Precipitation titration

1. Lecture 7 Precipitation titration Associate prof. L.V. Vronska Associate prof . M.M. Mykhalkiv

2. Outline • Essence and classification of methods precipitation titration • Titration Curves • Argentometry • Thiocyanatometry • Mercurometry • Sulphatometry • Hexacianoferratometry

3. 1. Essence and classification of methods precipitation titration A reaction in which the analyte and titrant form an insoluble precipitate also can form the basis for a titration - a precipitation titration.

4. Requirements to reactions and defined substances: • The defined substance should be dissolved in water and give an ion which would be active in precipitation reaction. • The received precipitate should be practically insoluble (Ksp<10-8  - 10, S<10-5 mol/L). • Results of titration should not be deformed by the adsorption phenomena (coprecipitation). • Precipitate should form enough quickly. • There should be a possibility of fixing of an equivalence point.

5. Classification of methods precipitation titration(ontitrant): • Argentometry • Thiocyanatometry • Mercurometry • Sulphatometry • Hexacianoferratometry

6. 2. Titration Curves Curve plotting of titration is based on a rule of solubility product. And accordingly

7. Precipitation titration curve for 50.0 mL of0.0500 M Cl– with 0.100 M Ag+. (a) pClversus volume of titrant; (b) pAg versusvolume of titrant.

8. The factors which define value of inflection points of titration on curves of precipitation titration • Concentration of titrant solutions and a defined ion (than more concentration, the titration inflection point is more) • Solubility of a precipitate (than solubility less, the titration inflection point is more)

9. Influence of precipitate solubility on titration inflection point

10. Determinate the end-point by potenthiometric way

11. Influence of different factors on inflection point inprecipitation titration • Temperature (than more temperature, the solubility of a precipitate will be more and the inflection point is less) • Ionic strength of a solution (than more ionic strength of a solution, the solubility of a precipitate will be more and the inflection point is less)

12. 3. Argentometry This is a precipitation titration in which Ag+ is the titrant. X- + Ag+ = AgX where: X- = Cl-, Br-, I-, CN-, SCN-, etc.. • Titrant: AgNO3 – secondary standard solution • Standardization оn primary standard solution of Sodium chloride AgNO3 + NaCl =AgCl+ NaNO3 • Indicator by standardization - 5 % Potassium chromate K2CrO4 (to appearance reddish-brown precipitate of Silver chromate): 2AgNO3+ K2CrO4=Ag2CrO4+ 2KNO3.

13. Argentometry: • without indicator: - Gay-Lussac method (method of even turbidity) - method to point enlightenment • with inicator: - Mohr method - Fajans – Fisher – Khodacov method - Volhard method

14. Gay-Lussac method (method of even turbidity) If solution NaBr titrate by solution AgNO3 (or on the contrary) there is a reaction: Br- + Ag+ = AgBr↓ • For fixation of equivalence point it is necessary to select two identical portions of a solution before the titration end. To one of them add a drop of AgNO3 solution, on another - a drop of NaBr solution at the same concentration. Titration finish when will be identical intensity of turbidity in both portions of solution.

15. Method to enlightenment point • The method of titration to an enlightenment point can be applicable when insoluble compounds is in colloidal state. For example, definition of І- ions by silver nitrate, AgІ forms, it is adsorbing І- and receive negative charges (colloidal solution of AgІ forms). As more and more І- ions react with Ag+ ions, particles AgІ gradually lose adsorbed by them І- ions, and their charge decreases. In the end of titration occur coagulation of particles and their sedimentation. The solution thus is absolutely clarified.

16. Mohr method • Titrant: AgNO3 – secondary standard solution • Stanardization on primary standard solution of sodium chloride NaCl (by a measured volume of primary standard solution): AgNO3 + NaCl =AgCl + NaNO3 • Indicator - 5 % potassium chromate K2CrO4 (to formation precipitate of reddish-brown Ag2CrO4): 2AgNO3 + K2CrO4 = Ag2CrO4+ 2KNO3 • Determinate substance: chloride Cl-, bromide Br-. • Medium: рН~ 6,5-10,3. • Usage: quantitative definition of sodium chloride, potassium chloride, sodium bromide, potassium bromide, etc.

