3. Limit Tests for Iron ($Fe$) & Lead ($Pb$)

The Limit Test for Iron is based on the reaction between iron ($Fe^{2+}$ and $Fe^{3+}$) and Thioglycolic Acid in an alkaline ammoniacal medium buffered with Citric Acid to produce a water-soluble, stable purple-colored coordination complex of Ferrous Thioglycolate. Citric acid is indispensable because it forms a soluble coordination complex with iron at neutral pH, preventing the unwanted premature precipitation of insoluble brown Ferric Hydroxide [$Fe(OH)_3$] when excess ammonia is added. Thioglycolic acid reduces $Fe^{3+}$ to $Fe^{2+}$, ensuring all iron is measured. The Limit Test for Lead is based on the extraction of trace lead ions ($Pb^{2+}$) from an alkaline aqueous solution (pH 9.0 to 10.0) with a chloroform solution of Dithizone (Diphenylthiocarbazone) to form a brilliant violet-red Lead Dithizonate chelate complex soluble in chloroform.