silver nitrate is the standard precipitate-inducing reagent used specifically in the pharmaceutical limit test for chloride.