pH Electrode Performance

pH electrodes are electrochemical sensors used by many industries ranging from water and wastewater, manufacturing, enviromental protection to food production applications. The sensor itself is similar to a battery as it generates a voltage output and has a useful useful and shelf life. There are many types and variations of pH electrodes used in field, lab and process environments.

OFFSET - Theoretically, when placed in 7.00 buffer at 25 °C a pH electrode produces zero millivolt (mV) which the pH meter reads as 7.00 pH. The difference between these perfect readings and the electrode's actual reading is called the offset error.
SPAN - A perfect pH electrode, at 25 °C produces 59.12 mV 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. This is due to different glass property, composition and may vary from batch constuction from one lot to another lot.


Normal Aging
As electrodes are used or stored for some periods they will experience shifts in these new electrode specifications. Offsets may change and span error increases; 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 (within meter tolerance) 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.

An electrode's response time increases as it ages. Even though the electrode can be calibrated, sluggish response can limit its useful life. Also, certain application conditions like elevated varying temperatures and use in aggressive chemicals that electrode is not designed for, will cause electrodes to have shorter useful lifespan.


Speed of Response
An electrode's speed of response can be affected by the impedance (resistance) of the pH glass measuring surface and the condition of this surface. The type of pH responsive glass used and the size, shape and thickness of its surface all affect impedance characteristics. When selecting pH electrodes there are tradeoffs to consider. These includes shapes of glass bulb, glass sensor type, reference junction type and coatings.

Shapes Of Glass Bulb
A spherical shaped (round) 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 (semi-round) bulb is a stronger 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 for fast response.
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 at 95% in less than 5 seconds.

Glass Sensor Type
GX-1 is especially suited for low temperature, nonaqueous solutions, and pH measurements under 12 pH.
GX-2 is best suited for most pH measurements where minimal or no Na+ is present. It is a low-resistance glass with a very fast and stable response and is designed for pH ranges of 0 to 13 and temperatures of 0 to 100 °C.
GX-3 is especially formulated for continuous, long-term use at high temperatures, particularly in strong alkaline solutions above pH 11. It experiences negligible Na+ ion error above 13 pH. The impedance of the glass is much higher than other glasses, and a slower response will be experienced at room temperatures and below.
GX-V is specially designed for low ionic strength and high pH measurements, and offers superior accuracy and repeatability with range of 0 to 14 pH and temperature up to 135 °C.

Electrode Reference Junction
Reference junctions refers to the porous material that contacts the sample. The common types available are: ceramic, open junction, fibre-glass and porous plugs. Its material property and ability to clogging issue undermine the electrolyte flow rate and measurement stability.

Coatings can result a sluggish speed response problem; therefore a used pH glass measuring surface should be cleaned periodically 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 glass and housing material. The glass surface should never be cleaned in a manner that would scratch the glass surface. Scratches will result in a slow response and shorter working life. When wiping the surface always use clean, non-abrasive and non-electrostatic materials to blot dry.