Indicators for complexometric, redox and precipitation titrations
Eriochrome Black T, redox indicators, the Mohr, Volhard and Fajans indicators and starch: transition ranges, colour changes and uses. Transition ranges are computed from each indicator’s equilibrium constants.
| Eriochrome Black T (pH 10) | metallochromic | pMg 4.4–6.4 | wine red → blue | Mg²⁺, Ca²⁺ + Mg²⁺ (water hardness) |
|---|---|---|---|---|
| Eriochrome Black T (pH 10) | metallochromic | pCa 2.8–4.8 | wine red → blue | Ca²⁺ alone: changes too early; add Mg–EDTA |
| Diphenylamine sulfonate | redox | 0.82–0.88 V | colourless → red-violet (on oxidation) | Fe²⁺ with K₂Cr₂O₇ (add H₃PO₄) |
| Ferroin | redox | 1.09–1.21 V | red → pale blue (on oxidation) | Fe²⁺ with Ce⁴⁺ and similar |
| 5-Nitroferroin | redox | 1.19–1.31 V | red-violet → pale blue (on oxidation) | Fe²⁺ with Ce⁴⁺ and similar |
| Potassium chromate (Mohr) | precipitation | Ag₂CrO₄ forms; about 0.005 M CrO₄²⁻, pH 7–10 | yellow → brick-red precipitate | Cl⁻ and Br⁻, direct titration |
| Iron(III) (Volhard) | complex formation | [FeSCN²⁺] ≈ 6.4 × 10⁻⁶ M visible, nitric acid | colourless → red | back titration of Ag⁺; filter off AgCl for chloride |
| Dichlorofluorescein (Fajans) | adsorption | precipitate surface turns positive after equivalence | yellow-green → pink (on the precipitate) | Cl⁻ and others; keep colloidal, avoid light |
| Starch | specific | deep-blue complex with I₃⁻ (about 610 nm) | deep blue → colourless (iodometric endpoint) | iodometry and iodimetry; add when pale yellow |
Eriochrome Black T ranges are lg K′(MIn) ± 1 at pH 10; redox indicator ranges are E° ± 0.059/n V versus SHE.