Titration involving precipitation at end of process is called as precipitation titration. Most of metallic halides are titrated by precipitation method.
It is also called as argentimetric titration. There are three methods used for determining end point in precipitation titration.
Mohr’s method:
Neutral halides are titrated with silver nitrate using potassium chromate as indicator and this method is called as Mohr’s method as developed by Mohr scientist. At the end of titration brick red precipitates of silver halides are formed.
Volhard’s method:
Acidic halides are treated with excess amount of silver nitrate to form precipitates of halides which are filtered and the filtrate is then titrated with standard ammonium thiocyanate with ferric ammonium sulphate as indicator. At the end of titration permanent red color is obtained due to formation of ferric thiocyanate. This method is developed by Volhard so called as Volhard’s method.
Fajan’s method:
Adsorption indicators are used in this method and developed by Fajan so called as Fajan’s method. At the end point, indicator is absorbed by the precipitate and during the process of adsorption, a change in color of indicator take places so they are called as adsorption indicator.
Example: Fluorescein
Dichlorofluorescein
Eosin
Di-iodo diethylfluorescein
Tartarazine
Phenosaframine
Rhodamine 6 G
Application of Precipitation titration:
Use for determination of halides and pseudo-halides
To determine solubility constant of compounds
To determine electrode potential
For determining chloride, cyanides and thiosulphites