Precipitation Titrations

 Chapter 7 

Precipitation Titrations 

Introduction 

In titrimetric, the end point of any titration occurs at the completion of reaction of titrant with titrate. The reactions may be of various types such as neutralization, oxidation-reduce ion or complexation. Similarly, the reaction may end with formation of precipitate. Thus, any titration in which the end product formed at the completion of reaction is insole le precipitate, the titration is called as 'perc imitation   titrate ion'. Precipitation is the combination of two ionic species to form a very insoluble product.

Precipitation Titrations 

Precipitates are insoluble le ionic solids that are formed in aqueous solute ion by comb ination of cations and anions. However, all reactions in aqueous solutions are not necessarily produce precipitates. Factors that determine the formation of precipitates vary. Some reactions depend on temperature, whereas other reactions depend on so solute ion.

Solubility Product and Precipitation 

Cossid er an aqueous solution of a slightly soluble salt BA in equilibrium with excess of the solid at constant temperature. The equilibriums can be represented by:

 Were, BA (Jsr present's the solid phase. In dilute aqueous solute ions essentially Noun dissociated BA will be present in the sole tio n. Since the activity of solid is constant, the equilibrium constant for equation may be written as:                               

                                                           K,p      [B”] [A]

K,p is the solubility product, which is a constant for a given solute, solvent and temperature

Calculation of Solubility and Solubility Product 

The solubility product of a slightly soluble electrolyte can be calculated if its solubility is known and vice versa.

 Calculation of solubility product:

1.     Write the equation for the dissociation of the electrolyte, the solubility product of which is to be calculated. 
2.    Express the solubility product of the electrolyte as the product of the concentration of its ions.

1.                              MgCO₃          Mg” ” + CO3 "

2.                                             Kit (MgCO3)          [Mg* *] [CO3 "]

Factors Influencing Solubility Fractional Precipitation

The solubility of any slightly soluble salt can be decreased by adding an excess of either

of its ions, e.g., the dissociation of a slightly soluble salt BA is:

BA(s)          B”  A"

and                                          K,p      [B*] [A"]

This is the equilibrium condition. If, however, an excess of either B* cation or A- are added in the form of another salt (whose solubility) is greater than that of BA then the product of ionic concentrations [B*] [A"] will exceed the solubility product and hence BA will precipitate. The common ion effect provides a valuable method for controlling the concentration of the ions furnished by a weak electrolyte

Detection of End Point 

 Mohr's Method

Mohr ‘s method involves determination of the chloride ion concentration of a solution by titration with silver nitrate. In this method, the silver nitrate solution is slowly added to chloride solutions to produces a precipitate of silver chloride.

When all the chloride ions are precipitated, the excess of silver ions react with the chromate ions from potassium chromate which is the indicator. A red brown precipitate of silver chromate will be formed which gives the end point of the titration

Volhard's Method 

Volhard's method involves the determination of concentration of chloride ions in a solution using a back titration with potassium the isocyanate e. Initially an excess volume of a silver nitrate solution is added to the solute ion containing chloride ions. This is done before the titration, and this produces a precipitate of silver chloride. The add it ion of excess volume of silver nitrate allows the complete reaction of chloride ions present in the solute ion with silver nitrate

Modified Volhard's Method

In Volhard's method, a slight modification is done to prevent the interference of silver chloride with the titration process. Hence, it is termed as modified Volhard's method. This method is used for, especially determination of NaCl or KCI. When silver nitrate is reacted with sodium chloride, silver chloride is formed, and the excess of silver nitrate is back titrated with potassium or ammonium thiocyanate using ferric alum as an indicator. However, the silver chloride may react with the potassium or ammonium Thio cyanate, thus, int refer with the titration of excess silver nitrate with potassium or ammonium thiocyanate.

 Fujan's Method 

Fujan's method involves the titration of chloride ions with silver ions using adsorption on indicators. K Fujan, introduced assort ion type of indicator for precipitation ti tragions. These indicators are basically dyes that adsorb or desorb on the surface of the precipitate at the equivalence point and produces the color change. These nidi actors are either acid Desheng. fluorescing, eosin etc. or basic dyes, e.g. rhodamine e series. The best indicator known is fluorescing for the titration of chloride ions with silver.

Estimation of Sodium Chloride (NaCl) 

Assay:

Sodium chloride can be estimated using Volhard 's method, Mohr's method and Fagan's method. The principals involved in all three methods have already been explained in the previous sections. As per Indian pharmacopoeia, Sod iu m chloride in a Samp le is determined by the following method.

Weigh accurately about 0.1 g of the sample and transfer into a 250 ml glass-stoppered flask. Dissolve the contents using SO ml of water. Add 50.0 ml of 0.1 M silver nitrate, 5 ml of 2 M nitric acid and 2 ml of dibutyl phthalate, shake well and titrate with 0.1 M ammonium thiocyanate using 2 ml of ferric ammonium sulphate solute on as indicator, until the cooler becomes reddish yellow. 1 ml of 0.1M silver nitrate is equivalent to 0.005844 g of NaCl.

Alternatively, Chlora id e ions in NaCl are stim acted using Volhard's m ethos which involves a back tit ration with potassium thiocyanate. Initially, an excess amount of silver nitrate solute on is added to the solution containing chloride ions prior r to the titration which form's a precept ate of sil veer chloride. The unreacted silver nitrate e is then titrated with thiocyanate