DEMONSTRATION: Reaction of oxalic acid
with potassium manganate (VII).
In this experiment the oxalic acid [ethanedioic acid] (the acid found in rhubarb leaves) reacts
with an acidified solution of potassium manganate (VII) slowly at room temperature but much
more quickly at higher temperatures. The time taken for the solution to change from purple to
colourless is recorded.
Chemicals needed
Make up 2 solutions.
SOLUTION A made up of:
100 mls of distilled water,
50 mls of 0.02M potassium manganate and
50 mls of 2M sulphuric acid.
SOLUTION B made up of
50 mls of 0.02M oxalic acid [ethanedioic
acid].
PROCEDURE
Pour solution A into a burette.
Run 20 mls of solution A from the
burette into a boiling tube.
Add 5 mls of solution B into a test
tube.
Heat solutions A and B in a water bath
to a known temperature.
Remove solution A from the water
bath.
Put the thermometer into solution A
Add 5mls of solution B to solution A
Record the temperature of the
solution.
Record the time taken for the purple
colour to become colourless.
Repeat at various temperatures .e.g
room temperature, 30oC, 40oC, 50oC,
60oC, etc
Record the results in an
appropriate table(see below)
Temperature
room temperature
Time taken for solution Using this information plot a graph of time
to go clear
(in secs) vs temperature (in oC)
secs
30oC
secs
40oC
50oC
secs What information can you get from
the graph ?
secs
60oC
secs
FOR THE TECHNICIAN
measuring cylinder
boiling tube
water bath
bunsen burner
stop clock
0.02M potassium manganate (VII)
0.02M ethanedioic acid
burette
-10 – 110 oC thermometer
test tube
triopod and gauze
bunsen mat
Dilute sulphuric acid
HAZARD WARNINGS
Dilute sulphuric acid
Corrosive
Causes severe burns
0.02M potassium manganate (VII)
An oxidising agent
Avoid contact with combustible material.
Harmful if swallowed.
Ethanedioic acid (oxalic acid)
Harmful when swallowed