How do I measure the conductivity ?

With a suitable instrument, electrical conductivity (EC) measurements are relatively quick and easy. EC measures the ability of water to conduct electricity, which in turn depends on the ion concentrations in the solution. For this reason, EC provides useful information about the solution and can be used to estimate total dissolved solids (TDS).

The conductivity of water is measured with a probe that is inserted into the water. With the electrodes in the probe and the electronics in the meter, the meter can measure conductivity and give a temperature compensated conductivity value (units µS / cm ). To ensure an accurate result, the meter is usually calibrated with several standards before measurement.

Calibration of the measuring device

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

Factors influencing the conductivity

There are three main factors that affect the conductivity of a solution: the ion concentrations, 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. Since each ion can carry an electric charge, water with more ions present can 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 transfer 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 more slowly, but small ions can often attract water molecules more strongly, resulting in a slow-moving hydrated ion. For example, the light ion Li + electricity moves only about half as much as the heavier K + ion due to its stronger interaction with water molecules [2].

Organic substances tend to form poorer electrolytes than inorganic substances – mainly because they have a relatively weak tendency to dissociate into ions. For example, acetic acid is a weak acid with a tendency to remain in its uncharged CH 3 COOH0 form rather than separating into the hydrogen (H +) and acetate (CH 3 COO-) ions. Since many organic substances are weak acids, the conductivity of the solutions containing them tends to increase with increasing pH. This is because organic acids tend to convert to their ionic forms when the solution becomes more basic.

3) Temperature. This is a relatively small but significant effect. Since ions can move faster in warmer water, the conductivity of water increases with increasing temperature. Conductivity increases by about 1.9% [1] for every 1 ° C increase in temperature (or by slightly more than 1% for every 1 ° F difference), so temperature must be equalized in order to compare different conductivity measurements.

Temperature compensation

To facilitate comparison of results for samples measured at different temperatures, conductivity measurements are usually reported as temperature-compensated values. This means that the specified value corresponds to the conductivity when the temperature is 25 ° C. For example, the actual conductivity of a solution tested at 20 ° C is lower than the specified temperature-compensated value. Temperature compensation is normally automatic with a thermistor built into the conductivity probe. If the conductivity readings are not temperature compensated, especially if the temperature is far from 25 ° C, the results will not be reliable.

Can conductivity be determined without using a conductivity instrument?

As described above, the conductivity of water depends on the type and amount 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 conductivity properties of each ion. This is most easily achieved with specialized chemical software such as PHREEQC [3]. However, it is usually easier and more direct to measure conductivity with an appropriate meter.


[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.