Chapter 7 Reference
7.1 Conductivity measurement
72
●
Measuring conductivity
“Conductivity” is an index that expresses the ease with which electric
current flows through a material. Conductors are categorized either as
“electron conductors” (such as metals and other substances which use
free electrons to conduct electricity) or “ion conductors” (such as
electrolytic solution or fused salt, which use ions to conduct electricity).
This section deals with the kind of conductivity that pertains to ions,
especially the conductivity of electrolytic solution that uses water as the
solvent. As shown in Fig. 1, two pole plates with an area A (expressed in
m
2
) are positioned parallel to each other, separated by distance l
(expressed in m). Then solution is poured into the cell until it is full and
alternating current is run between the plates.
Fig. 1 Type of conductivity cell
Each positive and negative ion in the solution will migrate toward the
oppositely charged pole. The result is that current flows through the
solution by means of ion conductivity. When this occurs, resistance R
(expressed in
Ω
) is in inverse proportion to the area A (expressed in
m
2
) of the pole plates, as is the case with metal and other conductors,
and is proportional to the distance l (expressed in m) between the two
pole plates. These relationships are expressed by equation 1, below.
R = r x l/a = rJ (Equation 1)
R: Resistance (
Ω
)
r: Specific resistance (
Ω⋅
m)
a: Pole plate area (m
2
)
l: distance between pole plates (m)
J: Cell constant (m
-1
)
Solution
Polar plates a m
2
x2
