FAQs

In the early 1900’s, Danish biochemist Soren Paul Lauritz Sorensen defined the term pH as the negative logarithm of the hydrogen ion concentration. To this he introduced the term “pH” or pondus Hydrogenii.

The output of a pH electrode varies per the ratio of temperature. See chart below. Most pH analyzers have automatic temperature compensation to correct the output to the theoretical response of 59.16 mV/pH unit at 25ºC.
Theoretical values of mV/pH unit for different temperatures
E = E0 – k · T · pH
0°C = -54.2 mV/ per pH unit
20°C = -58.2 mV/ per pH unit
25°C = -59.2 mV/ per pH unit
50°C = -64.1 mV/ per pH unit
75°C = -69.1 mV/ per pH unit

The general rule of thumb applicable to all pH glass electrodes is that for every 25ºC step change the life of the electrode will be cut in half. For example, if the life of an electrode is benchmarked at one year at 25ºC, life expectancy will be 6 months at 50ºC, 3 months at 75ºC and 1.5 months at 100ºC continuous.

Always store your electrode in a wetting solution. For mono measuring glass electrodes a pH 4.0 buffer solution is ideal. But because most electrodes are combination types you need a salt solution for the reference half-cell. An optical storage solution for combination electrodes is a pH 4.0 buffer (clear, not pink) with 225 grams of KCl per liter. Table salt, NaCl, can be used if KCl is not readily available. Never store your electrode in distilled or deionized water. DI water is for rinsing only during buffer calibration.

Electrodes with fixed cable are considered water resistant and will usually survive months of submersion service. However, this is not a recommended method of installation unless installed with an extension pipe with waterproof coupling. Electrodes with TOP68 connection are rated IP68.

A solution ground is simply a conductive contact to the process liquid used to drain any voltage present to earth ground. When incorporated into a pH electrode this contact can also be used by some pH meters as a measuring point for electrode diagnostics or as a reference point for a differential amplifier circuit. When used with our OPM2x3 transmitters the solution ground is used for potential matching to utilize the Sensor-Check-System (SCS) to alarm for deviations of the glass or reference impedance from normal ranging signaling coating, blockage or breakage.

The instrument manufacturer decides on how to make the compensation circuit and therefore which type of RTD or thermistor is needed. With process instrumentation the most commonly used RTD is PT100, followed by PT1000 and 3K ohm thermistor. Our OPM2x3 transmitter uses PT100 for glass electrodes and PT1000 for ISFET electrodes.

The most common cause for this problem is an electrical ground loop in your system. To verify this problem, remove the electrode and calibrate it with a known buffer solution in a beaker. If the electrode measures within specification, place a copper wire in the beaker and the other end in your process fluid. If the reading become unstable or shifts, a ground loop is your problem. The source of the ground loop could be any motor, pump, conductivity probe, or other electrically powered device in the media with the pH electrode. Do not attach the conductivity probe or any other electrically powered device to the same ground on your meter or controller as the pH electrode. Our ProbeSaver differential amplifier module will minimize most effects from ground loops.

The most common possibilities to explain these observations are as follows:

1. The electrode sensing surfaces may be coated.
2. The electrode may be reaching the end its useful life.
3. The media may be low ionic strength (conductivity below 100 microsiemens).

It is best to use a cleaning chemical that will dissolve the coating on the pH glass and junction but not harm the materials of construction.

1. Prepare the following soaking solutions: 5% HCl, 4% NaOH, and 4 buffer saturated with KCl.
2. Pour each solution into a separate beaker.
3. Soak sensor in 5% HCl for 5 minutes. Begin by “swirling” the sensor to create agitation. Repeat this step in the 4% NaOH solution.
4. Rinse in DI water for 2 minutes. Visually inspect. If the glass membrane or Teflon junction is not clean, repeat step 3. Rinse in DI water for 2 minutes.
5. Place sensor in the beaker of 4 buffer saturated with KCL, and let soak for 48 – 72 hours and retest.

If the electrode is allowed to dry out by sitting on a bench or left installed into a dry flow cell there are two concerns. 1) The junction becomes very high impedance and you will see drift or unstable readings. 2) The outer gel layer on the glass membrane (not visible) needs to be reformed by soaking in water or buffer. In most cases performance can be restored by soaking in KCl at ambient for 24 hours or at 60C for 6 hours. Calibration will be required to determine functionality.

You must consider all the possible sources of errors to define accuracy. You can accumulate errors from the following:

a. Uncertainty in potential measurement
b. Uncertainty in calibration buffers
c. Nonlinear electrode response
d. Inaccuracy in temperature compensation element
e. Non-parallel behavior of the pH and reference half-cells
f. Electrode memory effects
g. Junction potentials

These are just examples and the degree of the error can be much higher. Still based on the above factors your lowest realistic uncertainty is 0.05 to 0.10 pH units.