Volhard

An example of back titration, the Volhard method, named after Jacob Volhard, involves the addition of excess silver nitrate to the analyte; the silver chloride is filtered, and the remaining silver nitrate is titrated against ammonium thiocyanate,^[1] with ferric ammonium sulfate as an indicator which forms blood-red [Fe(OH₂)₅(SCN)]²⁺ at the end point:

Ag⁺ (aq) + SCN⁻ (aq) → AgSCN (s) (K_sp = 1.16 × 10⁻¹²)

Fe(OH)(OH₂)²⁺
₅ (aq) + SCN⁻ (aq)→ [Fe(OH₂)₅(SCN)]²⁺ + OH⁻

Mohr

In the Mohr method, named after Karl Friedrich Mohr, potassium chromate is an indicator, giving red silver chromate after all chloride ions have reacted:

2Ag⁺ (aq) + CrO²⁻
₄ (aq) → Ag₂CrO₄ (s) (K_sp = 1.1 × 10⁻¹²)

The solution needs to be near neutral, because silver hydroxide forms at high pH, while the chromate forms Ag₂Cr₂O₇ or AgHCrO4 at low pH, reducing the concentration of chromate ions, and delaying the formation of the precipitate. Carbonates and phosphates precipitate with silver, and need to be absent to prevent inaccurate results.

The Mohr method may be adapted to determine the total chlorine content of a sample by igniting the sample with calcium, then ferric acetate. Calcium acetate "fixes" free chlorine, precipitates carbonates, and neutralizes the resultant solution. Ferric acetate removes phosphates. All chlorides are dissolved out of the residue, and titrated.^[1]

Fajans

In the Fajans method, named after Kazimierz Fajans, typically dichlorofluorescein is used as an indicator; the end-point is marked by the green suspension turning pink. Prior to the end-point of the titration, chloride ions remain in excess. They adsorb on the AgCl surface, imparting a negative charge to the particles. Past the equivalence point, excess silver(I) ions adsorb on the AgCl surface, imparting a positive charge. Anionic dyes such as dichlorofluorescein are attracted to the particles, and undergo a colour change upon adsorption, representing the end-point. Eosin (tetrabromofluorescein) is suitable for titrating against bromide, iodide, and thiocyanate anions, giving a sharper end-point than dichlorofluorescein. It is not suitable for titrating against chloride anions because it binds to AgCl more strongly than chloride does.^[2]