Dnyanasadhana College, Thane. Department of Chemistry T. Y. B. Sc Dnyanasadhana College, Thane. Department of Chemistry T.Y.B.Sc. Analytical Chemistry Paper-IV Sem-V Precipitation Titrations Dr.G.R.bhagure

Some basic concept Solution Unsaturated solution Saturated solution Super saturated solution Ionic Product Solubility Product Precipitation

Defination of Precipitation NaCl---------Na + + Cl- NaCl + AgN03----- +AgCl+AgN03 Ag + + Cl- ----AgCl ------- AgCl Cl- Ag+ Cl- Ag+ AgCl

Condition for Precipitation Reaction I.P. less than Ksp Solution is unsaturated I.P. is equal to Ksp Solution is saturated I.P. greater than Ksp Solution is supersaturated Precipitation Reaction takes place

Topics Argentimetric titrations Construction of titration curve Mohr’s method Volhards method Adsorption Indicators Theory and application

Argentimetric titrations In the precipitation titrations of halide most common reagent AgN03 is used, hence the titrations are called as Argentimetric titrations

MOHR’S METHOD Product AgCl AgCr04 Titrant Titrand AgN03 K2Cr204 Indicator K2Cr204 Titrand Halide ion Cl- Product AgCl End point Product AgCr04

MOHR’S METHOD + + Product AgCr04 Titrand Titrant K2Cr204 AgN03 AgCl Halide ion Cl- Indicator K2Cr204 Titrant AgN03 + + AgCl End point Product AgCr04

AgCl AgCl Ag2Cro4 Ag2CrO4 Ksp=1.2x10-10 More Soluble Precipitate Precipitate first Ag2Cro4 Ksp=1.7x10-12 Ag2CrO4 Precipitate At the End point Less Soluble

Lets consider the titration of 0. 1M NaCl solution with 0 Lets consider the titration of 0.1M NaCl solution with 0.1M AgNO3 solution by using potassium chromate as indicator . The following equilibrium exist; Ag++ Cl- A AgCl Ksp= 1.2 x10-10 Ag+=

Ag+= KSp=[Ag+]2 [Cr04-2]= 1.7 x 10-12 Similarly the another equilibrium 2Ag++ Cr204 Ag2 Cr204 KSp=[Ag+]2 [Cr04-2]= 1.7 x 10-12 Ag+=

To precipitate AgCl and AgCr04 Ag +ion concentration required for precipitation of AgCl (1.2x10-9) is less than the precipitation of AgCr04 (2.4x10-5)

Similarly concentration of Chloride and Chromate ion can also be calculated At the equivalence point Chloride ion concentration will be [Ag+]=[Cl-] =1.1 x 10-5 Introduce the value of chloride in the above equation we have ;

The concentration of chromate ion should be large i. e The concentration of chromate ion should be large i.e.0.014M,however in actual practice more dilute solution are used [0.003-0.005M] because high concentrated solution will impart colour which makes difficult to detect appearance of red silver chromate To avoid this excess of silver nitrate is added for the appearance of red silver chromate ppt.

Limitations Titration should be carried out in neutral or alkaline [pH 6.5-9]. As solubility of silver chromate increases with increase in temperature to overcome this titration should be carried out at room temperature . Titration of iodide and thiocyanate are not accurate .

VOLHARD,S METHOD Ferric Alum Titrand Excess AgNO3 Titrant KSCN Indicator Ferric Alum AgNO3 Titrand Halide ion Cl- Titrant KSCN Excess AgNO3 Titrated with End point Fe(SCN)3

Sequence of precipitation reaction Cl+ AgNO3 AgCl+ AgNO3(Excess) AgNO3(Excess)+ KSCN AgSCN (White) Fe+3 + SCN- [Fe(SCN)]+2

