46
SECTION 4,
continued
4.9
Theory of Conductivity Measurement
Conductivity is the ability of a material to conduct current.
Positive and negative ions in a solution will move to the
oppositely charged electrode when an electric charge is applied to
the solution, thus conducting current. In addition to the current
applied, ion movement is affected by the solvent properties
(temperature, viscosity) and the physical properties of the ion
(size, charge, concentration...). As temperature increases, ions
move faster and conduct more current. As viscosity increases, the
ions move slower and conduct less current.
In theory, a conductivity measuring cell consists of two, 1-cm
square electrode surfaces spaced 1 cm apart. The cell constant (K)
is determined by the cell length (L) and cross-sectional area (A)
(K = L
÷
A). The theoretical cell just described has a cell constant
of K = 1.0 cm
-1
. Cells with larger/smaller electrodes or electrodes
spaced at a different distance are characterized by a different
cell constant.
The Hach Conductivity measuring system has an innovative
two-cell probe design. With this design, a single probe can take
measurements within the full, dynamic range of the instrument.
Less advanced conductivity measurement systems that use
single-cell probes require the user to purchase several probes,
each of which measures only a portion of the instrument’s range.
Electrolytic conductivity is not the same as specific conductivity.
Electrolytic conductivity is a property of the solution being
measured; specific conductivity includes the property of the
measuring cell, partially defined by its physical design. By
defining the physical parameters of the cell, a standard measure is
created. This standard measure (specific conductivity) is
reciprocal of the resistance (1/ohm), measured between the
opposing faces of 1 cm cube of liquid at a specific temperature.
mg/LTDS
g/L TDS
mg/L TDS x 0.001
mg/L TDS
gpg TDS
mg/L TDS x 0.05842
g/L TDS
gpg TDS
g/L TDS x 58.42
µS/cm
ohms•cm
1,000,000 ÷ µS/cm
mS/cm
ohms•cm
cm 1,000 ÷ mS/cm
Table 9 Conversions (Continued)
