Conductive titration of mixture(strong acid +weak acid) with
strong base
Upon adding a strong base to a mixture of a strong acid and weak
acid(HCl,CH3COOH), the conductance falls until the strong acid is
neutralize, the rises as the weak acid is converted into its salt ,and finally
rises more steeply as excess of alkali is introduced .Such a titration curve
is showen as S in diagram .The three branches of the curve will be
straight lines except in so far as:
a- increasing dissociation of the weak acid results in a rounding off the
first end- point.
b-Hydrolysis of the salt of the weak acid causes a rounding off at the
second –point, usually extrapolation of the straight portions of the three
branches leads to definite location of end-points .Here also titration with a
weak base such as aqueous solution, is frequently preferable to strong
alkali for reasons already mentioned in discussion.
Procedure:
1- Prepare (50ml) of NaOH(0.1N) in volumetric flask.
2-Wash electric cell by distilled water ,then measure conductance of it.
3-Put (25ml) of HCl and (25ml) of CH3COOH in beaker and measure
conductance of mixture.
4-Add (1ml) of NaOH(0.1N) as portions and measure conductance after
each addition until reach first-end point.
5-Add (0.1ml) as portions and measure conductance after addition in
order to reach second-point.
Calcuation:
1-Plot graphic relation between volume of:base and conductance
2-Find first-end point of strong acid by using this equation:
N.VHCl=N.VNaOH
N.25=0.1.plot
3-Find normality of weak acid from this equation:
V=V2-V1
N.VCH3COOH=N.VNaOH
N.25=0.1.V