Precipitation Titration PDF

Precipitation Titration Precipitation Titration  A special type of titrimetric procedure involves the formation of precipitates during the course of titration.  The titrant react with the analyte forming an insoluble material and the titration continues till the very last amount of analyte...

Precipitation Titration Precipitation Titration  A special type of titrimetric procedure involves the formation of precipitates during the course of titration.  The titrant react with the analyte forming an insoluble material and the titration continues till the very last amount of analyte is consumed.  The first drop of titrant in excess will react with an indicator resulting in a color change and announcing the endpoint of the titration. Example : AgNO3 + NaCl  AgCl + NaNO3 Principle of Precipitation Titration The main principle of the precipitation titrations is that the quantity of the added precipitating reagent or precipitant is equivalent to the substance being precipitated.  Argenmetric Titration (Argentometry) Titration involving precipitation with a standard solution of sliver nitrate  Precipitates should be practically insoluble.  Precipitation reactions should be rapid and quantitative  Precipitate should not interfere in the sharp detection of the end point  Titration results should not differ appreciably due to adsorptions on the precipitate (co-precipitation) Types of Argentometry  THREE classical methods based on color indicators can be used for end point detection: Mohr method Fajans method Volhard method  Mohr Method  Chloride is titrated with AgNO3 solution. A soluble chromate salt is added as the indicator. This produces a yellow color solution.  As the silver nitrate solution is slowly added, a precipitate of silver chloride forms.  The end point occurs when the precipitation of the chloride is complete. Yellow  Then additional of silver ion (Ag+) reacts with the indicator to form a red precipitate of silver chromate: 2 Ag+(aq) + CrO42–(aq) → Ag2CrO4(s) Yellow red ppt  Fajans Method – adsorption of a colored indicator on the precipitate at the end point.  Fajan’s method is the most recent and accurate silver halide method, which is generally used for the quantitative analysis of halide ions or thiocyanate ions.  The precipitation titration in which silver ions is titrated with halide or thiocyanate ions in presence of adsorption indicator.  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.  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. Ag+ + FI-  AgF Reddish ppt *** Fluorescein and its derivatives are adsorbed to the surface of colloidal AgCl. After all chloride is used, the first drop of Ag+ will react with fluorescein (FI) forming a reddish color. Adsorption indicator  Volhard Method  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 precipitate 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)  Volhard Method Advantanges  This method is useful where the titrations have to be performed at low pH.  The strong acidic environment give advantage for halide analysis because ions like carbonate, oxalate and arsenate do not interfere.  Give accurate results due to back titration. Limitations  Can not be used where the solution has to be neutral.  Time consuming.  Sometimes adsorption of Ag+ may give false end point.