1. Precipitation titrations
Week 9

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 sedimentation 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 (on titrant):
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 of 0. 0500 M Cl– with 0
Precipitation titration curve for 50.0 mL of M Cl– with M Ag+. (a) pCl versus volume of titrant; (b) pAg versus volume 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 in precipitation 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. AgNO3 + NaCl = AgCl + NaNO3
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. AgNO3 + NaCl = AgCl + NaNO3 2AgNO3 + K2CrO4 = Ag2CrO4+ 2KNO3
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.

15. 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

16. Surface of precipitate
Colour
A number adsorption abilities of anion at рН~7 on a precipitate surface of Silver chloride
I- > CN- > SCN- > Br- > eosine > Сl- > dichlorofluoroscein > NO3- > ClO4-
Indicator
Solution
Surface of precipitate
dichlorofluoroscein
greenish yellow
pink
eosine
yellowish-red
redish - violet

17. 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 point is necessary to spend slowly
Titration with adsorption indicators spend in a diffused light
Use:
Quantitative definition of chlorides, bromides, iodides, thiocyanides, cyanides.

18. AgNO3 + NH4SCN = AgSCN + NH4NO3
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 of AgNO3:
AgNO3 + NH4SCN = AgSCN + NH4NO3
Indicator by standardization of ammonium or potassium thiocyanide with iron (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

19. 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

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

21. Hg2(NO3)2 + 2NaCl = Hg2Cl2 + 2NaNO3
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 solution of NaCl:
Hg2(NO3)2 + 2NaCl = Hg2Cl2 + 2NaNO3

22. 6. Sulphatometry
Sulphatometry – this is a precipitation titration in which SO42- is the titrant
Sometimes bariummetry – this is a precipitation titration in which Ba2+ is the titrant
Reaction of method:
Ba2+ + SO42- = BaSO4↓
determinate titrant
substance
Titrant: secondary standard solution of H2SO4, Ba(NO3)2, BaCl2.
Stardadization: solution of H2SO4 on primary standard solution of Na2B4O7 or Na2CO3 (crystal.) with methyl orange; Ba(NO3)2 or BaCl2 on secondary standard solution of H2SO4 with nitrchromazo or orthanilic А.

23. BaInd + H2SO4 = BaSO4 + H2Ind
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

24. 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).

25. 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.