practical pharmaceutical analytical chemistry - ii …practical pharmaceutical analytical chemistry...
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Practical Pharmaceutical Analytical Chemistry - II
Second LevelFirst Semester 2018-2019
Section 3
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Redox Indicators
✓ They are highly colored organic compounds that change their
color when being oxidized or reduced.
✓ The color change depends on the change in the redox
potential (E) of the system during titration.
✓ The half-reaction responsible for the color change of the
indicator can be written as follows:
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➢ Requirements (specifications) of general redox
indicators:
1. Has a standard redox potential intermediate between
that of the sample and that of the titrant.
Eºsample < Eºindicator < Eºtitrant OR Eºsample > Eºindicator > Eºtitrant
So that the titrant reacts first with the sample and then with
the indicator at the end point.
2. Exhibits a sharp, readily detectable color change.
3. The transition potential of the indicator (i.e. the
potential range at which the indicator changes its color)
should be close to the potential at the equivalence point.
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➢ Examples:
• 1,10-Phenanthroline indicator: { USP }
Used for the titration of ferrous {Fe2+} # cerric sulfate
titrant {Ce(SO4)2}.
• Diphenylamine indicator:
Used for the titration of ferrous {Fe2+} # pot.
dichromate titrant {K2Cr2O7}.
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Determination of Ferrous Salts
(FeSO4.7H2O)
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Determination of Ferrous with
0.1 N Ceric Sulphate using
1,10-Phenanthroline Indicator
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Principle
✓ Ferrous salts can be determined by titration with 0.1 N Ceric
Sulphate using 1,10-Phenanthroline as a redox indicator.
✓ 1,10-Phenanthroline-ferrous complex (ferroin) is an intense
red colored complex, which is reversibly oxidized (with strong
oxidizing agents) to phenanthroline-ferric complex ion (ferrin),
which has a pale blue color. The complex is used as an
indicator in the titration of ferrous by ceric sulphate.
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✓ During Titration:
Fe2+ + Ce4+ → Fe3+ + Ce3+
✓ At End point:
N
N
3 + Fe2+
N
N
Fe
3 3
N
N
Fe
3+2+
1,10-Phenanthroline Ferroin Ferrin
base intense red pale blue
-e
+e
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Procedure
10 ml Ferrous sulphate sample
+ 2 drops of 1,10 phenanthroline
0.025 N St.
cerric sulphate
At the E.P. the color in the conical
flask changes from orange color to
the first pale blue color
Titrate Ferrous
Sulphate with St.
Cerric Sulphate
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Calculation of equivalent:
278 0.025 Each 1 ml 0.025 N Ce(SO4)2 = ------------------- = 0.00695 g FeSO4.7H2O.
1000
E.p eq. 100
Conc. of ferrous = ---------------------------- = % w/v
10
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Determination of Ferrous with
0.1 N Potassium Dichromate
using Diphenylamine Indicator
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Principle✓ Ferrous salts can be determined by titration with 0.1 N Potassium Dichromate
using Diphenylamine as a redox indicator.
✓ During Titration:
Cr2O72- + 6 Fe2+ + 14H+ → 2Cr3+ + 6 Fe3+ + 7H2O
✓ At End point:
The diphenylamine (I) undergoes oxidation first into a colourless diphenylbenzidine (II) which
is the real indicator and is reversibly further oxidized to diphenylbenzidine violet (III).
N
H
2 N
H H
N
Diphenylamine Diphenylbenzidine (colourless)
+ 2 H+ + 2 e
N N
Diphenylbenzidine (violet)
+ 2 H+ + 2 e
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✓ The oxidation potential of the system diphenylamine/diphenylbenzidine
(colourless)/diphenylbenzidine (violet) = 0.76 volt
✓ The oxidation potential of Fe3+/Fe2+ = 0.77 volt
✓ The oxidation potential of Cr2O7 2-/ 2 Cr3+ = 1.36 volt
➢ It is obvious that there is an overlapping between Eind & EFe3+
/Fe2+
➢ For this, diphenylamine is only able to function as indicator in this
reaction when phosphoric acid is present in the solution.
➢ What is the role of Phosphoric acid?
❑ Formation of a colorless complex with the produced ferric ions leading to:
1. Decreasing the molar concentration of Ferric and hence reduces the actual
potential of (Fe3+/Fe2+ ) system so that Fe2+ ion will be oxidized before the
indicator.
2. Removing the dark yellow color of Fe3+ ion giving a more clear color change.
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Electrochemical series (Eº values at 25°C)
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Procedure
10 ml Ferrous sulphate sample
+ 5 ml dil. H2SO4
+ 1 ml phosphoric acid
+ 7 drops of diphenylamine
0.1 N St.
Potassium
dichromate
The color in the conical flask changes
from greenish blue color to greenish
grey color then at the E.P. changes
into intense violet blue color
Titrate Ferrous
Sulphate with St.
Potassium
dichromate
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Calculation of equivalent:278
Each 1 ml 0.1 N K2Cr2O7 = ------------------- = 0.0278 g FeSO4.7H2O. 10 1000
E.P eq. 100
Conc. of ferrous = ---------------------------- = % w/v
10
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