How to judge the quality and life span of pH meter electrode
Generally, the detection electrode of the pH meter mainly judges the quality of the electrode by the slope of the electrode, and can also be judged by the zero potential mV value of the electrode. For electrodes in industrial occasions, the factory standard is a slope of 95% to 105%. Zero potential: ±15mV, zero potential is a range, within this range are very good electrodes, not 8.2mV given by some meter manufacturers, limit the zero potential of electrodes to around 8.2mV. After using for a period of time, the zero potential will change, and it is allowed within ±60mV. But the slope should not be lower than 70%, if required, it should not be lower than 80%.
The pH measurement adopts the potential analysis method, the so-called potential analysis method, that is, the electrode potential of the electrochemical cell is measured by the zero current method. The main feature of the pH electrode is high internal resistance, generally tens of megohms to hundreds of megohms. Therefore it requires the pH meter to have very high input impedance. The input impedance of a general pH meter should be at least 1000 times the internal resistance of the electrode. Therefore, the input impedance of the pH meter should be >5×1011Ω to 1×1012Ω, but because the internal resistance of the glass is not a constant, it is an exponential function of temperature. For example, an electrode has an internal resistance of 100 megohms at 28°C, and 800 megohms at 7°C, and 1600 megohms at 0°C.
Therefore, if the input impedance of the pH meter is not high enough, the displayed value will drift and become unstable. At the same time, a current will be generated in the measurement circuit, which will polarize the electrode and destroy the reference electrode. Therefore, when designing the pH meter, the input impedance must reach 1012Ω or more, but this is not enough. In practical applications, the input impedance of some instruments The sealing of the part is not good, or it is placed in a place prone to moisture. After a period of time, the input impedance will not meet the requirement of 1012Ω or more, and this problem is recessive. When the displayed value is often inaccurate, First of all, it was suspected that the electrode was the problem. This is really wronged to the electrode!
Regarding the life of the electrode of the ph meter, the national standard is "the guarantee period of the electrode, starting from the date of manufacture marked on the electrode, when it is unpacked and used within one year of validity, the manufacturer shall be responsible for repairing or returning it." Therefore, the quality assurance of the electrode The term is based on the premise that it has not been used, and the term is one year. Generally speaking, the life of the electrode is one year, which is not correct. The service life of the electrode has a lot to do with the medium used, and the service life of different media is completely different. In many severe cases, it may only be used for 2 months. And some better media can be used for about a year or even longer.
