 Guidebook: Conductivity
Reading guide： What are the factors that affect the conductivity value? What is the working principle and method of conductivity measurement? Learn more now.

## What is conductivity?

Conductivity, a physical concept, can also be called electrical conductivity. The product of this quantity and the electric field intensity E is equal to the conduction current density J in the medium. For isotropic media, conductivity is a scalar; for anisotropic media, conductivity is a tensor. In ecology, conductivity is the numerical ability of a solution to conduct an electric current. The unit is expressed in (S/m).

Conductivity is a parameter used to describe the ease of charge flows in a substance. In the formula, conductivity is represented by the Greek letter σ. The standard unit of conductivity σ is S/m, which is the reciprocal of resistivity ρ, that is, σ=1/ρ.

The conductance of an object is 1 S when a current of 1 ampere (1 A) is passed through its cross-section and a voltage of 1 volt (1 V) is present. Siemens is actually equivalent to 1 Amp/Volt. If σ is conductance, I is current (in amperes), and E is voltage (in volts), then: σ = I/E

Typically, the current in such a DC circuit is proportional to the conductance when the voltage is held constant. If the conductance doubles, the current also doubles. If the conductance is reduced to 1/10 of its original value, the current will also be reduced to 1/10 of its original value. This rule also applies to many low-frequency AC systems, such as household circuits. In some AC circuits, especially high-frequency circuits, the situation becomes very complicated because components in these systems store and release energy.

## What are the factors that affect the conductivity value?

### 1. Temperature

Conductivity has a great dependence on temperature. The electrical conductivity of metals decreases with increasing temperature. The conductivity of semiconductors increases with temperature. Over a range of temperature values, conductivity can be approximated as proportional to temperature. In order to compare the conductivity of substances at different temperatures, a common reference temperature must be set. The dependence of conductivity on temperature can often be expressed as the slope of a plot of conductivity versus temperature.

### 2. Doping degree

The degree of doping of solid semiconductors can cause large changes in conductivity. Increasing the doping level results in an increase in conductivity. The conductivity of an aqueous solution depends on the concentration of solute salts or other chemical impurities that break down into electrolytes. The conductivity of a water sample is an important indicator for measuring the salt content, ion content, impurity content, etc. of water. The purer the water, the lower the conductivity (higher resistivity). The electrical conductivity of water is often reported in terms of conductance; conductance is the conductivity of water at a temperature of 25°C.

### 3. Anisotropy

Some substances will have anisotropic conductivity, which must be represented by a 3 X 3 matrix (in mathematical terms, a second-order tensor, usually symmetric).

## What is the principle and method of conductivity measurement?

The measurement of conductivity is usually that of a solution. The resistivity of a solid conductor can be measured by Ohm's law and the law of resistance. The measurement of the conductivity of the electrolyte solution generally uses an AC signal to act on the two electrode plates of the conductivity cell, and the conductivity σ is obtained from the measured conductivity cell constant K and the conductance G between the two electrode plates.

The earliest used in conductivity measurement is the AC bridge method, which directly measures the conductivity value. The most commonly used instruments are set with a constant regulator, a temperature coefficient regulator, and an automatic temperature compensator. The primary instrument part is composed of a conductivity cell and a temperature sensor, which can directly measure the conductivity of the electrolyte solution.

Measuring Principle

The principle of conductivity measurement is to put two polar plates (or cylindrical electrodes) parallel to each other and with a fixed distance L into the solution to be measured, and apply a certain potential to both ends of the polar plates (in order to avoid electrolysis of the solution, Usually a sine wave voltage with a frequency of 1~3 kHz). Then the conductance between the plates was measured by a conductivity meter.

The measurement of conductivity requires two pieces of information. One is the conductance G of the solution, and the other is the cell constant Q of the solution. Conductance can be obtained by measuring current and voltage.

According to the relationship K=Q×G, the value of conductivity can be obtained. This measurement principle is widely used in direct display measuring instruments.

And Q = L/A

A - the effective plate area of the measuring electrode

L - the distance between the two plates

This value is called the cell constant. In the case of a uniform electric field between the electrodes, the cell constant can be calculated from the geometrical dimensions. When two square plates with an area of 1 cm are separated by 1 cm to form an electrode, the constant Q of this electrode is 1 cm. If the conductance value G=1000 μS is measured with this pair of electrodes, then the conductivity of the measured solution is K=1000 μS/cm.

In general, the electrodes often form part of the non-uniform electric field. In this case, the cell constant must be determined with a standard solution. The standard solution generally uses a KCl solution. This is 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°C is 12.88 mS/cm.

The so-called non-uniform electric field (also known as stray field, or leakage field) has no constant but is related to the type and concentration of ions. Therefore, a pure stray-field electrode is the most complicated electrode, which cannot meet the needs of a wide measurement range with a single calibration.

## What is the conductivity benchmark?

A high-purity potassium chloride with a purity better than 99.99% is used as the conductivity reference material in line with international recommendations, and the reference solution prepared by it should have an internationally recommended conductivity value. Taking the conductivity of the solution at 25°C as the starting point, measure each conductivity constant accordingly, and then calculate the conductivity constant K at other temperatures according to the formula
K=K0 (1-at)

In the formula, K0 is the conductivity cell constant at 0°C; a is the linear expansion coefficient of the glass used to make the conductivity cell; t is the solution temperature, in °C.

The above formula is the result of approximate derivation, and the difference between it and the complex situation will not exceed plus or minus 1xl0 at most. Then, according to the measured resistance values of each solution on the corresponding conductivity cell at different temperatures, the conductivity of each solution at different temperatures is calculated accordingly. Because the temperature coefficient of the relative change of the conductivity cell constant is -8.49x10°C, while the temperature coefficient of the conductivity of the KCl solution is about +2x10°C-.

Therefore, if the measured conductivities of 1D, 0.1D, and 0.01D solutions at 18°C and 20°C are consistent with the international recommended values, it can be considered that such a relative measurement method is reliable, which will be used in future international sample comparisons. Has been verified. Among them, the international recommended value of 20°C is the recommended value of IUPAC in 1972 and 1976.

## Conductivity Products     