Engineering Chemistry S Datta

218 ENGINEERING CHEMISTRY
Highlights:
• The change in volume throughout the course of titration must be kept small by
using a titrant which is at least 100 times more concentrated than the solution
being titrated.
• Measurements of conductance in the region of equivalence point are of little value
due to hydrolysis, dissociation or solubility of the reaction products. For this reason,
the graph shows a curvature rather than a clear intersection of the two straight
lines [Fig. 9.10(b)].
(a) Neutralisation Reactions
In the titration of a strong acid with a strong base, there is always an initial decrease in
conductance. The conductance then rises beyond the equivalence point. In the titration of
NaOH with HCl, the decrease in conductance is due to the replacement of OH – ions (molar
ionic conductivity = 199) by Cl – ions (molar ionic conductivity = 76). Beyond the equivalence
point, the increase in conductivity is due to excess of H + ions with very high ionic molar
conductivity (350).
In the titration of HCl with NaOH, the initial fall in conductance is due to the replacement
of H + ions (350) with Na + ions (50) and the rise beyond equivalence point is due to excess OH –
ions (199) [Fig. 9.10(a)].
Highlight:
The stronger the solution used for the titration, the sharper is the end point
(Fig. 9.10).
Conductivity falls as
–
OH (aq) ions are
neutralized
Conductivity
Conductivity of
+ –
Na Cl (aq)
Conductivity rises
due to excess
+
H (aq) ions added
End point
Conductance
Volume of HCl (aq) added
Volume of alkali
Fig. 9.10 (a) Changes in conductivity during titration Fig. 9.10 (b) Changes in conductivity
of NaOH(aq) with HCl(aq). In water H + (aq) during titration of a weak acid
and OH – (aq) ions are very mobile, so they (CH 3
COOH) (aq) with a strong
conduct electricity very well. base (NaOH) (aq).
(b) Precipitation Reactions
Conductometric titrations of this type may be performed if the reaction product is
sparingly soluble salt. The precipitate should be formed fairly rapidly and should not have
strong adsorbent properties. The titration of KCl solution vs AgNO 3
solution satisfies the above
conditions. The nature of the titration curve is as in Fig. 9.10 (c).
The point of intersection of the two curves is sharp. So, end point of the reaction can be
determined easily.