PH MONITOR
ORP Electrode Calibration
8-162 Aquastorm™ Series Options Guide Rev. 1
8
ORP Electrode Calibration
Introduction To define proper electrode operation and maintain linearity during the electrode
life-span review the offset, and slope checks.
Offset (STD) Offsets occur when a reference junction is clogged or reference solution/wire is
aged or contaminated.
Check Offsets in a pH 7 buffer saturated with Quinhydrone @ 25 °C (77 °F).
Theoretical output is +87 mV. Quinhydrone is the oxidizer measured by the ORP
electrode and required for calibration. To guarantee buffer saturation, mix 1/8 g
Quinhydrone per 50 mL of pH buffer.
Any deviation is the ORP electrode offset (i.e. +008 mV). Electrode offsets
greater than ±50 mV indicate the electrode requires cleaning or replacement.
Slope (SLP) Contamination of the platinum electrode surface is the common cause of ORP
slope errors. Cleaning usually restore proper values.
To minimize calibration reference solutions when both pH and ORP calibration is
required, use pH 7 and 4 buffers for pH calibration first. Then add Quinhydrone to
the same buffers to complete ORP calibration.
Common ORP Values
Reaction E0 (V)
Cr → Cr
2+
+ 2é –0.913
Fe → Fe
2+
+ 2é –0.440
Cr2 → Cr
3+
+ é –0.407
4OH → O
2
+ 2H
2
O + 4é –0.401
2l → I
2
+ 2é –0.400
Ti
2
→ Ti
3+
+ é –0.37
Ni → NI
2+
+ 2é –0.250
Pb → Pb
2+
+2é –0.126
Fe → Fe
3+
+3é –0.037
H
2
→ 2H
+
+ 2é 0.000
Fe
2
→ Fe
3+
+ é +0.771
Ag → 2H
+
+ 2é +0.799
Pb → PB
4+
+ 4é +0.80
3Br → Br
3
+ é +1.06
2Br → Br
2
+ 2é +1.066
ClO → ClO
2
+ é +1.16
Pt → Pt
2
+ 2é +1.188
Ag → Ag
2
+ 2é +1.369
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