Comprehensive Guide to Precipitation Titrations in Analytical Chemistry

1. SatishPrdhanDnyanasadhana College, Thane.Department of ChemistryS.Y.B.Sc. Analytical ChemistryPaper-I Sem-IVPrecipitation TitrationsDr.G.R.Bhagure

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

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

4. 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

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

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

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

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

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

10. 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+=

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

12. 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)

13. 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 ;

14. 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 chromateTo avoid this excess of silver nitrate is added for the appearance of red silver chromate ppt.

15. 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 .

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

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

18. 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

19. 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

20. 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

21. 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.

22. How Adsorption Indicators Work Adsorption Indicator Precipitate of AgCl

23. This method of end point indicationis 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)

24. 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

25. 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.

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

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

28. 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-

29. All the Best Cl-- Ag+