Reasons That ORP Measurement May Be Inaccurate
There are a variety of reasons that ORP values taken in aquaria may be inaccurate. The primary one is that the platinum electrode may become fouled in a variety of ways. For example, it is expected that the platinum will get a coating of materials on it that can include
oxygen,
sulfur compounds, and
organics. All of these will impact absolute ORP values. If an electrode is moved from a fluid containing one set of these compounds that bind to the electrode surface to a different fluid with a different set, the ORP may take a long time, even hours, to stabilize.[SIZE=-1]2[/SIZE]
Additionally, as soon as bacteria grow on it, they can lower ORP be reducing the local O[SIZE=-1]2[/SIZE] concentration. Likewise, if the probe is in a lit area, algae can coat it and release O[SIZE=-1]2[/SIZE] locally onto the platinum. The end result is a greatly increased ORP. Occasional cleaning eliminates some of these concerns, but because of these issues, one should not put too much emphasis on the precise ORP readings in any given aquarium.
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[SIZE=-1]++[/SIZE]/Fe[SIZE=-1]+++[/SIZE] reactions described above.
