The reverse titration of silver ion with chloride ion using chromate as indicator is not feasible, the flocculated Ag2CrO4 formed initially, reacts slowly with chloride especially near the end point. Titration of iodide; and of thiocyanate is not successful because silver iodide and silver thiocyanate adsorb chromate ions so strongly that a false and indistinct end point is obtained. B- Formation of a soluble coloured compound (Volhard method): Used for chlorides, bromides, and iodides indirect method determination ferric nitrate or of ferric ammonium alum Indicator Medium Nitric acid solution Titrant standard potassium or ammonium thiocyanate Excess of standard silver nitrate solution is added, and the excess is back-titrated with standard thiocyanate solution. Ag+ + SCN− AgSCN → Fe+++ + SCN− [FeSCN]++ ( reddish color) → For the chloride estimation, we have the following two equilibria during the titration of excess of silver ions: Ag+ + Cl− AgCl → Ag+ + SCN− →AgSCN The two sparingly soluble will be in equilibrium with the solution hence: When the excess of silver has reacted, the thiocyanate may react with the silver chloride, since silver thiocyanate is the less soluble until the ratio [Cl−] / [SCN−] in the solution is 170. AgCl + SCN− +Cl− →AgSCN It is therefore absolutely necessary to prevent the reaction between the thiocyanate, and the silver chloride. This may be affected in several ways: The silver chloride is filtered off before back titration. After the addition of the silver nitrate, the suspension is boiled for about 3
The reverse titration of silver ion with chloride ion using chromate as indicator is not feasible, the flocculated Ag2CrO4 formed initially, reacts slowly with chloride especially near the end point. Titration of iodide; and of thiocyanate is not successful because silver iodide and silver thiocyanate adsorb chromate ions so strongly that a false and indistinct end point is obtained. B- Formation of a soluble coloured compound (Volhard method): Used for determination chlorides, bromides, and iodides indirect method Indicator ferric nitrate or of ferric ammonium alum Medium Nitric acid solution Titrant standard potassium or ammonium thiocyanate Excess of standard silver nitrate solution is added, and the excess is back-titrated with standard thiocyanate solution. Ag+ + SCN− → AgSCN Fe+++ + SCN− → [FeSCN]++ ( reddish color) For the chloride estimation, we have the following two equilibria during the titration of excess of silver ions: Ag+ + Cl− → AgCl Ag+ + SCN− →AgSCN The two sparingly soluble will be in equilibrium with the solution hence: When the excess of silver has reacted, the thiocyanate may react with the silver chloride, since silver thiocyanate is the less soluble until the ratio [Cl−] / [SCN−] in the solution is 170. AgCl + SCN− →AgSCN +Cl− It is therefore absolutely necessary to prevent the reaction between the thiocyanate, and the silver chloride. This may be affected in several ways: The silver chloride is filtered off before back titration. After the addition of the silver nitrate, the suspension is boiled for about 3
part_2_colored.docx