GF Signet 2764-2767 Differential DryLoc pH_ORP Electrodes - Manual (Page 4)

4 Signet 275X, 276X, 2777X Series pH and ORP Electrodes
4. ORP Electrode Calibration
ORP electrodes are designed to ensure linearity during their lifespan. The following sections defi ne proper electrode operation.
4.1 Offset (STD)
• Electrode offsets are usually caused by clogged reference junctions or by an aged or contaminated reference solution/wire.
• Offsets should be checked in pH 7 buffer saturated with quinhydrone @ 25 °C. The theoretical output is +86 mV.
Any deviation from +86 mV is the ORP electrode offset (e.g., +90 mV).
• Quinhydrone is the oxidizer measured by the ORP electrode and is required for calibration. To measure ORP electrode offset,
saturate 50 mL of pH 4 and pH 7 buffers with ⅛ grams of quinhydrone:
4 pH w/Quinhydrone 7 pH w/Quinhydrone
Temp: 20 °C 25 °C 30 °C 20 °C 25 °C 30 °C
ORP: 268 mV 263 mV 258 mV 92 mV 86 mV 79 mV
A new ORP electrode measures these values ± 15 mV. The electrode continues to be functional until the offset from these values
exceeds 50 mV. Electrodes with offset greater than 50 mV should be cleaned and replaced if necessary.
3.3 Response Time/Stability
Response time and stability are affected by the condition of the pH glass measuring surface (for an ORP electrode, the platinum surface),
the reference junction and the reference solution. Electrodes may sometimes be restored to acceptable levels by cleaning the measuring
surface and reference junction.
Electrode mV values should remain stable ± 3 mV. Conditions that may cause fl uctuations are:
• Electrode
coating
• Clogged reference junction
• Ground fault: If proper operation is observed in the beaker, but readings are unstable in the application, a ground fault may exist.
• Using instrumentation with isolated inputs and outputs may restore stable operation.
• Solution grounding may also restore stable operation.
3.2 Slope (SLP)
Electrode slope is the ratio of mV per pH unit. At 25 °C the theoretical slope is 59.16 mV per pH. Temperature has an appreciable affect
on electrode slope. Reliable instrumentation includes temperature compensation. The graph below illustrates potential pH error when a
temperature compensated instrument is not used.
Recommendations:
• Calibrate temperature before calibrating the standard and slope.
• The mV offset will track across the entire pH range.
The slope is usually not affected by offset changes.
(e.g., pH 7 = +10 mV, pH 4 = +187 mV); slope = 59 mV
Slope = (pH 4 mV value - pH 7 mV value) ÷ Span between pH 7 and pH 4
Slope = (187 - 10) ÷ 3 = 59 mV
• Coatings on the glass may affect sensor slopes. See section 5: Maintenance and Cleaning.
• A constant output near 0 mV in all buffer solutions indicates a shorted electrode that must be replaced.
°C pH Error
2 3 4 5 6 7 8 9 10 11 12
15 0.15 0.12 0.09 0.06 0.03 0 0.03 0.06 0.09 0.12 0.15
25 0 0 0 0 0 0 0 0 0 0 0
35 0.15 0.12 0.09 0.06 0.03 0 0.03 0.06 0.09 0.12 0.15
45 0.3 0.24 0.18 0.12 0.06 0 0.06 0.12 0.18 0.24 0.3
55 0.45 0.36 0.27 0.18 0.09 0 0.09 0.18 0.27 0.36 0.45
4.2 Slope (SLP)
ORP slope errors are generally caused by contamination of the platinum electrode surface. Cleaning the electrode surface will usually
restore proper values, response time, and stability. See section 5: Maintenance and Cleaning.
Many systems require both pH and ORP calibration. To conserve calibration reference solutions, use pH 7 and 4 buffers for pH
calibration fi rst. ORP calibration can be performed with the same buffers by adding quinhydrone.