Engineering Chemistry S Datta

214 ENGINEERING CHEMISTRY
R
T
R
S
C
A
a
X
b
B
Platinum
electrodes
A.C. Source
Fig. 9.8 Determination of conductivity of solutions;
C is a compensating condenser (adjustable).
Fig. 9.9 Conductivity cell.
The modifications are: (a) the use of A.C., (b) the use of telephone detector or magic eye
indicator, (c) the electrolyte is taken in a cell containing platinised Pt electrodes (Fig. 9.9).
The solution whose conductance is to be measured is put into the conductivity cell (see
Fig. 9.9). When the sound in the telephone is minimum or zero reading on the appropriate
meter, specific resistance ρ is given by the following equation:
ρ
R = a b
Since the conductivity of a solution rapidly increases with temperature, the cell should
be placed in a thermostat. Conductivity water (double distilled water) should be used throughout.
Different cells are in use, the common form is shown in Fig. 9.9. Connections are made by Pt
wires.
Calculation of Specific Conductance (K)
If necessary ρ is calculated from the observed resistance, if the area of the electrodes
and the distance between them are accurately known. But the procedure is to calculate a
factor for each cell known as cell constant (x).
Specific conductance = Observed conductance × Cell constant
When l is the distance between the electrodes and s, the area of cross-section of the
electrode, the resistance R of liquid enclosed between the electrodes is:
where l/s = x is cell constant.
R = ρ × l s = ρ × x
NI The cell constant is known by determining the conductance of
HG
50 K J KCl solution. The
K value of the solution has been accurately determined by Kohlrausch to be 0.002768 ohm –1 . cm –1
at 25°C.
Specific conductance K = x × Observed conductance.
F
= x × 1 ρ = x × b a × 1 R