Total dissolved solids (TDS): The conductivity of water, the inverse of the resistance of water, is often used to indicate the purity of water. Conductivity is the ability of an object to conduct current. The measuring principle of the conductivity measuring instrument is to place two parallel plates into the solution to be measured, add a certain potential (usually a sine wave voltage) at both ends of the plate, and then measure the flow between the plates. Current.

According to Ohm's law, the inverse of conductivity (G)-resistance (R) is determined by voltage and current. The basic unit of conductivity is Siemens (S), formerly known as Mohm, which takes the inverse ohms of the resistance units. Because the geometry of the conductivity cell affects the conductivity value, the standard measurement is expressed in terms of unit conductivity S/cm to compensate for the difference in electrode size.

The 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 two plates, and A is the area of ​​the plate . =ρl=l/σ

(1) Define or explain the inverse of the resistivity as conductivity. σ=1/ρ

(2) Unit: In the SI system, the unit of conductivity is Siemens/meter.

(3) The physical meaning of conductivity is the performance of the material's conductivity. The higher the conductivity, the stronger the conductivity, and vice versa.

Conductive electrode types and uses:

Conductivity electrodes are generally classified into two-electrode type and multi-electrode type. The two-electrode conductivity electrode is currently the most used type of conductivity electrode in the country. The structure of the experimental two-electrode conductivity electrode is to sinter two platinum sheets on two parallel glass sheets or on the inner wall of a round glass tube to adjust the platinum sheet. Area and distance can be made into conductivity electrodes with different constant values. There are usually K = 1, K = 5, K = 10 and other types.

The two-electrode conductivity electrode used on the on-line conductivity meter is often made into a cylindrically symmetrical electrode. When K=1, graphite is often used. When K=0.1, 0.01, the material may be stainless steel or titanium alloy. The multi-electrode type conductivity electrode generally has several ring-shaped electrodes on a support body, and different constant conductivity electrodes can be made by different combinations of series electrodes and parallel electrodes of the ring electrodes. The material of the ring electrode may be graphite, stainless steel, titanium alloy, and platinum.

The conductivity electrode also has a four-electrode type and an electromagnetic type. The advantage of the four-electrode conductivity electrode is that it can avoid the measurement error caused by the polarization of the electrode. It is widely used in foreign experimental and on-line conductivity meters. Electromagnetic conductivity electrode is characterized by being suitable for measuring high-conductivity solution, generally used in industrial conductivity meter, or using its measurement principle to make single-component concentration meter, such as hydrochloric acid concentration meter and nitric acid concentration meter.

Measure conductivity cell constants and calibrate constants:

According to the formula K=S/G, the electrode constant K can be obtained by measuring the conductance G of the conducting electrode in a certain concentration of KCL solution, at which time the conductivity S of the KCL solution is known. Since the concentration and temperature of the measuring solution are different, and the accuracy and frequency of the measuring instrument are also different, the conductivity constant K sometimes has a large error. After a certain period of time, the electrode constant may also change. Therefore, the newly purchased conductivity Electrodes, as well as conductivity electrodes after a period of use, should be re-calibrated.

When measuring conductivity constants, note the following points:

1. Measurements should be made using a conductivity meter that is used in conjunction with, and do not use other types of conductivity meters.

2. The temperature of the KCL solution for measuring the electrode constant is preferably close to the temperature of the actually measured solution.

3. The concentration of KCL solution for measuring the electrode constant is preferably close to the concentration of the actually measured solution.