Precipitation titration Titrations with precipitating agents are useful for determining certain analytes e. g. Cl- can be determined when titrated with AgNO3. Detection of end point: •Chemical –Precipitation Type – Mohr’s method –Adsorption – Fajan’s method –For silver analyses –Volhard method •Sensors –Potentiometric or amperometric The chemical types are also classified into: 1. Indicators reacting with titrant forming specific color. 2. Adsorption indicators. Indicators reacting with the titrant:
Two methods will be discussed where this type of indicators are applied; namely: Mohr and Volhard. I) Mohr method for determining chloride: Chloride is titrated with AgNO3 solution. A soluble chromate salt is added as the indicator. This produces a yellow color solution. When the precipitation of the chloride is complete, the first excess of Ag+ reacts with the indicator to precipitate red silver chromate: 2 Ag+(aq) + CrO42–(aq) > Ag2CrO4(s) Yellow red ppt The Mohr method must be performed at a pH about 8.
This method is useful for determining Cl- in neutral or unbuffered solutions such as drinking water. II) Volhard titration: This is an indirect titration procedure for the determination of anions that precipitate with silver like CL-, Br-, I-, SCN-, and it is preferred in acid (HNO3) solution. A measured excess of AgNO3 is added to ppt the anion, and the excess of Ag+ is determined by back titration with standard potassium thiocyanate solution: Ag+(aq) + Cl–(aq) > AgCl(s) + excess Ag+ excess Ag+(aq) + SCN–(aq) > AgSCN(s)
The end point is detected by adding iron III (Fe3+) as ferric ammonium sulfate which forms a soluble red complex with the first excess of titrant. Fe3+(aq) + SCN–(aq) > [FeSCN]2+(aq) These indicators must not form a compound with the titrant that is more stable than the precipitate or the color reaction would occur on addition of the first drop of titrant. Adsorption indicators: The indicator reaction takes place on the surface of the precipitate. The indicator, which is a dye, exists in solution as the ionized form, usually an anion.
Principle of adsorption: Consider the titration of Cl- with Ag+. Before the equivalent point, Cl- is in excess and the primary layer is Cl- (go back to precipitation process in gravimetry). This repulses the indicator anions; and the more loosely held the secondary (counter) layer of adsorbed ions is cations, such as Na+ : AgCl : Cl- : : Na+ Beyond the equivalent point (end point as well), Ag+ is in excess and the surface of the precipitate becomes positively charged, with the 1? ayer being Ag+. This will now attract the indicator anion and adsorb it in the 2? (counter) layer: AgCl : Ag+ : : indicator- The color of the adsorbed indicator is different from that of the un-adsorbed indicator, and this difference signals the completion of the titration. The degree of adsorption of the indicator can be decreased by increasing the acidity.
The titration of chloride using this kind of indicator is called Fajan’s Method. Fajan’s method is the most recent and most accurate silverhalide method. It is based on the adsorption of dichlorofluorescein (DCF) on the surface of the positively charged silver chloride particles formed in the precipitation titration when Ag+ ion is in excess.