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Ch 6 Precipitation Titrations, Sec 6-5 and 6-6

Ch 6 Precipitation Titrations, Sec 6-5 and 6-6. Titration of a Mixture. e.g. A mixture of Cl - and I - is titrated with Ag + . Which halide precipitates out first? K sp AgCl = 1.8 x 10 -10 , K sp AgI = 8.3 x 10 -17. Measuring the Titration Curve in Precipitation Titrations. Fig 6-5.

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Ch 6 Precipitation Titrations, Sec 6-5 and 6-6

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  1. Ch 6 Precipitation Titrations, Sec 6-5 and 6-6

  2. Titration of a Mixture e.g. A mixture of Cl- and I- is titrated with Ag+. Which halide precipitates out first?Ksp AgCl = 1.8 x 10-10, Ksp AgI = 8.3 x 10-17

  3. Measuring the Titration Curve in Precipitation Titrations

  4. Fig 6-5

  5. Extracting Results From Figure 6-5 In curve (a) in Figure 6-5, 40.00 mL of unknown solution containing both I- and Cl- were titrated with 0.0845 M Ag+. Calculate the Molarity of each ion.

  6. There are three common types of precipitation titrations - • Mohr Method - CrO42- indicator(yellow to red) • Volhard Method - back titration of excess Ag+ with SCN-, Fe3+ indicator(clear to red) • Fajans Method - adsorption indicator = dichlorofluorescein anion (pink at endpoint)

  7. Mohr Method Titration of a halide (Cl-, Br-, I-) with AgNO3 to form a precipitate using K2CrO4 as the indicator. e.g. Ag+ + Cl-→ AgCl eq. pt. color change to red The indicator combines with excess Ag+ at the eq. pt. Before the eq. pt., the solubility of the Ag-halide precipitate must be less than the Ag-indicator, otherwise the latter would precipitate out during the titration! 2 Ag+ + CrO42- → Ag2CrO4(s) red Ag+ + Cl- → AgCl(s) white

  8. Volhard Method • Excess AgNO3 is added to precipitate out all of the halide. • The excess Ag+ is back-titrated with KSCN using Fe3+ as the indicator • e.g. Ag+ + Cl-→ AgCl(s) • added in excess • then Ag+ + SCN-→ AgSCN(s) • excess titrant • after the endpoint - • Fe3+ + SCN- → FeSCN2+ • [Cl-] = total Ag+ - amount backtitrated

  9. Fajans Method The indicator adsorbs onto the colloid particles after the eq. pt. and changes color to pink. Adsorption Indicator - Dichlorofluorescein

  10. before the eq. pt. the lattice ion in excess is the halide (Cl- etc) so the colloid is negatively charged after the eq. pt. the lattice ion in excess is the titrant Ag+ so the colloid is positively charged and then the negative adsorption indicator will bind and change color.

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