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.

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.