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? K sp AgCl = 1.8 x 10 -10, K sp 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 - 1.Mohr Method - CrO 4 2- indicator (yellow to red) 2.Volhard Method - back titration of excess Ag + with SCN -, Fe 3+ indicator (clear to red) 3.Fajans Method - adsorption indicator = dichlorofluorescein anion (pink at endpoint)

7. Mohr Method Titration of a halide (Cl -, Br -, I - ) with AgNO 3 to form a precipitate using K 2 CrO 4 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 + + CrO 4 2- → Ag 2 CrO 4 (s)red Ag + + Cl - → AgCl(s)white

8. Volhard Method 1.Excess AgNO 3 is added to precipitate out all of the halide. 2.The excess Ag + is back-titrated with KSCN using Fe 3+ as the indicator e.g.Ag + +Cl - → AgCl(s) added in excess thenAg + +SCN - → AgSCN(s) excesstitrant after the endpoint - Fe 3+ + SCN - → FeSCN 2+ [Cl - ] = total Ag + - amount backtitrated

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

11. 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.