SENSOREX

The purpose of this article is to provide a better understanding of how to evaluate the performance of both new and used pH electrodes. Also discussed are some of the many factors which can affect pH electrode performance.

Theoretically, when placed in 7.00 buffer at 25 (C a pH electrode produces zero millivolts which the pH meter reads as 7.00 pH. The difference between these perfect readings and the electrode's actual reading is called offset error. A perfect pH electrode, at 25 (C produces 59.12mV per pH unit. The difference between this perfect reading and the electrode's actual reading is called the span error. These theoretical values are not always achieved, even with brand new electrodes. New pH electrode performance specifications should meet the following criteria:

Offset: 7.00 +/- 0.2 pH (+/- 12mV) SPAN: Better than 95% of theory; i.e. between 56.2 and 59.2 mV

Normal Aging

As electrodes are used they will experience some shifts in these new electrode specifications. OFFSETS will change and SPAN error will increase; i.e., the span will become shorter. By using the calibration controls these errors can be corrected. If an electrode is able to be calibrated and it is stable and responsive, it is still a functional electrode and may be used in service even though it no longer meets "new" electrode specifications.

As described later in this article, an electrode's response time becomes longer as it ages. Even though the electrode can be calibrated, sluggish response can limit its life. Also, certain application conditions, elevated temperatures, for example, will cause electrodes to have shorter service lives.

Speed of Response

An electrode's speed of response is affected by several things; mainly, by the impedance (resistance) of the pH glass measuring surface. The type of pH responsive glass used and the size, shape and thickness of its surface determines impedance characteristics. When selecting pH electrodes there are tradeoffs to consider. Here is a comparison of the three most common shapes:

SPHERICAL BULB HEMI-SPHERICAL BULB FLAT pH GLASS

A spherical shaped bulb will provide 95% response in less than one second. It has low impedance and fast response but is relatively fragile. pH electrodes with spherical shaped surfaces are designed so that the bulb is recessed inside the electrode body. Such designs protect the glass bulb against breakage.

A hemispherical shaped bulb is a stringer shape mechanically and, as a result, it has a higher impedance and slightly slower response. These shapes are often used in a fully exposed manner.

A flat measuring surface is the most durable of all the shapes. It makes good sample contact, is easily cleaned, is very strong mechanically but has the highest impedance and the slowest speed of response-95% in less than 5 seconds.

Coatings can mimic a sluggish speed response problem, therefore a used pH glass measuring surface should be cleaned before assuming that the electrode is no longer functional. Normally, the electrode may be cleaned with whatever chemical the coating material is soluble in provided the chemical will not attack the electrode's materials of construction. The glass surface should never be abraded; when wiping the surface always use clean, non abrasive materials and cloths.

Effects of Temperature

The impedance issue previously discussed is also a factor when measuring samples at temperatures other than 25(C. For every 10(C decrease in temperature the glass impedance will increase about 2.5 times. Therefore a spherical electrode (which has lower impedance) will offer better speed of response in lower temperatures than a flat electrode will.

Use of electrodes in elevated temperatures will cause pH electrodes to experience shorted service life than if used at ambient temperature. High temperatures accelerate both the natural aging of pH glass and chemical attack of the glass. These factors cause impedance to increase and the surface to loose its ability to respond to hydrogen ion activity (which is what a pH electrode actually measures!).

High and Low pH Measurements

Very strong acidic or caustic solutions will accelerate aging of a pH electrode leading to shorter service life. Some of these solutions will actually etch the pH glass surface with a resulting loss of response to hydrogen ions.

Low Ionic Strength Measurements

pH measurement in low ionic or low conductivity solutions may create several problems for standard pH electrodes. Typical difficulties include:

Slow sluggish or drifting readings
Unrepeatable readings.
Premature electrode demise.

Special design electrodes have been developed and are often used in low ionic strength applications. They feature:

Reference junctions- the porous material that contacts the sample- are made of porous polyethylene so that there is a very large surface area where the junction contacts the sample.

The reference junction is peripheral; that is, it surrounds the glass stem onto which the pH bulb is blown. This design minimizes streaming current effects which can generate spurious reference junction potentials.

The built-in reference electrode is of a double junction design. The inner chamber contains the usual high (3.5M or higher) salt concentration solutions so that stable outputs are generated. The outer chamber, which contacts the sample through the porous reference junction, is filled with 0.1M KCl. This lower ionic strength material more closely matches that of the sample and further reduces spurious potentials.

Single vs. Double Junction References

For many applications, a single junction reference electrode is satisfactory. However, if samples contain proteins, sulfides, heavy metals or any other material which interacts with silver ions, unwanted side reactions may occur. These reactions can lead to erroneous reference signals or to precipitation at the reference junction leading to a short service life.

A double junction reference design affords a barrier of protection to combat the above interactions. When in doubt about using single or double junction designs, the safest approach is to use the double junction; they can be used anywhere a single junction design can be used. Conversely, single junction designs should not be used where double junction designs are needed.

Storage/Shelf Life

Since pH electrodes have limited lives, it is important to keep one or more spare electrodes on hand. An important aspect of the performance of any spare pH electrode is will it work when go to use it. Electrodes supplied in soaker storage bottles have longer shelf lives than those supplied dry or with small vinyl caps. The special solution in soaker bottles allows the electrode to be instantly ready for use as well as ensuring a long storage life without reference junction or pH glass dehydration.

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