Tech Insights
Why should pH compensation be used with caution in residual chlorine monitoring?
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Residual chlorine monitoring is a key link in the evaluation of water quality disinfection effectiveness, and is commonly used in the fields of drinking water, sewage and swimming pool water treatment. The pH compensation function is widely mentioned in electrochemical residual chlorine sensors. Its principle is to obtain the measured value of the pH electrode to compensate for the response deviation of the residual chlorine electrode caused by changes in the pH value of the solution. However, in many scenarios, relying on pH compensation may introduce larger errors or cause residual chlorine over-limit alarms, so it should be used with caution.
Scenarios and reasons for using pH compensation with caution
1. pH electrode drift scenario
pH compensation is very dependent on the measurement accuracy of the pH electrode. However, in the pH range of 7 to 8 (most drinking water scenarios), the hypochlorous acid dissociation curve is very steep (as shown in the figure below). Near pH 7.5, the pH changes by 0.1, and the hypochlorous acid concentration changes by about 20%. Once the pH electrode measurement value drifts by 0.1, it will cause serious residual chlorine measurement errors. If the pH electrode fails, the residual chlorine value after compensation will cause drastic changes, which can easily cause an over-limit alarm.
Therefore, if you need to use the pH compensation function, it is recommended to first choose a pH electrode of reliable quality and frequently verify the accuracy of pH measurement.
2. Scenario where maintenance personnel are unprofessional
When the instrument adopts pH compensation, the operation and maintenance work must be very careful. First of all, the order of instrument sensor maintenance cannot be wrong. The pH electrode must be calibrated first, and then the residual chlorine electrode. If the operation is reversed, or there is a large deviation when calibrating the pH electrode, it will artificially cause a large error in the residual chlorine measurement. In China, the path from manufacturer to middleman to general project contractor to project construction party to user is often relatively long.
In addition, some people do not like to read instructions carefully, which can easily lead to many problems when using the product. It ultimately affects the reputation of enterprises in every link of the industrial chain.
Therefore, if you need to use pH compensation, it is recommended that you or the operation and maintenance personnel downstream of your industrial chain first receive professional operation and maintenance training.
3. Other scenarios
Different residual chlorine electrodes have different sensitivities to pH changes, and the theoretical hypochlorous acid dissociation formula cannot be fully used for pH compensation. In addition, different water bodies have different compositions and require different pH compensation curves, which requires a large amount of on-site operation experience.
For example, when there is a high concentration of ammonia nitrogen (NH₄⁺-N) in the water body, sodium hypochlorite (NaClO) will react with ammonia to generate combined chlorine such as monochloramine and dichloramine, and the pH compensation algorithm may not be able to accurately distinguish the concentrations of free chlorine and combined chlorine. pH compensation may cause the total chlorine concentration to be misjudged as free chlorine, and the actual disinfection effect depends on the concentration of free chlorine, thus affecting the adjustment of the disinfection process.
Suggestions for residual chlorine compensation function
In summary, the pH compensation function of residual chlorine needs to be used with caution. If conditions permit, it is more recommended to use a coated residual chlorine electrode with less impact on pH.
YuanGan Technology's digital membrane-covered residual chlorine sensor supports pH compensation, but due to the above reasons, the digital residual chlorine sensor has the pH compensation function turned off by default. If pH compensation is required, the user can start residual chlorine compensation by setting the relevant registers, and can adjust the compensation parameters at any time.