Calibrating for Reliable ORP Measurement Systems

Like pH, conductivity/TDS, and other electrochemical measurements, ORP (oxidation-reduction potential) measurements are calculated using a specialized piece of equipment and an electrode in the solution. Each ORP electrode has unique characteristics that cause variability in the electrodes' signal to the meter.

Both the offset and slope characteristics of the pH electrode must be compensated by calibration to the meter in order to obtain accurate readings. ORP electrodes and measurements present a different set of calculation considerations.

ORP Electrodes

Offset characteristics of ORP electrodes can present issues in calculating a reliable measurement. Whereas the slope characteristics of ORP electrodes are less variable than pH electrodes since the ORP sensors are made of noble (more or less non-reactive) metals such as platinum or gold and do not change much with use. These sensors ' response times, which are sometimes confused with slope, can vary greatly depending on the surface area, size, construction, and cleanliness.

For most ORP applications, the absolute accuracy is far less important than the speed and relative changes measured in the system. Many procedures and specifications call for target ORP values with tolerances of ±25 mV, ±50 mV, or they specify changes in ORP, such as a 400 mV drop in the value with a target endpoint value.

ORP has a variety of uses with methods that have their own specialized target readings or reading changes that are based on experience. This makes it difficult to elaborate on the offset allowances for specific applications. It is sufficient to say the precision required for pH and other electrochemical measurements typically does not apply for ORP, and so calibration for ORP electrodes and meters is not common.

Calibration Solutions for ORP

There are some high-quality ORP solutions in the laboratory science market. Careful assessment of these reveals they are not calibration solutions, rather, they are described as checking or testing solutions. These types of solutions can assist in the performance evaluation of your electrode and testing meter. These solutions typically have very wide tolerances, ±35 mV or more, for the actual values one can expect when checking their ORP.

Two-Point Check

In these procedures, the desired result is to take two readings that are expected to have a specified differential value with tolerances of +/-10 mV possible. If your ORP readings have a differential close to this specified value, the electrode and meter system are considered to be in working order.

We recommend using household bleach and water as an alternative to a manufactured solution. For a quick check of whether your ORP measurement system is working properly, simply take a reading of your tap water. This will typically be a value well below 200 mV, and then take a reading of the same tap water adding an equal volume of bleach.

In both tests, you will need to allow the electrode to stabilize at a reading which may take up to 30 minutes. When the bleach is added, you should get a much higher reading. Up to 800 mV is possible. Though lower values of 300 to 600 mV are more typical.

If your test of the bleach and water mixture gives a higher reading, the ORP measurement system is working. If the second reading with bleach included remains close to the reading for tap water alone, there is likely a problem with the ORP measurement system. For example, the electrode sensing metal could be dirty, or the reference junction might be clogged. In these cases, the electrode can be cleaned using Orion™ General Purpose Electrode Cleaning Solutions.

Another reference option for ORP is to measure using a different measurement system and a sample of the liquid to be measured. Often an ORP measurement method and target values have some protocol established using another ORP electrode and meter. When the ORP electrode is replaced, electrode offset differences between the old electrode and the new one will cause differences in the readings that are not errors but can add some confusion.

For example, if your method calls for a target value of 310 mV defined with your previous electrode and meter system, the new electrode and meter system may read 325 mV in the same liquid.