Instant Notes: Analytical Chemistry

C10 – Conductimetry 105

Similar rules apply to anions. Also, potassium ions and chloride ions have very

similar mobilities. This is useful when ‘salt bridges’ are needed (see Topic C2).

If ions are present in other solvents, such as liquid ammonia, they will

conduct in a similar way, but their mobilities will be different.

Conductance Ohm’s lawstill applies to relate the current, I, to the applied voltage, E, and the
resistance, R, or to the conductance, G =1/R.

I =E/R =EG

Gis related to the concentration of ions and has units of ohms-^1 or siemens (W-^1

or S).

In order to measure the conductance of a solution, a conductance cell is used.

It has two electrodes, often platinum, coated with platinum black, separated by

a fixed distance. Connecting these to a conductance meter, or more simply to a

Wheatstone bridge, the conductance may be measured directly. In order to

prevent electrolysis (see Topics C2, C9) taking place, which would change the

concentrations in the solution, the bridge uses alternating current. Plastic and

flow-through conductance cells are also available

In any conductor, the conductance depends on the cell dimensions and on the

characteristics of the conducting medium. For a cell with electrodes of area A,

set a distance lapart:

G =k/(l/A)

where kis called the conductivity of the solution with units of W-^1 m-^1 or S m-^1.

So that the results from different cells and solutions may be compared, the cell

constant, K=(l/A) is measured using a solution of known conductivity, often

0.1 M aqueous potassium chloride, which has a conductivity k=1.288W-^1 m-^1

at 25∞C.

Conductivity Although it is often possible to work directly with conductance, it is useful to
know the conductivity and molar conductivity when results are to be compared.
From the equations above:

k=G(l/A)

Since the analyst is concerned with molar concentrations, and the conductivity

depends mostly on the amounts of ions present, the molar conductivity Lmis

defined by:

Lm=k/c

where Lm has units of W-^1 m^2 mol-^1 provided that the concentration cis

expressed in mol m-^3 (=1000 M).

Example

When immersed in a 0.1 M aqueous solution of KCl at 25∞C, a conductance cell

had G =8.59¥ 10 -^3 W-^1. For 0.1 M HCl, using the same cell, the conductance was

measured as 2.57¥ 10 -^2 W-^1. Calculate the conductivity and molar conductivity

of the HCl.

For the KCl calibration:

k=G(l/A) =8.59¥ 10 -^3 ¥ (l/A)=1.288 W-^1 m-^1

Thus (l/A) =149.9 m-^1 and for HCl, k=3.85 W-^1 m-^1