Page 26 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

• Requirements of typical redox indicator

1. The transition range of redox indicator must fall within the
equivalence point break of the titration curve

2. The potential of the indicator should be between E0 values

of the two systems used in titration (analyte-titrant)
For example: In the titration of ferrous sample using Ce4+/Ce3+ sys

Ce4+/Ce3+ 1.44 V

Ferroin 1.11 V

Fe2+ 0.7 V

Ferroin is weaker reducing agent than Fe2+ sample. Otherwise, it

will react (oxidized) first before Fe2+ sample

3. The indicator reaction must be rapid and reversible -------
sharp and intense color change

FERROIN

▪ It is a complex of Iron (II) with 1,10-phenanthroline
(orthophenanthroline)

▪ Three molecules of 1,10-phenanthroline (bidentate) combine with

1 Fe2+ , formulated as (phen)3Fe2+

N

2+

Fe

N

3

▪ The complexed iron in ferroin undergoes a reversible redox

reaction (phen)3Fe2+ E0 =1.11 V

(phen)3Fe3+ + e–

Pale blue (Ferriin) Red (Ferroin)

▪ It is useful for many titrations with cerium (IV)

▪ For Ferroin, E0 = 1.11 V.

▪ ( Question: Calculate the transition range of ferroin)

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