Cleaning of redox electrode

[panou]

Hello.
I have installed a Intrac mechanism for a combined pH/redox electrode with automated cleaning system. I have checked that after cleaning the pH reading is coming to its normal value after 1 minute but for the redox(ORP) reading there is always an offset of +40 mV. The temperature before and after cleaning is increased by 1oC(from 44 to 45, for example). Is there any reason for this or can i take some action to avoid this offset ?
Thanks in advance for your time.

 

[m777182]

I am sorry that I do not know the Intrac mechanism. I assume that it works with some mechanic action-jet or vortex of fine polystyrene particles or a gentle brush. The pH response is intrinsically quite slow because of very large time constant of the system solution-membrane interface-inner electrode and electrical resistivity times capacitance.
The ORP should be faster because of small impedance. I used to clean both electrodes with a gel type tooth paste that contains SiO2 as an abrasive and the ORP electrode potential came back very quickly. Maybe the 40 mV offset is not the offset but the true value that you have with fresh electrodes and your normal reading has an offset 40mV to inverse direction. The possible offset may be assigned to an ORP potential of a layer of freshly adsorbed solution. Check with some standard potential solution or ORP end point tiration with known concentrations.
I do not quite understand the problem with temperature, you say "The temperature before and after cleaning is increased by 1oC(from 44 to 45, for example)". Is it the rise immediately before the moment that cleannig action starts?
m777182