Calibrating ORP

Randy Holmes-Farley

Reef Chemist
Premium Member
I discuss ORP calibration solutions here, but not a DIY recipe:

ORP and the Reef Aquarium

from it:

ORP Standards

Many ORP meters do not permit calibration, but some do, and for detailed ORP measurements, including situations where ORP is being controlled (such as when using ozone), it is worthwhile calibrating (or checking the proper operation of) the meter. Usually, the calibration is quite easy given commercial ORP standards. A variety of standards are available, including Zobell solution which can be obtained from Cole Parmer for $21. It is a solid that is reconstituted from deionized water, and has an ORP of 231 ± 10 mv. Another common standard ORP solution involves putting quinhydrone into pH 7 and 4 calibration solutions to make standards of 86 and 263 mv, respectively. A third standard is Light's solution, which uses the Fe++/Fe+++ reactions described above.

The link to lights solution in it is this one:


Old School Reefer
Thanks, I've already seen the article you cite. I was hoping you had a DIY recipe for Light's. I don't want to use Zobell's because I'd rather not have cyanide around, and the quinhydrone solutions have to be made fresh each time. So the Light's sol'n would have been best. I already have the chemicals in my lab, just needed a recipe.


Old School Reefer
Duh, sorry for my ignorance. Looking a bit more closely at the two articles you sent, I can just calculate the equilibrium potential for the Fe+++/Fe++ ratio using the Nernst equation cited in the article. And by adjusting that ratio I can set the mV to my own preference, which in my tanks is 315 mV. Thanks for that reference.