Precipitation titrations are a form of titration useful in the determination of halides such as chlorides, bromides and iodides. These titrations involve the use of a precipitating agent such as silver nitrate, and are therefore also known as argentimetric titrations. Depending on the method of detecting the end point of the titration, there are three methods in precipitation titrations: Mohr’s method, Volhard’s method and Fajan’s method.
The Mohr’s method involves the use of a silver nitrate solution as the titrant for the determination of chlorides and bromides in the presence of potassium chromate indicator. When a chloride containing solution reacts with a standard solution of silver nitrate, it results in the formation of silver chloride. When all the chloride existing in solution is completely precipitated in this manner, the next excess drop of the titrant leads to reaction between silver and the indicator ions. This formation of silver chromate gives a visible end point when the color of the solution changes from yellow to a red precipitate.
Volhard’s method involves the titration of chlorides, bromides and iodides in an acidic medium. Here, a known excess amount of silver nitrate solution reacts with the chloride in solution. When all the chloride is converted to silver chloride, the silver nitrate left behind is estimated by back titration against a standard solution of potassium thiocyanate. After all the silver is consumed in the reaction with thiocyanate, the next excess of thiocyanate reacts with the ferric ammonium sulfate indicator and gives a red color caused by formation of the ferrous thiocyanate complex.
Fajan’s method makes use of a reaction between the indicator and the precipitate formed during the titration. A dye such as dichlorofluorescein is the indicator, and exists as an anion in solution. In a solution of chloride, since chloride ions are in excess, they form the primary layer on the precipitate, with the cations of sodium held as the secondary layer. On completion of the reaction, at the end point, the silver ion is in excess. As a result, the primary layer is now the silver ion which is positively charged and attracts the anion of the indicator to form the secondary layer. The color of the free indicator is different from that of the adsorbed indicator. This provides a visible end point to signal that the reaction is complete.
Mohr’s method is used for the determination of chloride in neutral solutions. Under acidic conditions, the chromate ion is protonated to form chromic acid, which does not produce the precipitate at the end point. Too alkaline a solution results in the formation of silver hydroxide, which has a brown color that interferes in detection of the end point. Volhard’s method gives best results in an acidic medium. In neutral solutions, the ferric ion of the ferric ammonium sulfate indicator is precipitated as iron hydroxide, which interferes in the reaction.