Two equilibria of precipitation reaction Cl-+ Ag+ AgCl (White) Ag ++SCN- AgSCN (White) Fe3+ + SCN- ------ FeSCN2+ (Red complex) As both salt are in equilibrium so added KSCN reacts with AgCl instead of reacting with Fe+3 to give end point FeSCN2+(Red complex ion) AgCl+ KSCN--AgSCN+ KCl

Minimization of error: After this added SCN- ion reacts with indicator and forms red colour to indicate end point. Fe3+ + SCN-  FeSCN2+ (Red complex). There fore back titration of excess of Ag+ ion will give further reading more than ( true reading). Which creates error in the titration. Minimization of error: i) AgCl precipitate is filtered off before back titration. ii) The precipitate should be boiled for few minutes to coagulate it so as to remove adsorbed Ag+ ion. Then ppt should be filter ,cold filterate should be used for titration

Minimization of error: i) AgCl precipitate is filtered off before back titration. The precipitate should be boiled for few minutes to coagulate it so as to remove adsorbed Ag+ ion. Then ppt should be filter ,cold filtrate should be used for titration. ii) KN03 is added as coagulant after excess addition of AgN03, The precipitate should be boiled for few minutes, Then filter ,cold filtrate should be used for titration. iii) An immiscible liquid Nitrobenzene is added to form coating over AgCl precipitate which prevents reaction with thiocyanate

Use of Adsorption Indicator FAJAN'S METHOD The Fajan’s method is a direct titration of chloride with silver ions (from silver nitrate) using dichloroflurescein as the indicator: Ag+ + Cl---------AgCl(s) Ksp = 1.8× 10 10 How Adsorption Indicators Work Adsorption indicators are dyes that adsorb, or desorbs from, the surface of a precipitate near the equivalence point in a precipitation titration. To be useful for end point indication, this must result in a distinct change in colour, either on the surface of the precipitate.

How Adsorption Indicators Work Precipitate of AgCl

This method of end point indication is sometimes called the Fajan's method after its originator. The best–known adsorption indicator is fluorescein, which is used to indicate the equivalence point in the titration of Cl– with Ag+. Fluorescein is a weak acid, which partially dissociates in water to form fluoresceinate a Fluorescein ------ Fluoresceinate ion (weak acid)

The fluoresceinate anion has a yellow–green colour in solution The fluoresceinate anion has a yellow–green colour in solution. Although fluoresceinate anion forms an intensely coloured precipitate with Ag+, it is used as an indicator at concentrations that do not exceed the solubility product of the silver fluoresceinate salt. AgCl.Ag+ + Ag–( fluoresceinate anion)  AgCl.Ag+ : A– Greenish yellow yellow–green to red or pink

When Cl– is titrated with Ag+ in the presence of fluorescein, the negatively charged fluoresceinate anions are initially repelled by the negatively charged AgCl colloidal particles, with their primary adsorption layer of Cl– ions. Thus the fluorescein remains in a yellow–green solution prior to the equivalence point. At the equivalence point, the colloidal AgCl particles undergo an abrupt change from a negative charge to a positive charge by virtue of Ag+ ions adsorbed in the primary adsorption layer. The fluoresceinate ions are strongly adsorbed in the counter–ion layer of the AgCl colloids, giving these particles a red colour and providing an end point colour change from yellow–green to red or pink.

AgCl FAJAN'S METHOD when AgN03 is added to chloride ion solution Primary layer of chloride ion is form. ‘[ Cl- AgCl Cl- Cl- Cl-

when all Chloride ion precipitate ,with excess addition of AgN03 secondary layer of Ag+ ion is FAJAN'S METHOD Ag+ AgCl Ag+ Ag+ Ag+

AgCl Fl- Fl- Fl- Fl- FAJAN'S METHOD Fl- ion is get attracted to secondary layer of Ag+ ion is form yellow–green to red or pink on the surface of precipitate. FAJAN'S METHOD Fl- Ag+ Ag+ AgCl Fl- Ag+ Ag+ Fl- Ag+ Fl-

All the Best Cl-- Ag+