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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?
Ksp AgCl = 1.8 x 10-10, Ksp AgI = 8.3 x 10-17
Measuring the Titration Curve in
Precipitation Titrations
Fig 6-5
Extracting Results From Figure 6-5
In curve (a) in Figure 6-5, 40.00 mL of
unknown solution containing both I- and Clwere 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 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
 Ksp 
S AgCl   1 1 
 1 1 
S Ag2CrO4
1
11
 Ksp  1.8x10 10  1.3x10 5
 Ksp 
  1 1 
 2 1 
1
21
1
3
 2.0x10-12 
  7.9x10 5
 

4


Volhard 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+
+
excess
SCN-
→
AgSCN(s)
titrant
after the endpoint Fe3+ + SCN- → FeSCN2+
[Cl-] = total Ag+ - amount backtitrated
Fajans 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.