Lecture 11 Precipitimetric Titration PDF

Mohr’s method - It is a direct method used for determination of soluble halides salts by AgNO3 in neutral or slightly alkaline solution (pH 6–9) using dil. K2CrO4 as indicator. - At the end point, After complete precipitation of halide with AgNO3, the first drop of AgNO3 will precipitate CrO42- as A...

Mohr’s method - It is a direct method used for determination of soluble halides salts by AgNO3 in neutral or slightly alkaline solution (pH 6–9) using dil. K2CrO4 as indicator. - At the end point, After complete precipitation of halide with AgNO3, the first drop of AgNO3 will precipitate CrO42- as Ag2CrO4 (Brick red ppt ). - During titration: NaCl + AgNO3 = AgCl + NaNO3 - At the end point : K2CrO4 + 2AgNO3 = Ag2CrO4 ↓+ 2NaNO3 Note that 1- The medium must be neutral or slightly alkaline because: a- In acidic medium, soluble silver acid chromate and dichromate are formed and no ppt formed at end point. 2 CrO42- + 2H+ 2(HCrO4-) Cr2O72- + H2O b- In strong alkaline medium, AgOH will precipitated 2 Ag+ + 2 OH- 2 AgOH Ag2O + H2O 2- Mohr’s Method is not suitable for determination of ammonium salts (NH4Cl or NH4Br) because in alkaline medium NH4OH is formed and dissolve the precipitate of AgCl or AgBr by formation of ammine complex [Ag (NH3)2]+.  Volhard’s Method It is an indirect method for determination of halides where a known excess of standard AgNO3 is added to the soluble halide, filter and residual AgNO3 is back titrated with standard SCN- in acidic medium (HNO3) and in presence of Fe3+ (ferric alum) as indicator. - End point is detected by appearance of blood red color of [Fe(SCN)6] -3.  During titration: NaCl + AgNO3 (xss) AgCl + NaNO3 AgNO3 (xss) + KSCN AgSCN ↓ + KNO3 At the end point: Fe3+ + 6 SCN- [Fe(SCN)6] -3 Blood red color Note that 1- The titration must be carried out in acidic medium to prevent precipitation of iron(III) indicator as Fe(OH)3. 2- The precipitate of AgCl must be filtered or coated with nitrobenzene because silver chloride is more soluble (higher Ksp) than silver thiocyanate, thus at the end point the following reaction will occurs AgCl (s) + SCN- (aq) ⇌ AgSCN (s) + Cl- (aq) 3- Volhard’s method can be used for direct titration of Ag+  Fajan’s Method It is a direct method for determination of soluble halides using standard AgNO3 as a titrant in presence of an adsorption indicator. Adsorption Indicators Adsorption indicators are dyes change their color when adsorbed to a surface of precipitate. Such compounds are called adsorption indicators. Types of adsorption indicators a- Weak acids: Fluorescein and Eosin (the anionic part is the active part). b- Weak bases: Rhodamine 6G (the cationic part is the active part). Fluorescein  Theory of adsorption indicator 1- During titration: Excess Cl- in the solution surrounds AgCl particle. 2- At the end point: All Cl- will be precipitated as AgCl 3- The first drop after the end point: Excess Ag+ surrounds AgCl particle 4- The indicator will be adsorbed producing color change due to electronic rearrangement in the molecules. Before the end point At the end point After the end point greenish yellow pink Note that 1- Colloidal precipitate maximize the quantity of indictor adsorbed. 2- Adsorbed part of the indicator must with charge opposite to that of titrant. 3- pH of the medium must assure ionization of the adsorption indicator. Weak acidic dye is used in slightly alkaline solution, while the weak basic dye is used in slightly acidic solution.  Liebig’s Method It is a direct method for determination of CN- with standard Ag+. End point is detected by the appearance of persistent turbidity. CN- + Ag+ = AgCN ↓ AgCN + CN- = [Ag(CN)2]- soluble complex At the end point [Ag(CN)2]- + Ag+ = 2AgCN ppt.  Denieg’s Modification In which KI solution is used as indicator in presence of ammonia solution. At the end point the yellow turbidity is produced instead of white turbidity in Liebeg’s (sharp end point) Ag+ + 2CN- = [Ag(CN)2]- soluble complex At the end point: KI + Ag+ = AgI + K+ yellow ppt  Comparison between Mohr’s, Volhard’s and Fajan’s Methods in determination of halides Mohr Volhard Fajan Type of Direct Indirect Direct titration Sample All halides except All halides All halides its ammonium salts Titrant AgNO3 NH4SCN AgNO3 Indicator K2CrO4 Ferric alum Fluorescein Medium Neutral or slightly Acidic Slightly alkaline alkaline End point Brick red ppt. Blood red Pink color color Filtration Not necessary Necessary Not necessary for Cl-