Randy's article on ORP

[gaboury]

Randy

I am a professor of environmental chemistry. I happened on your article on understanding ORP in aquaria. Good explanation for the non-specialist! But I am puzzled by one thing. All of your numbers seem to indicate that you are using pe0 values that are 3.3 units (or 193 mV) lower than standard ones. Does this have to do with the nature of ORP electrodes? or is there another reason?

[Randy Holmes-Farley]

Welcome to Reef Central and the Reef Chemistry Forum. :)


The standard for aquarium ORP seems to be to use Ag/AgCl reference electrodes, so it is off from the standard hydrogen electrode.

From the article:

"One difference between the Ag/AgCl electrode and the standard hydrogen electrode is that they do not have the same potential voltage. If they did, and the potential difference between them was measured, there would be no voltage difference. However, it turns out that there is a voltage difference of about 199 mv at 25°C. Consequently, if one wants to interpret ORP in terms of the EH, one has to add 199 mv to the ORP reading to get EH. "

Does that make sense, or do you think it is still off?


Good explanation for the non-specialist!

Thanks. :)

[gaboury]

Perfect. I guess I get a D for not reading the original article more carefully!

[Randy Holmes-Farley]

No problem. It took me a long time to figure that out. I made an ORP standard myself years ago out of Fe+++/Fe++, and it was way off for this reason. No one in the aquarium hobby ever mentions this offset, they just talk about ORP as if it were a constant thing. Most ORP probes sold to aquarists don't even tell you what the internal reference electrode is, or that there even is one!