Precipitation titration is a volumetric analytical method based on the formation of a sparingly soluble precipitate between the titrant and the analyte. The most common applications are the determination of halides (Cl-, Br-, I-) and other anions using silver nitrate as the titrant.
Principle of Precipitation Titration
A standard solution of silver nitrate (AgNO3) is added to a solution containing halide ions, forming an insoluble silver halide precipitate. The endpoint is detected when all analyte ions have been precipitated and excess titrant causes a detectable change. The solubility product (Ksp) of the precipitate determines the sharpness of the endpoint — AgCl with Ksp = 1.8 × 10^-10 gives a sharper endpoint than AgBr or AgI.
The Mohr Method
The Mohr method uses potassium chromate (K2CrO4) as the indicator for chloride and bromide determination. Before the endpoint, Ag+ reacts preferentially with Cl- to form white AgCl precipitate; after all Cl- is consumed, excess Ag+ reacts with CrO42- to form brick-red Ag2CrO4. The endpoint is detected by the appearance of a permanent reddish-brown color. The method requires neutral to slightly alkaline pH (6.5-9), as in acidic conditions CrO42- is protonated to HCrO4-, reducing chromate concentration and delaying the endpoint.
The Volhard Method
The Volhard method is an indirect titration method for halides using iron(III) alum (NH4Fe(SO4)2) as an indicator. For chloride, a known excess of AgNO3 is added to precipitate all Cl- as AgCl, and the unreacted Ag+ is back-titrated with standard KSCN or NH4SCN in acidic conditions. At the endpoint, excess SCN- reacts with Fe3+ to form red-brown [Fe(SCN)]2+, indicating the endpoint. For AgCl titrations, nitrobenzene is added to coat the AgCl precipitate and prevent its reaction with SCN-, which would cause a fading endpoint. The Volhard method is performed in acidic medium (0.1-1 M HNO3), which prevents interference from anions that form insoluble silver salts but are soluble in acid.
The Fajans Method
The Fajans method uses adsorption indicators that adsorb onto the precipitate surface at the endpoint, changing color. Common adsorption indicators include fluorescein (yellow-green to pink for chloride), dichlorofluorescein, and eosin (for bromide and iodide). The indicator is an organic dye that exists in solution as an anion; before the endpoint, the precipitate surface is negatively charged from adsorbing excess Cl-, and after the endpoint, excess Ag+ gives the precipitate a positive charge, attracting the indicator anion and causing a color change. The method requires careful pH control: fluorescein works at pH 7-10, dichlorofluorescein at pH 4-10, and eosin at pH 1-2.
Comparison of Methods
The Mohr method is simple and direct but limited to neutral pH and cannot titrate I- or SCN- because their silver salts adsorb chromate. The Volhard method works in acidic medium, is suitable for all halides and SCN-, and is more accurate for bromide and iodide, but requires back-titration. The Fajans method provides direct and sharp endpoints but requires careful selection of indicator and pH control.
Applications
Precipitation titration is used for determination of chloride in water, food products, and biological samples; analysis of bromide in pharmaceutical preparations and photographic chemicals; quantification of iodide in table salt, disinfectants, and nutritional supplements; determination of cyanide and thiocyanate in industrial wastewater; and silver assay in ores and alloys using the Volhard method.
