TURO TECHNOLOGY PTY LTD multiparameter water quality analysers and XBT systems

Technical Note
REDOX Potentials

T-611/T-612/T-613 ANALYSIS 
REDOX POTENTIAL
  
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.
 
pH  Buffer    Redox (mV) @20°C  Redox (mV) @25°C  Redox (mV) @30°C
Buffer 4.01   481               472.2             463.6
Buffer 7.00   305               295.2             284.6
 
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:

Ag, AgCl, 3MKCl
@ 20°C	@ 25°C	@ 30°C
213 mV	209.2 mV	205.6 mV

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:
pH  Buffer	Redox (mV) @20°C	Redox (mV) @25°C	Redox (mV) @30°C
Buffer 4.01	481	(263+209.2)		472.2				463.6
Buffer 7.00	305	(86+209.2)		295.2				284.6
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).