Precipitimetry Lecture 7 PDF

 Mohr Method for Cl - determination  Relies on Ksp differences for two insoluble silver salts Ag+(aq) + Cl-(aq)  AgCl(s) (titration reaction) White ppt. Ksp= 1.2 x 10-10 (S = 1.1 x 10-5 M)  The 1st excess of Ag+ reacts with In...

 Mohr Method for Cl - determination  Relies on Ksp differences for two insoluble silver salts Ag+(aq) + Cl-(aq)  AgCl(s) (titration reaction) White ppt. Ksp= 1.2 x 10-10 (S = 1.1 x 10-5 M)  The 1st excess of Ag+ reacts with Ind 2Ag+(aq) + CrO42-(aq)  Ag2CrO4(s) (End point) Brick red ppt. Ksp= 1.7 x 10-12 (S = 7.5 x 10-5 M) Although Ksp AgCl > Ksp Ag2CrO4 but AgCl is precipitated first; why? AgCl is less soluble than Ag2CrO4 , so it is precipitate first Volhard’s Method  Method Mohr’s Method Volhard’s Method Analyte Cl-, Br- Cl-, Br-, I- Type of tit. Direct tit. With Indirect titration AgNO3 with KCNS Ind. K2(CrO4) Fe(NO3)2 or CrO4- - Fe+3 Medium Neutral or fairly Acidic medium pH alkaline pH (6.3 – 9) (HNO3) Detection Secondary ppt of Soluble complex of end Ag2CrO4 Fe(CNS)++ point Orange red ppt. Reddish brown color soln (complex).  Volhard method - It is for the indirect determination (back titration) of halide ions (Cl-, Br-, I-). -A measured excess of standard AgNO3 solution is added to the halid sample (with Fe+3 ions as Ind and HNO3 as a medium), and - The AgCl precipitate must be filtered before titration (Why?), - the excess (unreacted) Ag+ ions is determined by back- titration with a standard CNS- solution. - the excess CNS- after Eq.P reacts with the Fe+3 as Ind producing a reddish brown solution of [FeSCN]++ complex.  Indirect Method (Back titrate) of excess Ag+ with SCN - Add excess silver nitrate  Ag+ + Cl- AgCl (white ppt) + Ag+(unreact) 20 ml 10 ml Ksp = 1.2 x 10-10 X ml 0.1 M ?? M 0.1 M Unreacted Ag+ is titrated with standard KCNS  SCN- + Ag+ unreact AgSCN (white ppt) 7 ml X ml Ksp = 1.0 x 10-12 0.1 M 0.1 M End point: with Fe+3 indicator  Fe+3 + SCN-  [Fe(SCN)]++ (reddish color complex) (NxV)Ag+ react = (NxV)Cl- VAg+ react = 20-x = 20-7 = 13 ml 0.1x13 = N x10  Disadvantages Only for Cl-: A very considerable error can be arise if titration of unreacted AgNO3 with CNS - carried out in presence of AgCl ppt. Why? AgCl ppt should be removed before addition of standard SCN - to prevent reaction of CNS - with AgCl. Disadvantage is due to the values of their Ksp Ksp (AgSCN) (1x10-12)