conductivity

Conductivity is the ability of a substance to transmit electrical current, as opposed to its resistance value. Conductivity represents the ability of an aqueous solution to conduct current. The conductivity of an aqueous solution is directly proportional to the concentration of dissolved solids, and the higher the concentration of solids, the greater the conductivity. The relationship between conductivity and dissolved solids concentration is approximately expressed as follows: 1.4 μ S/cm=1ppm or 2 μ S/cm=1ppm (per million units of CaCO3). The measurement unit is Siemens/cm (S/cm), where 10-6 is expressed in μ S/cm and 10-3 is expressed in mS/cm. Among them, 1ppm is equal to 1mg/l, which is the unit of measurement for total solid solubility.

Conductivity (L) is the reciprocal of resistance (R). Under certain conditions (temperature, pressure, etc.), the resistance of a conductor is not only determined by the nature of the material, but also by its cross-sectional area and length. The conductivity (L) of a conductor can be expressed as follows:

In the formula: K——1/ρ， Known as conductivity or specific conductivity;

Q——1/A， It is called electrode constant or conductivity cell constant.

For electrolyte solutions, conductivity refers to the electrical conductivity of a 1cm3 solution filled between two parallel electrodes 1cm apart. As can be seen from the above equation, when the electrode constant (Q) is known and the solution resistance (R) is measured, the conductivity can be obtained.

The electrode constant is often measured using a standard potassium chloride solution with known conductivity. The conductivity (25 ℃) of potassium chloride solutions with different concentrations is listed in Table 3-13.

The conductivity of a solution is related to factors such as temperature, polarization phenomenon on the electrode, and electrode distribution capacitance. Compensation or elimination measures are generally adopted in instruments.

Table 3-13 Conductivity of Potassium Chloride Solutions with Different Concentrations

The determination of water conductivity is mainly carried out using a conductivity meter (HJ/T97-2003), which consists of a conductivity cell system and measuring instruments. A conductivity cell is a container that holds or sends the measured solution. In the conductivity cell, conductive electrodes and temperature sensing elements are installed. The commonly used flat electrode in the laboratory, such as the 260 conductivity electrode, is made by fusing two smooth platinum plates or platinum black coated platinum plates with an area of 5 × 10mm2 onto a ring-shaped glass, with a distance of 6mm between the electrodes. Smooth platinum electrodes are used to determine solutions with low conductivity, while platinum black coated platinum electrodes are used to determine solutions with higher conductivity. The electrodes of industrial conductivity meters are often made of stainless steel or graphite in cylindrical or circular shapes; For the determination of electrical conductivity in highly corrosive media, non-contact electrodes can be used.

According to different principles of measuring conductivity, conductivity meters can be divided into balanced bridge conductivity meters, resistance voltage divider conductivity meters, current measurement conductivity meters, electromagnetic induction conductivity meters, etc.