The Concept of Electrical Conductivity and Its Measurement Principle

Electrical conductivity is the ability of a substance to conduct electric current. The principle of a conductivity meter is to place two parallel plates in the solution being measured, apply a certain potential (usually a sinusoidal voltage) across the plates, and then measure the current flowing between the plates. According to Ohm's law, conductivity (G) is the reciprocal of resistance (R) and is determined by the conductor itself. The basic unit of conductivity is the siemens (S), originally called the ohm. Because the geometry of the conductivity cell affects the conductivity value, standard measurements use unit conductivity S/cm to compensate for differences caused by various electrode sizes. Unit conductivity (C) is simply the product of the measured conductivity (G) and the cell constant (L/A). Here, L is the length of the liquid column between the two plates, and A is the area of the plates.

The conductivity of an aqueous solution is directly proportional to the concentration of dissolved solids; the higher the solid concentration, the greater the conductivity. The relationship between conductivity and dissolved solids concentration is approximately expressed as: 1.4 μS/cm = 1 ppm or 2 μS/cm = 1 ppm (per million units of CaCO3). The total hardness of water can be indirectly obtained using a conductivity meter or total dissolved solids meter. As mentioned above, for ease of conversion, 1 μS/cm conductivity = 0.5 ppm hardness. Conductivity is the ability of a substance to conduct electric current, relative to resistance, and is measured in units of Siemens/cm (S/cm). 10⁻⁶ of this unit is expressed as μS/cm, and 10⁻³ as mS/cm. However, it is important to note that: (1) the theoretical error of indirectly calculating water hardness using conductivity is approximately 20-30 ppm; (2) the conductivity of a solution determines molecular motion, and temperature affects molecular motion. To compare measurement results, the test temperature is generally set at 20℃ or 25℃; (3) using reagent testing can obtain a more accurate water hardness value.

The conductivity of water is related to the amount of inorganic acids, alkalis, and salts it contains. When their concentration is low, conductivity increases with increasing concentration; therefore, this indicator is often used to estimate the total concentration of ions or salt content in water. Different types of water have different conductivity. Fresh distilled water has a conductivity of 0.2-2 μS/cm, but after standing for a period of time, it increases to 2-4 μS/cm due to CO2 absorption; ultrapure water has a conductivity less than 0.10 μS/cm; natural water has a conductivity mostly between 50-500 μS/cm, while mineralized water can reach 500-1000 μS/cm; industrial wastewater containing acids, alkalis, and salts often has a conductivity exceeding 10,000 μS/cm; seawater has a conductivity of approximately 30,000 μS/cm.

The electrode constant is often determined using a standard potassium chloride solution with known conductivity. The conductivity (25℃) of potassium chloride solutions of different concentrations is listed in the table below. The conductivity of a solution is related to factors such as its temperature, polarization on the electrodes, and electrode distributed capacitance. Instruments generally employ compensation or elimination measures.

Water samples should be tested as soon as possible after collection. If they contain coarse suspended matter, oil, or grease that may interfere with the test, they should be removed by filtration or extraction.