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Precipitation Titrations






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Precipitation Titrations . Methods of titration which rely on reactions yielding compounds of low solubilitySilver salts are sparingly soluble and are useful examples to start with since .Solubility of AgI< AgBr< AgCl How soluble are we talking about ?Solid AgCl has [Ag ] and [Cl-] at about 10-
Precipitation Titrations

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1. Precipitation Titrations

2. Precipitation Titrations Methods of titration which rely on reactions yielding compounds of low solubility Silver salts are sparingly soluble and are useful examples to start with since . Solubility of AgI< AgBr< AgCl How soluble are we talking about ? Solid AgCl has [Ag+] and [Cl-] at about 10-5 M at most in equilibrium with the solid. So a 0.05 M NaCl solution should be measurable with only an error in the 5th decimal place, 0.05001 M, which is negligible.

3. Precipitation Titrations Titration of 50 mL of 0.05 M NaCl by 0.1000M AgNO3 Ways to follow the process A silver probe or electrode gives a response proportional to ?log 10[Ag+]. An Indicator could pick up the endpoint

7. Which corresponds to pCl of 1.60

12. Titration of 50 mL of 0.05 M NaCl by 0.1000M AgNO3 continued

13. Successful titrations of ions by Siver Nitrate See Figure 13.2

14. Successful titrations of Mixtures by Siver Nitrate See Figure 13.3

17. Endpoints for Argentometric Titrations Chemical Precipitation Type - Mohr?s method Adsorption ? Fajan?s method For silver analyses ?Volhard method Sensors ?Potentiometric or amperometric We will look at potentiometric sensors later.

19. Endpoints for Argentometric Titrations Chemical Precipitation Type - Mohr?s method Ag2CrO4 precipitation in neutral pH solution.. Product is coloured Colour forms just after AgCl or Ag I forms . Small error involved.

20. 2 Ag+(aq) + CrO42?(aq) ? Ag2CrO4(s) 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.

21. Endpoints for Argentometric Titrations Chemical Adsorption ? Fajan?s method A red dye attaches to the silver salt, on the surface of the analyte precipitate particle. This happens only when the silver ion Ag+ is in excess, ie just after the equivalence point.

22. Principle of adsorption: Consider the titration of Cl- with Ag+. Before the equivalent point, Cl- is in excess and the primary layer is Cl-. 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? layer 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. 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. ? ?

25. Endpoints for Argentometric Titrations Chemical method for silver analyses Volhard?s method using thiocyanate , CNS-, as titrant. Iron (III) is the indicator as it forms a red complex ion with thiocyanate , CNS-, Fe (CNS) 2+ The method can be adapted to Chloride analyses. AgNO3 is added in excess. The AgCl precipitate is often filtered off. Then the excess Ag+ backtitrated with thiocyanate , CNS-. Iron (III) acts as the indicator as above.

26. 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.

27. Homework In a precipitation titration of 46.00 mL of a chloride solution of unknown concentration, 31.00 mL of 0.6973 M AgNO3 were required to reach the equivalence point. Calculate the molar concentration of the unknown solution. A sample of NaCl weights 0.5 gram. 50 mL of 0.21M AgNO3 is added to precipitate AgCl . The excess silver nitrate is titrated with 0.28M potassium thiocyanate to give 25.5 mL at the end point. Find the percentage of NaCl in the sample. A sample weighing 0.340 gram containing a Cl- needed 48.2 mL of 0.10 M AgNO3 in Mohr?s titration. Calculate the percentage composition of Cl- in the sample. If 50 mL of 0.1 M NaCl is titrated with o.1 M AgNO3 . Calculate chloride ion conc. during titration at: (Ksp = 1.56x10-10) start after adding 10mL. 49.9mL equivalent point 60mL of o.1 M AgNO3 solution.


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