Testing Conductivity

With an appropriate instrument, electrical conductivity (EC) measurements are relatively fast and simple. EC measures the ability of water to conduct an electric current, which in turn depends on the concentrations of ions in the solution. Because of this, EC provides useful information about the solution and can be used to estimate its total dissolved solids (TDS).

The conductivity of water is measured using a probe that is inserted into the water. Using the electrodes in the probe and the electronics in the instrument, the instrument is able to measure the conductivity and report a temperature-compensated conductivity value (units of µS/cm are most typical). To ensure an accurate result, the instrument is usually calibrated with one of more standards prior to the measurement.

Calibration of the instrument

To make accurate measurements, a conductivity instrument is usually calibrated using potassium chloride (KCl) solutions of known concentration. Typically, a standard composed of 0.01 M KCl is used, which has a conductivity of 1412 µS/cm at 25°C [1], but a standard that has a conductivity similar to the solutions being analyzed is ideal. For greater accuracy over a wide range of conductivity values, up to 3-5 standards of different KCl concentrations can be used to calibrate the instrument.

Factors affecting conductivity

There are three main factors that affect the conductivity of a solution: the concentrations of ions, the type of ions, and the temperature of the solution.

1) The concentration of dissolved ions. An electrolyte consists of dissolved ions (such as Na+ and Cl-) that carry electrical charges and can move through water. As each ion is able to carry an electrical charge, water with more ions present is able to conduct a greater amount of current. This is the most important of the three main factors.

2) The types of ions in solution. Different ions have different abilities to transmit charge. Inorganic ions such as Na+, K+, Mg+2, Ca+2, HCO 3-, Cl- and SO4-2, tend to conduct electricity well, although each ion has a different ability to conduct electricity. This depends on factors such as the charge of the ion, its size, and its tendency to interact with water molecules. Heavier ions tend to move slower, but small ions can often attract water molecules more strongly, resulting in a slow-moving hydrated ion. For example, the lightweight ion Li + actually moves electricity only about half as well as the heavier K+ ion[2] because of its stronger interaction with water molecules.

Organic substances tend to make poorer electrolytes than inorganic substances largely because they have a relatively weak tendency to dissociate into ions. For example, acetic acid is a weak acid with a tendency to stay in its uncharged CH 3COOH0 form rather than separate into the hydrogen (H +) and acetate (CH3COO-) ions. Because many organic substances are weak acids, the conductivities of solutions containing them will tend to rise as pH increases. This is because organic acids tend to become converted to their ionic forms as the solution becomes more basic.

3) Temperature. This is a relatively small, but significant, effect. Because ions can move faster in warmer water, the conductivity of water increases with rising temperature. Conductivity will increase by approximately 1.9% for each 1°C increase in temperature [1] (or a little more than 1% for each 1°F difference), which makes it necessary to compensate for temperature so that different conductivity measurements can be compared.

Temperature compensation

To make it easier to compare results for samples tested at different temperatures, conductivity measurements are usually reported as temperature-compensated values. This means that the value reported is what the conductivity would be if the temperature was 25°C. For example, the actual conductivity of a solution tested at 20°C will be lower than the reported temperature-compensated value. Temperature compensation is usually done automatically with a built-in thermistor in the conductivity probe. If the conductivity readings are not temperature compensated, especially when the temperature is far away from 25˚C, the results would not be dependable.

Can conductivity be determined without using a conductivity instrument?

As described above, the conductivity of water depends on the type and amounts of charged ions in solution. If the chemical composition of a solution is known, and if the ions present are limited to well-characterized inorganic ions such as Na +, K+, Mg+2, Ca+2, HCO3-, Cl – and SO4-2 or some organic ions, the conductivity of the solution can be calculated based on the conductance properties of each ion. This is most easily accomplished using specialized chemical software such as PHREEQC [3]. However, it is usually simpler and more direct to measure the conductivity with an instrument.

References

[1] American Public Health Association (APHA) (2005) Standard methods for examination of water and wastewater, 21st edn. APHA, AWWA, WPCF, Washington.

[2] Haynes, W. M. (2009). CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data. Boca Raton: CRC Press.

[3] Parkhurst, D.L., and Appelo, C.A.J. (2013), Description of input and examples for PHREEQC version 3–A computer program for speciation, batch- reaction, one-dimensional transport, and inverse geochemical calculations: U.S. Geological Survey Techniques and Methods, book 6, chap. A43, 497 p.