17. Restrictions of usage of Mohr method: • It is impossible to use titration in acidic solutions: 2CrO42- + 2H+ = Cr2O72- + H2O • It is impossible to use titration in the presence of ammonia, etc. ions, molecules which can be ligands on relation to Silver ions • It is impossible to use titration in the presence of many cations (Ba2+, Pb2+, etc.) which form the painted precipitates with chromate ions CrO42- • It is impossible to use titration in the presence of reducers which reduce chromate ions CrO42- to Cr3+ ions • It is impossible to use titration in the presence of many anions (PO43-, AsO43-, AsO33-, S2- etc.) which with Silver ions give the painted precipitates

18. Fajans – Fisher – Khodacov method • Titrant: AgNO3 – secondary standard solution • Stanardization on primary standard solution of sodium chloride NaCl (by a measured volume of primary standard solution): • Medium: рН~ 6,5-10,3(by definition of clorides)and рН~ 2,0-10,3(by definition of bromidesandiodides). • Indicatorsof method: - dichlorofluoroscein(by definition of clorides) - eosine(by definition of bromidesandiodides)

19. The mechanism of indicators action AgNO3 + NaCl = AgCl + NaNO3 HInd H+ + Ind- Adsorption indicator whose color when adsorbed to the precipitate is different from that when it is in solution

20. Colour • A number adsorptionabilities of anion at рН~7 on a precipitate surface of Silver chloride • I- > CN- > SCN- > Br- > eosine > Сl- > dichlorofluoroscein > NO3- > ClO4-

21. Conditions of titration: • Acidity of solutions • Concentration of reacting solutions • The account adsorption abilities of indicators and ions which present in a solution • Titration near equivalence pointis necessary to spend slowly • Titration with adsorption indicators spend in a diffused light Use: • Quantitative definition of chlorides, bromides, iodides, thiocyanides, cyanides.

22. Volhard method • Titrant:AgNO3,ammonium or potassiumthiocyanideNH4SCN, KSCN - secondary standard solution • StardadizationAgNO3on primary standard solution NaCl,NH4SCN, KSCNon standardization solution AgNO3: AgNO3 + NH4SCN = AgSCN + NH4NO3 • Indicatorbystandardizationof ammonium or potassium thiocyanidewithiron (ІІІ) salts (NH4Fe(SO4)212H2O in presence of nitric acid) to the formation of the reddish colored Fe(SCN)2+ complex: Fe3+ + SCN- = [Fe(SCN)]2+

23. Medium:in presence of nitric acid • Indicators of method: iron (ІІІ) salts NH4Fe(SO4)212H2O in presence of nitric acid • Determinate substance: halogenides, thiocyanides, cyanides, sulphides, carbonates, chromates, oxalates, arsenatesetc.

25. Until equivalence point (e.p.) Hal-+ Ag+ (excess) = AgHal Ag+(residue) + SCN-= AgSCN After e. p. Fe3+ + SCN- = [Fe(SCN)]2+ (reddish) !!!At definition of iodides the indicator is added in the end of titration to avoid parallel: 2Fe3+ + 2I- = 2Fe2+ + I2 Advantages of Volhard method Titration possibility: • - In very acidic solutions • - In the presence of many cations which interfere by definition in Mohr method

26. 4. Thiocyanatometry • This is a precipitation titration in which SCN- is the titrant. • Titrant:ammonium or potassium thiocyanide NH4SCN, KSCN - secondary standard solution • Stardadization:on primary standard solution ofAgNO3: AgNO3 + NH4SCN = AgSCN + NH4NO3 • Indicatorbystandardizationof ammonium or potassium thiocyanidewithiron (ІІІ) salts: Fe3+ + SCN- =[Fe(SCN)]2+ • Medium:in presence of nitric acid • Indicator:iron (ІІІ) salts NH4Fe(SO4)212H2O in presence of nitric acid

