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 to the plates (normally a sine wave voltage), and the current is measured.
Conductivity (C), the inverse of resistivity (R) is determined from the voltage and current values according to Ohm’s law.
C = I/R = I (amps) / E (volts)
Since the charge on ions in solution facilities 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 below 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.
Units of Measurement:
The basic unit of conductivity is the siemens (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 electrode and A is the area of the electrodes (see Figure 1).
C = G x (L/A)
If the cell constant is 1 cm1, the specific conductivity is the same as the measured conductivity of the solution. Although electrode shape varies, an electrode can always be represented by an equivalent theoretical cell.
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Conductivity Temperature Compensation:
Conductivity measurements are temperature dependent. The degree to which temperature affects conductivity varies from solution to solution and can be calculated using the following formula:
Gt = Gt_{cal} {1 + (TT_{cal})}
where: Gt = conductivity at any temperature T in °C, Gt_{cal} = conductivity at calibration temperature T_{cal} in °C, = temperature coefficient of solution at T_{cal} in °C.
Conductivity Meter Calibration and Cell Maintenance:
Conductivity meters and cells should be calibrated to a standard solution before using. When selecting a standard, choose one that has the approximate conductivity of the solution to be measured.