Disadvantages
Conductance is a non-specific property; high concentrations of other electrolytes can be troublesome.
The electrical conductance of a solution is a measure of its current-carrying capacity and is therefore
determined by the total ionic strength. It is a nonspecific property and for this reason direct conductance
measurements are of little use unless the solution contains only the electrolyte to be determined or the
concentrations of other ionic species in the solution are known. Conductometric titrations, in which the
species of interest are converted to non-ionic forms by neutralization, precipitation, etc. are of more
value. The equivalence point may be located graphically by plotting the change in conductance as a
function of the volume of titrant added.
A conductance cell consists of two platinum electrodes of large surface area across which an alternating
low-voltage potential is applied. Generally, 5–10 V at 50–10 000 Hz is employed. A dc potential cannot
be used as the current flow would lead to electrolysis and hence changes in solution composition. The
cell is incorporated into one arm of a Wheatstone-bridge type of circuit and the conductance measured
by adjustment of a calibrated resistor to balance the bridge. Conductance measurements made in this
way have a precision of 0.1% or better but a limiting factor is the temperature-sensitivity of ionic
conductances. The cell temperature should be held constant to within ±0.1 K throughout a series of
measurements if a precision of better than 0.5% is required.
Ionic Conductances
Defined as the reciprocal of resistance (siemens, Ω–^1 ) conductance is a measure of ionic mobility in
solution when the ions are subjected to a potential gradient. The equivalent conductance λ of an ion is
defined as the conductance of a solution of unspecified volume containing one gram-equivalent and
measured between electrodes 1 cm apart. Due to interionic effects, λ is concentration dependent, and
the value, λ 0 , at infinite dilution is used for comparison purposes. The magnitude of λ 0 is determined by
the charge, size and degree of hydration of the ion; values for a number of cations and anions at 298.15
K are given in Table 6.6. It should be noted that H 3 O+ and OH– have by far the largest equivalent
conductances. For this reason and because H 2 O has a very low conductivity, acid-base titrations yield
the most clearly defined equivalence points. Some specific examples of titration curves are shown in
Figure 6.19. In all cases, the equivalence points are located at the intersection of lines of differing slope.
Curvature in these regions is due to partial dissociation of the products of the titration reaction.
Conductance readings must be corrected for volume changes unless the titrant is at least twenty times as
concentrated as the solution being titrated (cf. amperometric titrations). The shape of each curve can be
explained in terms of the λ 0 values given in Table 6.6.