Precipitation Titration

Precipitation Titration Techniques :

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.
 

Mohr’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 :

                       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 :

                   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.

Method Selection

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.

A titration is an analytical procedure in which a reagent called a titrant is slowly added to another substance. A rapid stoichiometric reaction takes place as the titrant is added, and both the addition and the reaction continue until one of the reactants is exhausted. Some process, device, or change in the solution indicates that this endpoint has been reached. The purpose of a titration is to determine the amount, or the concentration, of one of the reactants, which can be done if the amount, or concentration and volume, of the other reactant required to reach the endpoint of the titration is known.

In a precipitation titration, the stoichiometric reaction is a reaction which produces in solution a slightly soluble salt that precipitates out. To determine the concentration of chloride ion in a particular solution, one could titrate this solution with a solution of a silver salt, say silver nitrate, whose concentration is known. The chemical reaction occurring is

Ag⁺(aq) + Cl^-(aq)  AgCl(s).

A white precipitate of AgCl is deposited on the bottom of the flask during the course of the titration. Since the chemical reaction is one silver ion to one chloride ion, we know that the amount of silver ion used to the equivalence point equals the amount of chloride ion originally present. Since by the definition of molarity n = cV, the number of moles of either silver ion or chloride ion can be calculated from the number of moles of the other, and the molar concentration or the volume of added solution can be calculated for either ion if the other is known.

Endpoints :

In any titration, it is necessary to have some method of detecting when just enough of the titrant has been added -- a procedure known as detecting the endpoint of the titration. The endpoint of this titration can be detected if the rapid change in either the concentration of silver ion or the the concentration of chloride ion which occurs at the endpoint can be made apparent to an observer. Either instrumental methods or equilibrium methods can be used. The equilibrium methods are fairly straightforward. In this case we can use Ag₂CrO₄, because a solution of CrO₄^2- is yellow while a solution or precipitate of Ag₂CrO₄ is blood-red.

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