ERT207 Analytical Chemistry Precipitation Reactions and Titrations Pn Syazni Zainul Kamal

Types of titrimetric methods Classified into four groups based on type of reaction involve: 1. Acid-base titrations 2. Complexometric titrations 3. Redox titrations 4. Precipitation titrations

Precipitation Reactions  Precipitation is the formation of a solid in a solution solidsolutionsolidsolution  solid formed is called the precipitate  A precipitation reaction occurs when water solutions of two different ionic compounds are mixed and an insoluble solid separates out of solution. Eg KCl + AgNO 3 AgCl + KNO 3  The precipitate is itself ionic; the cation comes from one solution and the anion from another. Cl - solution Precipitating agentprecipitate

Solubility of precipitate Solubility of a compound = concentrations of a soluble species at equilibrium with its insoluble form. Solubility of a compound = concentrations of a soluble species at equilibrium with its insoluble form. If the compound is sparingly soluble, it will produce cation & anion. If the compound is sparingly soluble, it will produce cation & anion. Eg AgCl slightly dissolved in water. So AgCl has a specific solubility, s = solid phase aq = aqueous phase Eg AgCl slightly dissolved in water. So AgCl has a specific solubility, s = solid phase aq = aqueous phase AgCl (s) Ag + (aq)+ Cl - (aq)

The equilibrium constant for the reaction is known as solubility product constant. The equilibrium constant for the reaction is known as solubility product constant. K sp (AgCl) = [Ag + ][Cl - ] Concentration of any solid (AgCl) is constant and is combined in the equilibrium constant to give K sp Concentration of any solid (AgCl) is constant and is combined in the equilibrium constant to give K sp Solubility product constants are used to describe saturated solutions of ionic compounds of relatively low solubility. Solubility product constants are used to describe saturated solutions of ionic compounds of relatively low solubility. A saturated solution is in a state of dynamic equilibrium between the dissolved, dissociated, ionic compound and the undissolved solid. A saturated solution is in a state of dynamic equilibrium between the dissolved, dissociated, ionic compound and the undissolved solid.

Precipitation titration Precipitation titration is a titration method based on the formation of precipitate, which is slightly soluble Precipitation titration is a titration method based on the formation of precipitate, which is slightly soluble Titration with precipitating agents is useful for determining certain analytes, provided the equilibrium will take place rapidly and suitable means of detecting equivalent points is available Titration with precipitating agents is useful for determining certain analytes, provided the equilibrium will take place rapidly and suitable means of detecting equivalent points is available Titration curve – plot of the changes in analyte concentration againts titrant volumes. Titration curve – plot of the changes in analyte concentration againts titrant volumes.

Consider titration of Cl - with a standard solution of AgNO 3. Consider titration of Cl - with a standard solution of AgNO 3. Titration curve prepared by plotting pCl (-log[Cl - ]) againts the volume of AgNO 3 in a manner similar to acidbase titration. Titration curve prepared by plotting pCl (-log[Cl - ]) againts the volume of AgNO 3 in a manner similar to acidbase titration. Before titration started – only have Cl -. Before titration started – only have Cl -. pCl = - log[Cl - ] pCl = - log[Cl - ] Titration proceed – part of Cl - is removed from solution by precipitation as AgCl. Titration proceed – part of Cl - is removed from solution by precipitation as AgCl. pCl = -log [remaining Cl - ]

At equivalence point - we have solution a saturated solution of AgCl. At equivalence point - we have solution a saturated solution of AgCl. [Cl - ]= √K sp Excess AgNO 3 added – excess Ag +. [Cl - ] is determine from the concentration of Ag + and K sp. Excess AgNO 3 added – excess Ag +. [Cl - ] is determine from the concentration of Ag + and K sp. [Cl - ] = K sp /[Ag + ]

Example 1 Calculate pCl for the titration of ml M NaCl with M AgNO 3 for the addition of 0.0, 20.0, 99.0, 99.5, and ml AgNO 3. K sp AgCl is 1.0 x10 -10

solution a) Addition of 0.0 ml Ag + [Cl - ] = M pCl = -log [Cl - ] = -log = -log = 1 = 1

b) Addition of 20.0 ml Ag + Initial mmol Cl - = ml x M = 10.0 mmol mmol added Ag + = 20.0 ml x M = 2.0 mmol mmol Cl - left = 8.0 mmol [Cl - ] left = 8.0 = M (100+20) ml (100+20) ml pCl = -log [Cl - ] = -log = -log = 1.18 = 1.18

c) Addition of 99.0 ml Ag Initial mmol Cl - = ml x M = 10.0 mmol mmol added Ag + = 99.0 ml x M = 9.9 mmol mmol Cl - left = 0.1 mmol [Cl - ] left = 0.1 = 5.01 x10 -4 M (100+99)ml pCl = -log [Cl - ] = -log 5.01 x10 -4 = -log 5.01 x10 -4 = 3.3 = 3.3

d) Addition of ml Initial mmol Cl - = ml x M = 10.0 mmol mmol added Ag + = ml x M= 10.0 mmol Equivalence point is reached. The solution contain saturated AgCl solution K sp = [Ag + ][Cl - ] = 1.0 x [Cl - ] = √K sp = √1.0 x = 1.0 x = 1.0 x pCl = -log 1.0 x10 -5 = 5 = 5

Addition of ml Ag + Initial mmol Cl = ml x M = 10.0 mmol mmol added Ag + = ml x M =10.05 mmol mmol Ag + excess = = 0.05 mmol [Ag + ] excess = 0.05/200.5 ml = 2.5 x M K sp = [Ag + ][Cl - ] = 1.0 x [Cl - ] = K sp = 1.0 x = 4.0 x [Ag + ] 2.5 x M [Ag + ] 2.5 x M pCl = - log 4.0 x = 6.4 = 6.4