GF Signet 276 Differential Series Operating Instructions Manual page 8

pH System Calibration cont.
How to calculate a pH electrode slope effi ciency
Two pH buff ers are selected to check the pH probe performance, pH 4.01 and pH 7.00. The ratio of the measured potentials (E2-E1) to
the diff erence of pH (7.00 – 4.01) gives the slope of the straight line.
1. Read the mV potential generated by the electrode in two calibration buff ers
2. Determine the slope (the mV potential generated per pH unit)
3. Divide this number by the theoretical slope, 59.16 mV/pH @ 25 °C and multiply by 100.
Example 1
pH electrode generates -12 mV in pH 7.00 buff er
and +162 mV in pH 4.01 buff er.
160 mV - (-12 mV) = 172 mV
172 mV/ 3 = 57.33 mV/pH unit
57.33/59.16 x 100 = 96.9 % slope effi ciency
In Example 2, slope effi ciency of 90.1% is acceptable but the off set of -45 mV is not.
An off set shift can be the result of the reference electrode being contaminated or poisoned.
A slope shift can be the result of the glass being etched or coated with a hard build-up.
As the electrode ages, the slope decreases gradually, generally a slope between 85% and 105% is acceptable. If the off set value
(pH 7.00) is > +/- 45 mV, electrode should be replaced. When a great accuracy is required from the electrode, the off set and the slope
values need to be as close as possible to the theoretical values.
Refer to the Calibration Procedure section of the instrument manual for a 2 point pH calibration. If the sensor is used in a 4 to 20 mA
'blind application' refer to the Calibration section of the 2751 Smart Sensor Electronics for a 2-point EasyCal pH calibration.
ORP System Calibration
ORP electrodes do not incorporate a temperature sensor, so the only system calibration required is the electrochemical adjustment.
ORP electrodes should need less frequent calibration than a typical pH sensor, the redox potential is a characteristic of the interaction
between the platinum measuring electrode and the redox species in solution.
ORP measurement is only an indicator of the process solution reducing or oxidizing potential. Always perform a single point calibration
(standardization). Using the 9900 or 9950 transmitter, this can be performed under STANDARD. Disregard the SLOPE calibration.
If calibrating the ORP electrode under EasyCal (9900 Transmitters, Gen IV or later), one point calibration will be your only option.
A new ORP electrode measures the listed value +/- 20 mV. A two-point calibration will not increase the accuracy of the measurement
given the +/- 20 mV repeatability of the sensor itself. On top of that, the ORP is not temperature compensated, so if the measurement
is not done at 25 °C, any benefi t gained will be lost. A true two-point calibration is possible in very few applications where only one
redox couple is present in solution and its chemistry is known.
Calibration should be done using ORP premade standard solutions such as Zobell's solution and Light's solution, or solutions made
using the standard pH 4.01 buff er and pH 7.00 buff er with quinhydrone powder mixed in until saturation is reached (Table 3). If all the
quinhydrone does dissolve, continue to add small amounts and stir until a small amount of quinhydrone remains un-dissolved after
mixing. Quinhydrone is the oxidizer that is measured by the ORP electrode. Please note that Zobell's solution is not compatible with
the AutoCal function in Signet ORP instrumentation.
Pre-packaged or mixed Quinhydrone solutions are strong and may have a long lasting eff ect on the electrode, therefore after
calibration it's recommended to properly condition the electrode before returning to the process. It is also possible to calibrate the ORP
electronics based upon a known grab sample confi rmed with a lab meter.
Pick a solution with a mV value closest to the process value and if possible adjust the temperature of the solution to the process
temperature to minimize the off set.
The ORP electrode is functional until the off set exceeds 50 mV.
When the off set measures greater than 50 mV, the electrode should
replaced.
8 Signet 275X, 276X, 2777X Series pH and ORP Electrodes
Table 3:
ORP test solutions
*Saturate 50 mL of pH 4 or
pH 7 buff er with 1/8 g quinhydrone
Example 2
pH electrode generates -45 mV in pH 7.00 buff er
and +115 mV in pH 4.01 buff er.
115 mV - (-45 mV) = 160 mV
160 mV/3 = 53.33 mV/pH unit
53.33/59.16 x 100 = 90.1%
Zobell's Light's
solution solution
ORP at 20 °C
ORP at 25 °C 228 mV 469 mV
ORP at 30 °C
4 pH buff er 7 pH buff er
w/quinhydrone* w/quinhydrone*
268 mV 92 mV
263 mV 86 mV
258 mV 79 mV