Post on 28-Dec-2015
Ch 6 Precipitation Titrations, Sec 6-5 and 6-6
Titration of a MixtureTitration 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
Measuring the Titration Curve in Precipitation Titrations
Measuring the Titration Curve in Precipitation Titrations
Fig 6-5Fig 6-5
Extracting Results From Figure 6-5Extracting 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.
There are three common types of precipitation titrations -
1. Mohr Method - CrO42- indicator
(yellow to red)
2. Volhard Method - back titration of excess Ag+ with SCN-, Fe3+ indicator
(clear to red)
3. Fajans Method - adsorption indicator = dichlorofluorescein anion (pink at endpoint)
Mohr MethodMohr 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
51011
1
11AgCl 1.3x101.8x10Ksp11
KspS
53
1-1212
1
11Ag2CrO4 7.9x104
2.0x10
12
KspS
Volhard MethodVolhard Method
1. Excess AgNO3 is added to precipitate out all of the halide.
2. 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
Fajans MethodFajans Method
The indicator adsorbs onto the colloid particles after the eq. pt. and changes color to pink.
Adsorption Indicator - Dichlorofluorescein
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.