[ozonoyo]

I´m trying to measure ORP using electrodes.
I have 3 electrodes (Hanna, Cole-Parmer and a Japanesse one).
The electrodes were storaged for a long time. When I try to use them again, I have problems to stabilize the readings.
Each electrode give me different values and then, the values change so much when I left the electrode in the solution.
At the first time, I try with SnCl2 solution and the inicial reading was about -40mV. After some hours, the reading was 7,6mV. I left the electrode inmersed in the solution untill the next day and the new reading was 50mV.
Then, I try with dilluted NaClO solution. The inicial reading was 582mV. After 20 minutes, 743mV and after 4 hours, 824mV.
I try also with standard solution of 270mV and the stabilization was better. The reading was right.
I make several similar test with all the electrodes.
I attach a file with the data I measure. The vertical axis is mV measured, the horizontal is time (not real scale) and each color represents an different electrode.
Any suggestion or idea will be welcome.

Thank you

Sergio

[Borek]

Not sure I get you right - what kind of electrodes are these? Platinum foil?

What reference electrodes do you use?

[ozonoyo]

Each electrode has an internal reference of AgCl/Ag.
I´m not sure about the foil, but I guest, it is made of Pt.

[Borek]

I would suspect reference first. Platinum is platinum, it won't change much even if left for a long time. Reference electrodes are much more delicate.

Check if you don't have to refill internal solution.

Check if junction is not clogged.

[ozonoyo]

One of the electrodes (Pink triangles in the graphics), has new filling solution (I refill it the last week). The Cole Parmer electrode, hasn´t any hole to refill, the supplier told me, that some models aren´t refillable. I haven´t refilling solution for Hanna electrode.
Some days ago, I bring Hanna electrode to the supplier. He test it with two standards (250mV and 470mV) and the readings were corrects. I ask him to test also the Japanesse electrode in 470mV and the results was OK too.
Some people suggest me to deep the electrodes in 5% HCl for 5 or 10 minutes to clean it. I did it (just with Hanna and Cole Parmer because the japannese has plastic case) , but there wasn´t any improvement. Do you know another way to clean the junction?
I don´t understand why the electrodes change so much the readings when I deep then for some hours into the solution, and why the readings are so much differents between the electrodes. As shows the graphics, the readings between different elecrtodes dont seems to follow any pattern.
There is some parameter (like pH, some compound, conductivity or other) that turn unstable the ORP readings?

Thank you

[ozonoyo]

I´m sorry for the position of the confused smilie. I want to put it next to the question, but it was published after the thank you.

[Borek]

As for junction cleaning take a look here:

http://www.ph-meter.info/pH-electrode-cleaning

That's about pH electrodes, but junctions are similar.

Non-refilliable electrodes have in general shorter life-time than refilliable ones, that may explain problems with Cole-Parmer one (but it may not, I am just guessing).

What are time scales on the plots? Perhaps at least part of the changes is connected with dissolving oxygen/carbon dioxide?

If the electrodes were tested and found OK on some other potentiometer, perhaps there is something wrong with the device you are using?