What is the concept and measurement principle of conductivity?

The concept of conductivity and its measurement principle

Conductivity is the ability of an object to conduct current. The measuring principle of the conductivity measuring instrument is to put two parallel plates in the solution to be measured, add a certain potential (usually a sine wave voltage) to both ends of the plates, and then measure the flow between the plates Current. According to Ohm’s law, conductivity (G)-the reciprocal of resistance (R) is determined by the conductor itself. The basic unit of conductivity is Siemens (S), which was originally called ohm. Because the geometry of the conductivity cell affects the conductivity value, the unit conductivity S/cm is used in the standard measurement to compensate for the difference caused by various electrode sizes. The unit conductivity (C) is simply the product of the measured conductivity (G) and the conductivity cell constant (L/A). Here L is the length of the liquid column between the two plates, and A is the area of ​​the plate .

The conductivity of the aqueous solution is directly proportional to the concentration of dissolved solids, and the higher the solids concentration, the greater the conductivity. The relationship between conductivity and dissolved solids concentration is approximately expressed as: 1.4μS/cm=1ppm or 2μS/cm=1ppm (per million units of CaCO3). The total hardness value of water can be obtained indirectly by using a conductivity meter or a total dissolved solids meter. As mentioned above, for the convenience of approximate conversion, 1μs/cm conductivity = 0.5ppm hardness. Conductivity is the ability of a substance to transmit current. It is relative to the resistance value. The unit is Siemens/cm (S/cm). The unit of 10-6 is expressed in μS/cm, and when it is 10-3, it is expressed in mS/cm. But you need to pay attention:

  1. Indirect measurement of water hardness by conductivity, the theoretical error is about 20-30ppm
  2. The conductivity of the solution determines the movement of molecules, and temperature affects the movement of molecules. In order to compare the measurement results, the test temperature is general Set it as 20°C or 25°C
  3. Reagent testing can obtain a more accurate water hardness value.

The conductivity of water has a certain relationship with the amount of inorganic acids, alkalis, and salts it contains. When their concentration is low, the conductivity increases as the concentration increases. 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. The conductivity of fresh distilled water is 0.2-2μS/cm, but after a period of time, it absorbs CO2 and increases to 2-4μS/cm; the conductivity of ultrapure water is less than 0.10/μS/cm; the conductivity of natural water is more Between 50-500μS/cm, mineralized water can reach 500-1000μS/cm; the conductivity of industrial wastewater containing acid, alkali, and salt is often more than 10 000μS/cm; the conductivity of seawater is about 30 000μS/cm.

The electrode constant is often measured with a standard potassium chloride solution with known conductivity. The conductivity (25°C) of potassium chloride solutions with different concentrations is listed in the table below. The conductivity of the solution is related to its temperature, the polarization phenomenon on the electrode, the distributed capacitance of the electrode and other factors. Generally, compensation or elimination measures are adopted on the instrument. The water sample should be measured as soon as possible after collection. If it contains coarse suspended matter, oil and grease, which interferes with the measurement, it should be filtered or extracted.

Concentration (mol/L)Conductivity (μS/cm)
CM-230 conductivity TDS meter

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