Definition of Conductivity and theory

Definition of Conductivity

Conductivity is the ability of a material to conduct electric current. The principle by which instruments measure conductivity is simple—two plates are placed in the sample, a potential is applied across the plates (normally a sine wave voltage), and the current that passes through the solution is measured. Conductivity (G), the inverse of resistivity (R), is determined from the voltage and current values according to Ohm's law.

G =

1   R

=

I (amps)  E (volts)

Since the charge on ions in solution facillitates the conductance of electrical current, the conductivity of a solution is proportional to its ion concentration.
In some situations, however, conductivity may not correlate directly to concentration. The graphs shown here illustrate the relationship between conductivity and ion concentration for two common solutions. Notice that the graph is linear for sodium chloride solution, but not for highly concentrated sulfuric acid. Ionic interactions can alter the linear relationship between conductivity and concentration in some highly concentrated solutions.

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Units of Measurement
The basic unit of conductance is the siemen (S), formerly called the mho. Since cell geometry affects conductivity values, standardized measurements are expressed in specific conductivity units (S/cm) to compensate for variations in electrode dimensions. Specific conductivity (C) is simply the product of measured conductivity (G) and the electrode cell constant (L/A), where L is the length of the column of liquid between the electrodes and A is the area of the electrodes (see illustration).
C = G x (L/A)
If the cell constant(K) is 1 cm-1, the specific conductivity is the same as the measured conductivity of the solution. If other cell constants are used, most meters will automatically compensate for the change in cell geometry. To save room, cm-1 is not shown when cell constants are listed.
Although we specify conductivity ranges for our products in µS or mS, due to space limitations these ranges should be understood to reflect specific conductivity in µS/cm or mS/cm, respectively.
1 µS/cm = 0.001 mS/cm = 0.000001 S/cm = 1 µmho/cm
The following table shows optimum conductivity ranges for cells of three different constants:

Cell Constant(K)	Optimum Conductivity Range (µS/cm)
0.1	0.5 to 400
1.0	10 to 2000
10.0	1000 to 200,000

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Conductivity Meter Calibration and Cell MaintenanceConductivity meters and cells should be calibrated to a standard solution before using. Select a standard that is closest to the conductivity of the solution to be measured. Polarized or fouled electrodes must be replatinized or cleaned to renew active surface of the cell. In most situations, hot water with a mild liquid detergent is an effective cleanser. Acetone easily cleans most organic matter, and chlorous solutions will remove algae, bacteria, or molds. Do not use abrasives to clean an electrode. Replace this cell if all else fails.
The conductivity of some common solutions is shown in the table below.

Solution	Conductivity
Pure water	0.055 µS/cm
Power plant boiler water	1.0 µS/cm
Good city water	50 µS/cm
Ocean water	53 mS/cm
31.0% HNO3	865 mS/cm

Conversions:

Multiply	→	to get
to get	←	Divide
µS/cm	1	µmho/cm
mS/cm	1000	µS/cm
µS/cm	0.5	ppm