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Aquameter
™
Series Instruction Manual
10401-00188 Rev M
13.8. Converting ORP Readings to the Hydrogen Scale
Electrochemical measurements are ultimately referred to the so-called hydrogen scale, 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 ORP reference electrode used in Aquaread combination electrodes is a 3MPK1 silver
chloride type, and exhibits potentials on the hydrogen scale of:
Temperature
Potential
5
°
C
221 mV
10
°
C
217 mV
15
°
C
214 mV
20
°
C
210 mV
25
°
C
207 mV
30
°
C
203 mV
35
°
C
200 mV
40
°
C
196 mV
Thus, to refer an ORP potential value measured with the Aquaprobe to the hydrogen scale,
the appropriate value above should be added to the measured value.
© 2011 Aquaread Ltd.
www.aquaread.co.uk
Page 31 of 64