I have some problem measuring ORP-value in reverse osmosis permeate. The ORP value in the this RO permeate in all case was determined in the range 200-300 mV which is quite high. The calibration was OK. I've also used the same probe and instrument for surface water and boiler condensate, the values were 190 mV and 100 mV, respectively, which are correct values. The conductivity of the RO permeate is 10-14 uS/cm and 200-300 uS/cm for the surface water and 8-12 uS/cm for the boiler conensate. I do not understand why is this high values of the RO permeate because even sodium-metabisulfit is added to remove all the chorine constituent from the RO feed and no actiive chlorine was determined in the RO feed. It is related the low dissolved solid content? What is the reliability of the probe and the instrument in such a low dissolved solid concentation. Or this is because of the dissolved oxigen? Please give me some suggestions.
HI Gabor-
What kind of probe are you using? (Make/Model#)
Is there an internal electrolyte solution that needs to be refilled or changed?
Did you accidentally store the probe in a place that it could dry out or in the wrong type of electrolyte solution?
I doubt that this is due to high DO concentrations because you have added sodium metabisulfite, but you could test this by boiling a sample before taking a probe reading. You said the calibration is okay, so that is a good sign, but I still suspect that this is a probe issue, given the conductivity values you report... do you have the instrument's documentation info?
Good Luck,
Caitlin
This type of measurement is inherently a bit dicey. It is only used in water chemistry in the way that it is because it cheaply meets a need (provides some measure of oxidizing potential) that is more or less effective in practice for a large fraction of common feed chemistries. It's almost a kind of proxy measurement, as the underlying theory only applies if a number of assumptions are met. Is the pH unusual or is there anything unusual about the feed chemistry, such as high dissolved metals? Was anything done recently to clean up the membranes and associated equipment? Other than that, I would recommend following Caitlin's suggestions. You might also try another probe, even if this one seemed to work well previously.
Dear Caitlin, Dear Thomas, Thank you for your answers! I boil the sample and then try to determine again. The value of ORP decreased but not siginificantly. I've contacted to the supplier. The probe which was used is a common one, thus, it is not suitable to use in samples having low conductivity . Theoretically, adding KCl could help for the probe to have more ion strenght, however, to find a proper correlation is a bit challanging and not really reilable.
Thank you again!
Gabor
You can check about ORP reading of RO permeate and the impact of pH in the following link: http://www.ethanolproducer.com/articles/5833/common-mistakes-in-design-use-of-reverse-osmosis-systems.
- Badges
- Science topic
- Similar topics
- Phytochemistry
- Extracts