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Redox Equilibria IV  Redox Titrations
C
hem
F
actsheet
April 2003
Number 51
1
Exam Hint: 
At A2 level you need to learn some of the specific half
equations relevant to redox titrations – THESE HAVE BEEN MARKED
FOR YOU IN THIS FACTSHEET.
However, it would be a mistake to try to learn
all
the titration equations
– you need to use the METHOD
for working out the full equation using:
(a) E
ê
values and the anticlockwise rule,
(b) balancing equations using the ‘electron method’.
1) Potassium manganate(VII) titrations
N.B. Used to be called ‘potassium permanganate’.
Potassium manganate (VII) is usually found in laboratories as purple
crystals which are then dissolved in water to form the solutions used in
titrations.
The solution is a dark purple colour and is a
strong oxidising agent.
However, if it is not
acidified
(with dilute sulphuric acid) it produces a
brown precipitate of MnO
2
which makes accurate titrations impossible
(you cannot see the endpoint).
You must learn this half equation
MnO
4
−
(aq) + 8H
+
(aq) + 5e
−
→
Mn
2+
(aq) + 4H
2
O(l)
Note the following points about the halfequation:
(a) MnO
4
−
is the ion in KMnO
4
, potassium manganate(VII), which is
purple
.
(b) In MnO
4
−
the Mn has oxidation number = +7 (O = 2).
(c) 5e
−
are involved – this will be the number when we are balancing half
equations to give the full equation.
(d) The Mn in MnO
4
−
is REDUCED
to Mn
2+
(O.N. = +2) because it is
the OXIDISING AGENT.
N.B. Mn
2+
(aq) is faintly pink in colour but at the concentrations used in
the titrations the solution appears colourless.
In potassium manganate(VII) titrations there is
no indicator added
because the KMnO
4
acts as a
selfindicator
.
In the titration the purple KMnO
4
solution is in the burette and as it is
added to the solution in the conical flask the purple colour is ‘absorbed’.
At the endpoint
one drop
of KMnO
4
solutions produces a
pale pink
colour
in the flask (a very slight excess of the KMnO
4
solution).
Standardising potassium manganate (VII) solution
A ‘standardised solution’ is one whose concentration (mol dm
3
) is known
accurately by titrating it against an accurately madeup solution.
KMnO
4
(aq) is standardised by titrating it with
sodium ethanedioate, Na
2
C
2
O
4
, solution.
Before working through this Factsheet you should:
•
Understand and be confident with titration calculations and using E
ê
values (Factsheets
No.23 and No. 45).
After working through this Factsheet you will:
•
have met the specific reactions of potassium manganate (VII) and
sodium thiosulphate/iodine (which are the quoted examples for redox
titrations);
•
be able to use the anticlockwise rule and the ‘balancing equations
using electrons’ method to write titration reactions from halfequations;
•
have revised the method for calculations involving titrations.
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 Redox

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