By Mike Ross and Herb Feitler

While conventional pH electrodes are fine for many routine pH measurements, they experience problem when making pH measurements over wide temperature ranges. Long times are required for thermal equilibration and during these times pH readings appear to drift. Uncertainty about how long to wait for stable readings leads to inaccurate results. These problems are overcome by newer electrode designs such as the pHASE pH electrode described in this paper.

Conventional pH electrodes may require3, 5 or even as long as 10 minutes to provide stable readings when, for example, electrodes that have been at ambient temperature are placed in a 60(C sample. The causes of this long response time can be found in the design of both the pH and reference half-cells.

Conventional pH electrodes must have an unchanged potential regardless of the concentration changes of the sample to which it is exposed. This constant potential is achieved by placing the Ag/AgCl reference element in a fixed chloride ion solution or gel. Positioning the reference element this way protects it from the samples being measured. However, when exposed to temperature changes, heat reaches the reference element after being transferred through the electrode body and its solution or gel, necessarily a slow process.

On the other hand, the pH half-cell has a much faster response to thermal changes. Heat only needs to transfer through the very thin glass pH membrane rather than through the thick electrode body. So after a short time the pH half-cell's potential represents the value at the new temperature while the reference half-cell potential is representative of some intermediate value. The result of this asymmetrical temperature is erroneous, unstable readings.

Another source of error due to temperature changes are the buffers that are used inside conventional design pH half-cells. The pH of buffers changes versus temperature. So even when thermal equilibrium is reached there is still an error that is not overcome.

Many people are under the false impression that temperature compensators will correct for thermal effects. This is not true. Temperature compensators only correct the "Nernstian" temperature effects.

Applications in Quality Control, Research and Development, and other laboratory, plant and field applications which require pH measurement in samples of varying temperatures can benefit from the use of rapid temperature equilibration electrodes like pHASE.

pHASE pH electrodes are designed to overcome the effects of temperature changes that occur with conventional electrodes (other than the "Nernstian" effect; i.e.. temperature compensation is still needed). The pH value of the special solution inside the pHASE half-cell is virtually unchanged versus temperature so pHASE electrode provide more accurate readings. Also, the locations of the pH and reference Ag/AgCl elements have been arranged so that they respond equally to temperature changes. It indicates how rapidly and accurately a pHASE style electrode performs over wide changes of temperature.

Among the many applications that can benefit from using pHASE pH electrodes is verification of process pH system values. One way of verifying the process system's accuracy is to withdraw a sample from the system and check it with a portable laboratory pH meter. Because these systems are often at elevated temperatures, the errors notes above occur when conventional pH electrodes are used.

In fact, by the time a conventional electrode reaches thermal equilibration the sample may have cooled. Since pH electrodes measure the hydrogen ion activity, a change in temperature causes a change in apparent pH concentration (the meter reading). This temperature effect may cause operators to incorrectly think that the process systems reading is in error. Use of a pHASE style pH electrode will allow rapid thermal equilibration and assist users in eliminating thermal effect errors.

There are many other pH measurement applications that can benefit from the use if electrodes that have enhanced speed, accuracy and stability features. Any pH measurement system, regardless of cost, is only as good as the pH electrode providing the measurement information. The improved data resulting from the use of pHASE type electrodes can reduce maintenance, conserve on chemical additions and improve product quality.