| Conductivity Measurement Theory and Practice
Background
Electrical conductivity is an inherent property of most materials, and ranges from extremely conductive materials like metals to very non-conductive materials like plastics or glass. About halfway between the two extremes in conductivity are aqueous solutions, such as sea water and plating baths.
In metals, the electrical current is carried by electrons, while in water it is carried by charged ions. In both cases, the conductivity is determined by the number of charge carriers, how fast they move, and how much charge each one carries.
Thus, for most water solutions, the higher the concentration of dissolved salts, which will lead to more ions, the higher the conductivity. This effect continues until the solution gets "too crowded," restricting the freedom of the ions to move, and the conductivity may actually decrease with increasing concentration. (This can result in two different concentrations of a salt having the same conductivity.) See Table 1.
Some species ionize more completely in water than others do, and their solutions are more conductive as a result. Each acid, base, or salt has its own characteristic curve for concentration vs. conductivity.
Metals are extremely conductive because electrons move almost with the speed of light, while in water ions move much slowly, and the conductivity is much lower. Raising the temperature makes water less viscous, and the ions can move faster. Because the ions are of different sizes, and carry different amounts of water with them as they move, the temperature effect is different for each ion. Typically, the conductivity varies about 1-3% per degree C, and this temperature coefficient may itseif vary with concentration and temperature. See Tables 1 and 2.
Definitions
The conductivity of a material is an inherent property-that is, pure water at a particular temperature will always have the same conductivity. The conductance of a sample of pure water depends on how the measurement is made-how big a sample, how far apart the measuring electrodes are, etc. It is defined as the reciprocal of the resistance in ohms, measured between the opposing faces of a 1 cm cube of liquid at a specific temperature. See Figure 1. The basic unit of conductance is the Siemens (S) and was formerly called the mho. Because a measurement gives the conductance, techniques have been worked out to convert the measured value to the conductivity, so that results can be compared from different experiments. This is done by measuring a cell constant for each setup, using a solution of known conductivity.
Cell Conductance X K = Conductivity (Equation 1)
The cell constant is related to the physical characteristics of the measuring cell. K is defined for two flat, parallel measuring electrodes as the electrode separation distance (d) divided by the electrode area (A). Thus, for a 1 cm cube of liquid,
K = d/A = 1 cm -1 (Equation 2)
In practice, the measured cell value is entered into the meter, and the conversion from conductance to conductivity is done automatically. The K value used varies with the linear measuring range of the cell selected. Typically, a cell with K = 0.1 cm -1 is chosen for pure water measurements, while for environmental water and industrial solutions a cell with K of 0.4 to 1 cm -1 is used. Cells with up to K = 10 cm -1 are best for very high conductivity samples.
For some solutions, such as pure water, the conductivity numbers are so low that some users prefer to use resistivity and resistance instead. The resistivity is the reciprocal of the conductivity (R = 1/C), and the resistance is the reciprocal of the conductance. Resistance units are in ohms, and 1 ohm = 1/Siemens. From Eq. 1 and 2, it can be seen that conductivity units are in Siemens/cm, and therefore resistivity units are in ohm-cm.
measured temperature, temperature coefficient and reference temperature, will report sample conductivity
How is conductivity measured?
In the simplest arrangement (a 2-electrode cell), a voltage is applied to two flat plates immersed in the solution, and the resulting current is measured. See Figure 1. From Ohm's Law, the conductance = currentlvoltage. Actually there are many practical difficulties. Solution conductivity is due to ion mobility. Use of DC voltage would soon deplete the ions near the plates, causing polarization, and a higher than actual resistance. This can be mostly overcome by using AC voltage, but then the instrument designer must correct for various capacitance and other effects. Modern sophisticated 2-electrode conductivity instruments use complex AC waveforms to minimize these effects, and by using the cell constant, measured temperature, temperature coefficient and the reference temperature will report sample conductivity.
Figure 1. Conductivity Cell
Table 1 Some Conductivity Values of Typical Samples
| Sample at 25 o C |
Conductivity microScm -1 |
| Ultrapure Water |
0.055 |
| Power Plant Boiler Water |
1.0 |
| Drinking Water |
50 |
| Ocean Water |
53,000 |
| 5% NaCl |
223,000 |
| 50% NaOH |
150,000 |
| 10% HCl |
700,000 |
| 32% HCl |
700,000 |
| 31% HNO 3 |
865,000 |
Table 2 Some typical temperature coefficients
| Sample |
Percent per oC ( at 25 oC ) |
| Ultrapure Water |
4.55 |
| Salt Solution ( 5% ) |
2.12 |
| NaOH ( 5% ) |
1.72 |
| Dilute Ammonia Solution |
1.88 |
| HCl ( 10% ) |
1.32 |
| Sulphuric Acid ( 5% ) |
0.96 |
| Sulphuric Acid ( 98% ) |
2.84 |
| Sugar Syrup |
5.64 |
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