Page 30 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

Neutral or weakly alkaline solutions

MnO4− + 4H+ + 3e MnO2 (s) + 2H2O E0 = 1.69 V
Manganese dioxide

(brown solid)

Change in oxidation state of Mn: +7 to +4

Eq wt = Mw / 3

Strongly alkaline solutions (2 M NaOH)

MnO4− + e MnO42 − E0 = + 0.56 V
manganate (green)

Change in oxidation state of Mn: +7 to +6

Eq wt = Mw / 1

Applications in acidic solutions, 0.1M to 1M H2SO4

(Question: why not use HCl for acidity?)

The acid used for acidification is H2SO4 and not HCl as MnO4-
oxidizes Cl– in acid medium
For MnO4−/ Mn2+
E0 = +1.51 V

For Cl2 / Cl− E0 = +1.36 V

2 MnO4− + 10Cl− + 16H+ 2 Mn2+ + 5Cl2 + 8H2O

Direct Titrations

Determination of reducing species

[1] Determination of Fe2+ salts e.g. FeSO4

MnO4− + 5Fe2+ + 8H+ Mn2+ + 5Fe3+ + 4H2O

Cl- Interference

If an iron sample has been dissolved in HCl or if SnCl2 has been
used to reduce Fe3+ to Fe2+,

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