Theory
In this experiment the amount of Cu+2 in CuSO4.5H2O was determined with
the help of iodometric titration. This is an oxidation and reduction reaction,
as well as “Iodometric reaction”. Titration involving with iodine or dealing
with iodine liberated in chemical reaction is called Iodimetric and Iodometric
titration respectively. This reaction is iodometric because iodine is got from
KI. The main purpose is to
determine the amount of Cu+2
ions in copper sulphate (CuSO4.5H2O ) for this we first determined the
molarity (moles/dm3) of CuSO4.5H2O and then performed the mathematical calculations
to obtain the end result.
Apparatus
Burette, Burette
stand, beaker, stirrer, weight balance, measuring cylinder
Chemicals
1g KI, 0.1 M Na2S2O3,
Sample solution (CuSO4.5H2O)
Principle
Redox reaction
Reaction
equation
2 CuSO4 + 4KI
→ Cu2I2 + 2K2SO4 + I2
I2 + I- → I3-
Starch
+ I3- → Starch-I3 (dark blue)
2S2O3-2 + I2 → S4O62- + 2I-
Indicator
Starch solution
End point
Colourless
Procedure
1.
Make
of 0.1 M Na2S2O3 solution by dissolving 1.24g
of Na2S2O3 in distilled water in a 50ml beaker
and then make the volume of the solution up to the mark.
2.
Prepare
sample solution by taking small amount of CuSO4 and dissolve in distilled water make
solution up to 20 ml
3.
In
titration flask take 10ml of sample solution and add 1g of KI in it, then dilute
it with 10ml of distilled water
4.
Titrate
this solution with Na2S2O3 taken in burette until
the colour of the solution became pale yellow
5.
Add
2ml of 5% starch solution in the above solution the colour will change into blue
6.
Again,
titrate until colourless solution is obtained
Starch solution:
Make a paste of
5g of soluble starch with a little water and pour the paste with constant
stirring into 100 ml of boiling water and boil for 1 minute. Allow the solution
to cool and add 2-3g of KI. Keep the solution in a stoppered bottle.
Calculations:
Molarity of CuSO4.5H2O
= M2 = ………M (suppose its ‘A’ moles)
Cu+2 : CuSO4.5H2O
1
: 1
According to the above mole ratio 1 mole
of CuSO4.5H2O contains
1 mole of Cu+2
ions which means
A moles of CuSO4.5H2O = A mole of Cu+2
Result
Amount of Cu+2 in CuSO4.5H2O
is ………moles/dm3
Discussion:
In the
titration of iodine, starch must not be added until just before the end point
is reached. Apart from the fact that the fading of iodine colour is a good
indication of the approach at the end point. If the starch solution is added when
the iodine concentration is high, some iodine may remain adsorbed even at the
end point.