How to identify what ORP electrodes are expected to read on standards?


Dokument-ID TE8352


Veröffentlichungsdatum 12.07.2021
How to identify what ORP electrodes are expected to read on standards?
Expected results for Zobell's and Light's solutions with ORP electrodes
There are two factors that impact what an ORP electrode is expected to read in a standard assuming the probe is reading theoretical values and all other variables are constant. These two factors are temperature and type of reference system of the electrode.

For temperature, the standards have the expected values assigned at 25 °C. Variations in temperature will impact what mV result the probe reads in the standard. In order for the probe to read the expected value the standard must be at 25 °C.

The reference system that the electrode uses will also impact the expected result. Because of this, the potential versus a standard hydrogen electrode (SHE) would need to be known in order to know what that probe would be expected to read in a standard. What is potential versus Standard Hydrogen Electrode (SHE)? This means that two probes with different reference systems would be expected to read different mV values in the same standard at 25 °C.

According to Standard Methods for the Examination of Water and Wastewater 2580 B, Light's solution is +675 mV on SHE and Zobell's solution is +428 mV on SHE.

The IntelliCAL™ MTC ORP electrodes have a potential vs. SHE of +207 mV given in the Specifications section in the User Manuals (same for MTC101xx and MTC301xx manuals). With this information we know that the MTC ORP electrodes are expected to read 221 mV at 25 °C in Zobell's solution and 468 mV at 25 °C in Light's solution.

The certificate of analysis (COA) for these standards show different values in the specifications. This is because the quality testing performed on these standards is done using a probe that has a different reference system (Ag/AgCl - 4 M KCl) relative to the reference system that the MTC electrodes use (Ag/AgCl - 3 M KCl), and therefore have a different potential vs. SHE and a different expected result on the same standard.

The expected results as listed on the COA for these standards as measured with that reference system are 228 mV at 25 °C in Zobell's solution and 475 mV at 25 °C in Light's solution. These values match the table listed in standard methods and also the specifications for these standards on their COAs.

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