Table 6.6 Ionic conductancesTitration of Strong Acids or Bases (Figure 6.19(a))
Before the equivalence point, H 3 O+ is progressively replaced by Na+ which has a much smaller
equivalent conductance. After the equivalence point, the conductance increases linearly with increasing
concentration of OH– and Na+. Using LiOH in place of NaOH would give an even sharper change of
slope.
Titration of Weak Acids or Bases (Figures 6.19(b) and (c))
Curve (b) shows the titration of acetic acid, Ka = 1.75 × 10 –^5 mol dm–^3 at two different concentrations.
The initial additions of OH– establish a buffer solution in which the H 3 O+ concentration is only slowly
reduced. The resulting fall in conductance is increasingly counteracted by the addition of Na+ and the
formation of CH 3 COO– thus leading to a minimum in the curve. After the equivalence point, there is a
more rapid increase due to the addition of excess OH– and Na+. Concentrated solutions of weak acids
give more pronounced changes of slope at the equivalence point than dilute solutions. The change in
slope is well defined for very weak acids, curve (c), e.g. boric acid, Ka = 6 × 10 –^10 mol dm–^3 but for
those with values greater than 10–^5 mol dm–^3 , curvature in the buffer region renders detection of the
equivalence point impossible.
Titration of Salts of Weak Acids or Bases (Figure 6.19(d))
The curve shown is for the titration of sodium acetate with hydrochloric acid. Before the equivalence
point, there is a slight increase in conductance as Cl– replaces CH 3 COO– which is converted to
undissociated acetic acid. After the equivalence point, the conductance increases linearly with the
addition of H+ and Cl–.
The course of precipitation and complexometric titrations can also be followed conductometrically, but
changes of slope are generally less pronounced. Redox reactions are difficult to follow because of the
high