@ All rights reserved

Shanghai Joann Water Technology Co., LTD. - Technical Department 2024/3/15

Electrode selection - One (PH)

Due to the different working principles of the two types of electrodes, the scope of application under different working conditions is not the same, so the correct selection of electrodes is particularly important; Now the above two categories of water quality analysis electrode selection analysis, the specific content is as follows:

Electrochemical water quality electrode

As the name suggests, the electrochemical water quality electrode is an electrode that converts the corresponding chemical reaction into an electrical signal by a specific chemically sensitive material for analysis and measurement; Therefore, the tolerance, stability, repeatability and sensitivity of chemically sensitive materials under different working conditions are all factors that need to be considered in the selection of electrochemistry water quality analysis electrodes.

A. Composition of the measured medium:

Whether the measured medium contains components that will corrode, interfere with or affect the chemically sensitive material or shell of the electrode, resulting in the electrode not working properly or being damaged.

B. Temperature of the measured medium:

Whether the temperature of the measured medium exceeds the tolerance temperature of the electrode chemically sensitive material or the tolerance temperature of the electrode housing material, resulting in electrode damage.

C, the pressure of the measured medium:

Whether the pressure of the measured medium exceeds the pressure of the chemically sensitive material of the electrode or the pressure range of the electrode structure, causing the electrode to fail to work normally or be damaged.

D, electrode installation form:

In what form the electrode is installed to the measurement site, there are common pipeline-type installation, reactor pressure vessel installation, sunk installation, flow trough installation, etc.

In the above selection conditions, the composition of the measured medium is the most complex, and it is described in detail.

pH electrode:

a. Pure water

When the electrical conductivity of the water sample is below 20uS/cm, it is necessary to pay attention to the selection of electrodes. It is not suitable to measure the pH of the ordinary water sample, because the electrical conductivity of the water sample is very low, the ionic strength of the water sample is very weak, and the ordinary pH electrode can not work normally. The measurement data will fluctuate in a large range, which can not be stable and the error is great. It is necessary to choose a pH electrode that can measure pure water or ultra-pure water, and use it with a flow tank with a very low flow rate or flow rate, and it is recommended that the flow rate is about 6L/H (6 liters per hour). Such applications often appear in power plant boiler water, microelectronics factory process water, pharmaceutical injection water, food and beverage industry and other industries.

b. Water sample containing fluoride ions

Because fluoride ions in water will combine with hydrogen ions to form hydrofluoric acid (HF) in acidic environment, resulting in corrosion damage to pH glass electrodes, glass electrodes are not recommended in the environment of water sample testing containing fluoride ions. Specifically, when the concentration of fluoride ions is less than 3000ppm, the pH electrode is not recommended. When the pH value of the water sample is higher than 5, it is recommended to use a glass pH electrode that is resistant to hydrofluoric acid, and if the above conditions cannot be met, it is recommended to use a metal antimony pH electrode. Such applications often appear in solar panel processing plants, microchip processing plants, glass or glass processing plants, daily chemical plants containing fluorine components and other occasions.

c. Water sample with high concentration of sulfur ions

In the pH test of water samples with high concentration of sulfur ions, it is necessary to pay attention to the toxic damage of the reference electrode. Due to the chemical composition of silver chloride when the reference electrode is used, when the water sample under test contains high concentration of sulfur ions, it is necessary to consider the risk of sulfur ions penetrating into the reference electrolyte and then contaminating the reference electrode, resulting in precipitation of silver sulfide and failure of the silver chloride reference electrode. It is generally recommended that users choose pH electrodes for light chemical applications with dual liquid connections. Such applications often appear in thermal power plants, large boilers with coal burning, large boiler supporting coal washing plants, flue gas desulfurization equipment manufacturing engineering contractors.

d. High concentration organic solvent water sample

If the measured water sample contains a high concentration of organic solvent, it will cause crystallization of the electrode electrolyte, resulting in damage to the reference potential drift of the electrode, and it is necessary to select a pH electrode with a special electrolyte formula. Such applications are concentrated in pharmaceutical, chemical synthesis or chemical extraction and other applications.

e. Water sample containing heavy metal cyanogen

The heavy metal cyanogen contained in the measured water sample is easy to cause silver chloride poisoning of the reference electrode, thus damaging the reference electrode. It is generally recommended to choose the electrode of the gel electrolyte or the electrode of the double liquid junction to delay the reverse osmosis of the heavy metal cyanide root or the toxic effect on the reference electrode, so as to ensure the use effect and service life of the user. Such applications are concentrated in precious metals refining, electroplating wastewater treatment and other occasions.

f. Samples with high protein or fat content

The tested sample contains high protein or oil samples, it is recommended that users choose large surface liquid junction or special liquid junction materials. When testing the above samples, protein solidification or oil adhesion to the liquid junction will not easily block the liquid junction, resulting in the reference electrode can not play a normal role, resulting in electrode damage. It is also very easy to clean the electrode after use to ensure the subsequent normal use of the electrode.

g. High concentration of alkaline samples

In some chemical synthesis processes will be exposed to the measurement of high-concentration high-alkaline water samples, it should be noted that such water samples are easy to cause damage to the glass electrode bulb, on the one hand, high-alkaline samples are easy to damage the base metal base of the glass bulb, on the other hand, excessive concentration is easy to produce crystallization and adhesion phenomenon, it is generally recommended that users choose the electrode resistant to high-alkaline water samples. On the other hand, it is necessary for the user to clean and maintain the electrode frequently, such as soaking the electrode with 5% dilute hydrochloric acid to remove the crystallization on the electrode surface and ensure the cleanliness of the electrode glass bulb and liquid junction.

The above points are the pH electrode in use due to the particularity of the sample composition caused by the problem, need to pay special attention in the selection, the other is the conventional temperature, pressure, the viscosity of the sample to be tested, the particle size of the sample to be tested, installation form and other precautions, in the electrode selection need to ask the user to avoid unnecessary losses and trouble caused by selection errors.