2. Conductivity/Total Dissolved Solids
Introduction to Conductivity: Definition
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 is measured. Conductivity (G), the inverse of resistivity (R) is determined from the voltage and current values according to Ohm's law.
G = 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 sulphuric 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 cm-1, 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.
The table shows optimum conductivity ranges for cells of three different constants.
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 = Gtcal {1 + a(T-Tcal)}
where: Gt = conductivity at any temperature T in °C, Gtcal = conductivity at calibration temperature Tcal in °C, a = temperature coefficient of solution at Tcal in °C.
Common alphas (a) are listed in the table below. To determine that a of other solutions, simply measure conductivity at a range of temperatures and graph the change in conductivity versus the change in temperature. Divide the slope of the graph by Gtcal to get a.
All meters have either fixed or adjustable automatic temperature compensation referenced to a standard temperature - usually 25°C. Most meters with fixed temperature compensation use a of 2%/°C (the approximate a of NaCl solutions at 25°C). Meters with adjustable temperature compensation let you to adjust the a to more closely match the a of your measured solution.
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. The conductivity of some common solutions is shown in the table below.
A polarized or fouled electrode must be cleaned to renew the 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 colours solutions will remove algae, bacteria or molds. To prevent cell damage, abrasives or sharp objects should not be used to clean an electrode. A cotton bud works well for cleaning but care must be taken not to widen the distance of cell.
*Information Provided Courtesy of Eutech Instruments Pte Ltd Copyright © 1997