27. Determinate substance:drugs, which contain Silver (Albumosesilber, colloid silver - Kollargol, silver nitrate). !!! At the analysis of drugs which contain nonionic silver, preliminary it is heated with sulphatic and nitric acids (receive ionic compound). !!!At definition of iodides the indicator is added in the end of titration to avoid parallel: 2Fe3+ + 2I- = 2Fe2+ + I2

28. Advantages of Thiocyanatometry Titration possibility: - In very acidic solutions - In the presence of many cations which interfere by definition in Mohr method

29. 5. Mercurometry • Mercurometry – this is a precipitation titration in which Hg22+is the titrant 2Cl- + Hg22+ = Hg2Cl2 Ksp = 1,310-18 2I- + Hg22+ = Hg2I2 Ksp = 4,5 10-29 • Titrant: Hg2(NO3)2-secondary standard solution • Stardadization:on primary standard solutionof NaCl: Hg2(NO3)2 + 2NaCl = Hg2Cl2 + 2NaNO3

30. Indicators: 1) solution of iron (ІІІ) thiocyanide (from reddish to colourless) 2Fe(SCN)2+ + Hg22+ = Hg2(SCN)2 + 2Fe3+ !!! For the account of titrant volume which is used on indicator titration, do “blind test” 2) 1-2 % spirit diphenylcarbazone (to blue colour) !!! Indicator is added before end of titration • Determinate substance: chlorides and iodides. • Medium: very acidic (may be to 5 mol/L H+ ions).

31. Advantages of mercurometry: • Possibility usage titration in very acidic solutions • Titrant is cheaper • Mercury (I) Salts are less soluble, therefore accuracy and sensitivity of titration more, and inflection point on a curve is much more • Possibility of titration with the diphenylcarbazone indicator in the muddy and painted solutions • Probably reversive titration of Mercury (І) salts Lack: • Mercury (І) salts are very toxic.

32. 6. Sulphatometry • Sulphatometry – this is a precipitation titration in which SO42-is the titrant Sometimesbariummetry–this is a precipitation titration in which Ba2+is the titrant Reactionof method: Ba2+ + SO42- = BaSO4↓ determinate titrant substance • Titrant:secondary standard solutionof H2SO4, Ba(NO3)2, BaCl2. • Stardadization:solution of H2SO4on primary standard solution of Na2B4O7or Na2CO3(crystal.)with methyl orange; Ba(NO3)2or BaCl2on secondary standard solution of H2SO4withnitrchromazo or orthanilic А.

33. Indicators: use metallochrome indicator - nitrchromazo or (orthanilic С), orthanilic А. BaInd + H2SO4 = BaSO4 + H2Ind violet white roze precipitate solution Determinate substance: • Direct titration: - by sulphatic acid – content of Barium • by barium chloride or barium nitrate – content of sulphates !!! For completeness of sedimentation of barium ions use as solvent of 50 % ethanol !!! Sometimes, as the indicator use sodium rodizonate

34. 7. Hexacyanoferratometry Hexacyanoferratometry - this is a precipitation titration in which K4[Fe(CN)6]is the titrant • Titrant: secondary standard solution K4[Fe(CN)6] • Stardadization: on standard solution of KMnO4 in sulphatic acid 5K4[Fe(CN)6] + KMnO4 + H2SO4 = 5K3[Fe(CN)6] + MnSO4 + 3K2SO4 + 4H2O • Indicator by standardization - Methyl violet (from yellow-dreen to red-brown).

35. Indicators: • diphenylamine at the presence of a small amount of potassium hexacyanoferrate (ІІІ) for definition Zn2+, Mn2+ • 3,3-dimetilnaftizin for definition Cu2+, Zn2+, Cd2+, Pb2+, Fe2+, Ni2+ etc. • alizarin red S for definition Zn2+, Pb2+, Th(IV) • diphenylcarbazone for definition Cd2+ Determinate substance: metal cations (direct titration), displacement titration – phosphate ions.

36. Thanks for your attention!