Page 363 - Instrumentation Reference Book 3E

346 Chemical analysis: electrochemical techniques
to be due mainly to changes in the viscosity of (b) comparing the measured conductance with
water and the effect this has on the mobility and that obtained from a cell of known cell constant
hydration of the ions. when both cells contain the same solution at the
same temperature.
The only solutions whose conductivities are
17.3.3 Practical measurement of electrical known with sufficient accuracy to be used for
conductivity reference purposes are aqueous solutions of
From the foregoing, it can be seen that measure- potassium chloride. This salt should be of the
ment of electrical conductivity enables concentra- highest purity, at least analytical reagent grade,
tion to be determined. and dried thoroughly in an oven at 120°C before
preparing solutions by dissolving in deionized
water whose conductivity is less than 2 pSlcm at
17.3.3.1 Alternating current cells with contacl room temperature. The most accurate reference
electrodes
solutions are prepared by weight, and the two
Conductivity cells provide the means of conduct- most useful solutions are given in Table 17.4.
ing a small, usually alternating, current through a
precise volume of liquid whose conductivity we Table 17.4 Standard solutions for cell calibration
wish to know. At its simplest, this process
involves the measurement of the resistance Solution K at 18°C IE. at 25°C
between two electrodes of fixed shape and con- g KclllOOOg solution* S m-‘ s rn-I
stant distance apart. The relationship between the
specific conductivity K of the solution and the (A) 7.4191 1.1163 1.2852
resistance R across the electrodes includes a cell (B) 0.7453 0.12201 0.14083
constant “a” such that *All values are “mass in vacuo.”
6 = a1R
For many purposes a simpler procedure can be
If we express the conductivity in units of S cm-I followed. This involves weighing only the potas-
then the cell constant has the dimension of cm-’ . sium chloride and preparing solutions by volume
In order to simplify the electrical circuits of the at 20 “C; these details are given in Table 17.5.
measuring instruments it is customary to main-
tain the resistance of conductivity cells between
the limits of 10 and 100,000 R. The conductivity of Table 17.5 Standard solutions (volumetric) for cell
aqueous solutions varies from pure water with a calibration
conductivity of about 5 plm to those of concen-
trated electrolytes with conductivities as high as Solution K at 18°C IE. at 25°C
1000 S/m. In order to keep within these resistance (A’) 7.4365g KC1/1 1.1167s m-’ 1.2856s rn-l
limits it is necessary, therefore, to have cells with at 20°C
a range of cell constants from 0.01 to lOOcm-’. A (B’) 0.7440g Kclll 0.1221 S rn-l 0.1409s m-’
working guide to the most appropriate value of at 20°C
cell constant for any given range of conductivity (C’) 100ml of - 146.93 pS cm-’*
is shown in Table 17.3. solution B’
In order to measure the conductivity accurately made up to
it is necessary to know the cell constant accur- 1 liter at 20°C
ately. It is usual to determine the cell constant * For the highest accuracy the conductivity of the dilution water
by preferably (a) measuring the conductance should be added to this value.
when the cell is filled with a solution whose con-
ductivity is accurately known or, failing that, Calibration of conductivity cells by these solu-
tions requires considerable care if accurate values of
cell constants are to be determined. The importance
Table 17.3 Guide to cell constant for known conductivity of temperature control cannot be over-emphasized
range
since the conductivity of the potassium chloride
Conductivity range Cell constant solution will change by over 2 percent per Kelvin.
pS cm-’ cm- Alternatively, the cell constant can be determined
by the comparison technique with identical, rather
0.05 to 20 0.01 than standard conditions in both the “known” and
1 to 200 0.1 ”unknown” cell. Equally important as the effect of
10 to 2000 1
100 to 20 000 10 temperature is that of polarization in these cells
100 to 200 000 50 where the electrodes contact the solution and con-
duct a significant current.