Page 25 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

• Potentiometry Self-indicators

Some colored titrants serve as self-indicators

KMnO4 (strong oxidant used for determination of reducing species)

MnO4– + 5e– + 8H+ Mn2+ + 4 H2O

Purple colorless

The end point is located by first drop excess of MnO – which imparts
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the solution a definite pink color

Specific indicators

Starch is the indicator of choice for titrations involving iodine (Iodimetry
and iodometry)
Starch forms with I2 a reversible intense blue complex

Starch is not a true redox indicator because it responds specifically to
the presence of I2, not to a change in redox potential

True Redox indicators

Examples: 1- Ferroin 2- Diphenylamine sulphonic acid

Characteristics:
• These are highly colored dyes that are weak reducing or oxidizing
agents
• They change color upon being oxidized or reduced

• The color change depends on the changes in Esys throughout the
titration

• The half-reaction and Nernst equation responsible for color
change:

Inox + ne– Inred

0.059 [Inred]

Eind = E0ind – log

n [Inox]

The potential change required to produce color change "transition
range" is given by:

0.059

Eind = E0ind ±

n

i.e. the indicator exhibits a detectable color change WHEN the

titrant causes the system potential to shift from E0ind ─ 0.059/n V
to E0ind + 0.059/n V ( i.e. 0.118/n V)

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