7 Reference
7.1 Conductivity measurement
94
HORIBA
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
)
Specific resistance (expressed in
Ω⋅
m) is an index that
indicates the difficulty with which current flows and is a
constant determined according to the solution. The
inverse of r (expressed in
Ω⋅
m), which is L (and is equal
to 1/r), is called the “specific conductivity” and is widely
used as an index to express the ease with which current
flows. Specific conductivity L is generally referred to as
simply “conductivity” and is expressed in units of S/m.
Inserting conductivity L (expressed in S/m) into equation
1 results in equation 2, below.
R = J/L (Equation 2)
As is clear from equation 2, when a conductivity cell
having a cell constant J of 1 m
-1
is used (in other words,
when a conductivity cell having two pole plates that each
have an area a of 1 m
2
and are positioned parallel to
each other such that the distance l between the two
plates is 1 m is used) the inverse of the resistance R of
the solution (expressed in
Ω
) between both pole plates
is the conductivity. Conductivity is defined in this way,
but it changes according to the temperature of the
solution.
The conductivity of a solution is generally expressed as
the value when the solution is 25ºC.
