Place two parallel plates into the solution being tested, apply a certain potential (usually a sine wave voltage) to both ends of the plates, and then measure the current flowing between the plates. According to Ohm's Law, the reciprocal of conductivity (G) and resistance (R) is determined by voltage and current.
The electric field that causes ions to move in the measured solution is generated by two electrodes in direct contact with the solution. This pair of measuring electrodes must be made of materials that are resistant to chemical corrosion. Materials commonly used in practice include titanium. The measuring electrode composed of two electrodes is called a Kohlrausch electrode.
The measurement of conductivity requires two parameters: one is the conductivity of the solution, and the other is the geometric relationship of L/A in the solution. Conductivity can be obtained by measuring current and voltage.
Conductivity constant K=L/A
Among them: A - the effective electrode plate of the measuring electrode, L - the distance between the two electrode plates.
This value is called the electrode constant. In the presence of a uniform electric field between the electrodes, the electrode constant can be calculated through geometric dimensions. When two square plates with an area of 1cm2 are separated by 1cm to form an electrode, the constant K of this electrode is 1cm-1. If the conductivity value G=1000 μ S is measured using this pair of electrodes, then the conductivity of the measured solution K=1000 μ S/cm.
In general, electrodes often form partially non-uniform electric fields. At this point, the electrode constant must be determined using a standard solution. Standard solutions generally use KCl solution because the conductivity of KCl is very stable and accurate under different temperature and concentration conditions. The conductivity of 0.1 mol/l KCl solution at 25 ℃ is 12.88mS/CM.
The so-called non-uniform electric field (also known as stray field, leakage field) does not have a constant, but is related to the type and concentration of ions. Therefore, a pure stray field electrode is a complex electrode that cannot meet the needs of a wide measurement range through a single calibration. (Uneven electric field: It is an electric field in which the strength and direction of the electric field within the electric field region vary with spatial coordinates. Conversely, an electric field in which the strength and direction of the electric field are independent of coordinates is called a uniform electric field.)