Questions about ORP meters
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Our company has a denitration system that uses saline with a pH of 8 and a concentration of 200 g/l as the main medium. This saline contains some free chlorine; after the free chlorine is removed using sulfite, acid is added to adjust the pH to around 6, after which the mixture is fed into the denitration equipment. The key component of this system is the reverse osmosis membrane, and it is essential that there be no free chlorine present, as otherwise the membrane components will be damaged. The ORP setting for the interlock value is 150 MV; there are three ORP meters before the membrane, which monitor at different points. At the time of the incident, manual testing showed a free chlorine level of 30 ppm. However, all three devices show a value of less than 100 MV. Is an ORP meter suitable for detecting free chlorine in my application? Also, the actual value of free chlorine is much higher than the limit; what is the reason why the ORP meter shows a normal value? (The instrument is calibrated with standard solutions weekly.)
How is calibration done? Should both the zero point and the range be marked?
The last edit to this post was made by qugd on 2017-10-11 at 10:08. The ORP value is related not only to the concentration of oxidizing agents but also to pH; sometimes the influence of pH is even greater than that of the oxidizing agent concentration. It is recommended to conduct some basic experiments to determine an appropriate range for the relationship between pH and ORP values for reference. When the pH value changes from 6 to 8, the hydrogen ion concentration varies by a factor of 100; it is only meaningful to discuss the ORP value when the pH is kept constant. The oxidizing power of hypochlorite is related to pH, as hydrogen ions are involved in redox reactions.
If controlling the free chlorine level is a concern, it is recommended to choose a residual chlorine meter with pH compensation. ORP is correlated with residual chlorine, but it cannot reflect this parameter in its entirety.
Recently, I conducted an experiment in which sodium hypochlorite was added to saltwater (raising the pH), and the readings taken with an ORP meter turned out to be lower than those before adding it； So does this mean that the values displayed by an ORP meter under alkaline conditions cannot accurately represent the free chlorine content in saltwater (30PPM as measured on-site)? Or is it that the ORP value is only inversely related to pH? Then what is the purpose of an ORP meter? I work in the field of processing; I have little experience with instruments. I hope those with more experience can help me figure out whether there is something wrong with our instruments, or if this phenomenon indeed exists