Lecture 27 : Experimental methods in chemical kinetics
27.3
Conductometry
This method can be used when the conductance of the reaction mixture changes significantly during the course of a reaction. A common example is the saponification of an ester, ethyl acetate.
CH3 COOC2H5 + NaOH
CH3COONa + C2H5OH
(27.3)
In the reaction mixture, the conductances of the ester and the alcohol are negligible as they do not ionise. NaOH has a very high value of conductance and the conductance of sodium acetate is much smaller. The conductance of the reaction decreases as the reaction proceeds. The conductance (or the resistance) of the reaction mixture can be measured in a conductivity cell.
The above reaction is known to be second order and the rate constant can be determined by taking initially equal concentrations of NaOH and ethyl acetate. If the initial conductance is C0 final conductance is and the conductance at time t, UCt, then the rate constant can be obtained from eq (27.4)
k = (1/ t) x / [a ( a - x )]
(27.4)
Where a is the initial concentration (proportional to C0), x is the amount reacted ( = C0 - Ct ) and (a-x) is the amount remaining ( = Ct -
).
The operation of conductance cells and their calibration is done by using a conductivity bridge and do look up this material in reference books.
The measurements of polarization and conductance and several other physical properties are useful when the rate constants are small enough so that the values of these properties do not change during the time period of measurement (which is typically at least a few seconds.) The rest of the chapter deals with methods which are useful to study fast kinetics. As mentioned in the introduction, we will outline only some among the large number of techniques available in chemical kinetics.
CH3COONa + C2H5OH 
and the conductance at time t, UCt, then the rate constant can be obtained from eq (27.4) 
