No. 19

References:

1. "Standard Practice for Oxidation-Reduction Potential of Water", ASTM D 1498 - 00.
2. "Expressions of Performance of Electrochemical Analyzers", International Standard IEC 746-5.

These Redox values should only be used when the sensor (Ag/AgCI 3M KCl) is calibrating with a Redox Standard prepared with Quinhydrone.

When calibrating with 4.01 buffer mixture, the reading should be between +170 and +185 mV above the reading in the 7.00 buffer mixture, eg. 295.2 mY + l77mV = 472.2 mV @ 25⁰C. If a short span is found-less than a +170 mV change between the 7.00 and 4.01 buffers - the platinum surface may be coated. To remove the coating refer to clause 11.0 “General Care and Maintenance”, pH/ORP Sensor Maintenance.

Electrochemical measurements are ultimately referred to the so-called hydrogen scale (or Normal Hydrogen Electrode NHE), the convention for which is that the electrochemical potential of a hydrogen electrode in contact with hydrogen gas at one atmosphere partial pressure and a solution containing hydrogen ions at unit activity is zero at all temperatures. The T-611 reference electrode used is the 3M KCl Silver Chloride type, and this exhibits potential on the hydrogen scale of:

To refer a reduction-oxidation potential value measured with a 3M KCL Silver Chloride electrode to the hydrogen scale, the appropriate value above should be added to the measured value:

All potentials quoted are referred to the standard (“normal”) hydrogen electrode (NHE) and are expressed in mV (reference: International Standard IEC 60746-5, Oxidation-Reduction Potential or Redox Potential, Annex B).

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