Precipitation titrations Week 9

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

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.

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.

Classification of methods precipitation titration (on titrant): Argentometry Thiocyanatometry Mercurometry Sulphatometry Hexacianoferratometry

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

Precipitation titration curve for 50. 0 mL of 0. 0500 M Cl– with 0 Precipitation titration curve for 50.0 mL of 0.0500 M Cl– with 0.100 M Ag+. (a) pCl versus volume of titrant; (b) pAg versus volume of titrant.

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)

Influence of precipitate solubility on titration inflection point

Determinate the end-point by potenthiometric way

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)

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.

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

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.

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

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

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.

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

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

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

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

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.

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

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).

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.