Thermo Fisher Scientific CyberScan CON 10/CON 100/CON 200 User Manual
Page 57
54
Where
R = the resistance in ohms (
Ω
)
r = the resistivity (
Ω
-cm)
A = the surface area of the electrode (cm
2
)
L = the distance between the electrodes (cm)
k = the cell constant (cm
-1
)
The resistivity r is the index of how difficult it is for the current to flow through the solution. This
is a constant, determined for each electrolyte. The reciprocal of r is called the conductivity, C,
where C=1/r.
(In electrochemical terms, the conductivity is proportionally constant of the electrolyte, which is
an intrinsic property of the solution; and C is the conductance, the inverse of resistance for a
certain segment of the electrolyte).
However, we can use the terms conductance and conductivity interchangeably as an index of
how easily current can flow through an electrolyte, and C will be referred to as the
conductivity. This is an accepted general practice.
C is expressed in units of Siemens (S ohms
-1
) per cm (S/cm). Using C, equation1 can be
reformulated as equation 2.
R=k/C Equation 2
From equation 2, it is clear that we have a conductivity cell with a cell constant, k of 1 (i.e. 1
cm
-1
), then the inverse of the electrode resistivity, R, (in ohms) will be the conductivity, C, in
S/cm.
A conductivity cell with a cell constant of 1 is defined as an electro-chemical cell where each of
the electrode plates has a surface area of 1 cm
2
and the electrodes are aligned in parallel and
separated by a distance of 1 cm.
Conductivity can be defined in this way; however, since it will fluctuate depending on the
temperature of the electrolyte, in general, conductivity is specified at a standard reference
temperature of 25
°
C.
Since the conductivity of the electrolyte is based on ion movement, it is natural that the
concentration of ions in the solution will have a great bearing on this. Therefore, the
conductivity can provide us with valuable indicators in gathering data on the nature of the ions
in the solution. The results of conductivity measurements are widely used in the
electrochemical field.