Instant Notes: Analytical Chemistry

precipitation titrations (e.g., barium with sulfate, zinc with ferrocyanide) are less

commonly performed.

Indicators for silver-halide precipitation titrations are of two types. The first

react specifically when an excess of titrant becomes present immediately after

the end point -for example, if a small amount of potassium chromate is added,

it will react with excess silver ions to produce deep red silver chromate in

neutral solutions (Mohr’s method). In acid solutions, the silver is titrated with

potassium thiocyanate (KCNS) solution (Volhard’s method). Iron (III) ammo-

nium sulfate solution is added and reacts with an excesss of thiocyanate to

produce a deep red iron thiocyanate species.

Adsorption indicatorssuch as fluorescein adsorb onto the precipitate when

excess silver ions are present and the precipitate takes on a pinkish color.

As with other indicators, the change of color is detectable by eye over a range:

log (c(ion)) =± 1

A selection of indicators for precipitation titrations is given in Table 3.

Table 3. Indicators for precipitation titrations

Precipitation Color 1 Color 2 Ions detected

CrO 42 - Yellow Deep red Ag+

Fe^3 + Light brown Deep red CNS-

Fluorescein Green-yellow Pink Ag+

It is considerably easier to titrate mixed chloride and bromide in solution by

potentiometry. The use of a silver ion selective electrode, or even a silver wire,

together with a double junction reference electrode, since the chloride ions from

a calomel electrode would react, allows the determination of the silver ion

concentration. Other precipitation titrations may be followed using suitable ion

selective electrodes.

The major applications of precipitation titrations involve the determination

of halides with silver, or the reverse, or the determination of silver in acid

solutions with thiocyanate.

Example

An insecticide containing chlorine was digested in nitric acid to convert the

chlorine to soluble chloride. Silver nitrate was added in excess, and the excess

titrated with potassium thiocyanate by Volhard’s method. It is important to

know whether all the chlorine is converted to chloride.

Redox titrations Oxidation-reduction or redoxtitrations are used for determining metals with
two well-defined oxidation states,and indirect methods for the determination of
organic compounds.
For redox reactionsthe concentrations both of the oxidized species, Ox, and
of the reduced species, Red, will change simultaneously. Considering a cell with
a redox electrode and a reference electrode:

SCE ||a (Ox), a(Red) |Pt

the cell emf is given by:

E =En+(RT/F) ln (a (Ox)/a (Red)) -ESCE

C7 – Titrimetry II: complexation, precipitation and redox titrations 93