In many pH applications, the fragile glass electrode needs to be
replaced by a nonglass pH electrode alternative. For example, in food
processing a glass breakage can bring the whole process to stop, no
matter how small glass fragments.
The pH glass electrode technology has been available for a long time, and even it appears there have been few new developments the nonglass alternative has been on the market for more than two decades.
For a glass electrode, it is the probe tip,
the glass membrane, that is the hydrogen sensitive part. For the
nonglass pH electrode, the pH sensitive glass has been replaced by a
hydrogen ion-selective field effect transistor (ISFET).
The behavior of both ISFET and glass electrodes can be described by the Nernst equation. The sensitivity of the ISFET electrode is comparable with that of a glass pH electrode. However, the zero point of the ISFET electrode is different from that of most glass electrodes. Which mean pH 7 is not at zero mV but can be 100 mV or more. For more about sensitivity (slope) and zero point, see the calibration page.
ISFET uses a different mechanism for measuring pH from that of
traditional glass electrodes. The measurement principle is based on the
control of the current flowing between two semiconductor electrodes.
These two electrodes, the so-called drain and source electrodes, are
placed in a silicon chip together with a third electrode, the so-called
gate between them. The gate is in direct contact with the solution to be
The gate electrode consists of a special chemical layer, which is
sensitive to hydrogen ions. Materials like silicon oxide (SiO2), silicon
nitride (Si3N4) and aluminium oxide are used in the pH sensing layer.
Hydrogen ions will reside at the surface of the chemical layer in proportion to the pH. The positive charge of the hydrogen ions produces an electric field that influences the current between the source and drain. So when the pH value changes, the current through the transistor will change accordingly. To maintain the drain–source current at a constant value a control voltage has to be applied via the reference electrode.
The change in the control voltage (UGS) is a measure of the pH value of the sample.
reference electrode works the same as for a conventional glass
electrode. This means that for an ISFET electrode exist the same
problems and choices with the reference electrode as for the glass electrode.
ISFET-chip is sealed in a hermetic enclosure and mounted at the tip of
the sensor body. As the ISFET electrode is a nonglass pH electrode, the
electrode body is made of a plastic (epoxy or polymer). The plastic body
is a good choice for applications where a glass breakage is
An ISFET electrode can easily be made very small, as an ISFET chip is a lot smaller than a glass membrane. However, for normal use in a laboratory or process environment the size of ISFET electrodes matches the size of glass electrodes.
As with the glass electrode, most ISFET electrodes have a temperature sensor embedded in the body.
As already mentioned on the pH meter
page, a glass electrode has a very high internal resistance and to make
reliable measurements the input resistance of the pH meter has to be
even higher. However, this differs from an ISFET nonglass pH electrode.
An ISFET electrode combines in one device, the pH sensing transistor and
a signal amplifier which produces a high current, low impedance output.
This means you have to use a pH meter with an interface adapted to an
+ are virtually unbreakable and there is no risk of broken glass.
+ are very rugged and the pH sensing area can be easily cleaned with a toothbrush.
+ can be stored dry. pH glass electrodes must be stored in an aqueous environment to prevent dehydration.
+ have a short response time and give fast measurements.
+ can reduce the acidic or alkaline errors in extreme pH conditions without the need to resort to a special glass.
- are more expensive than pH glass electrodes.
- do not offer the same stability and accuracy afforded by glass electrodes.
- have a known drifting problem.
- are highly temperature dependent and there is a risk of damage by rapid cycling temperature changes.
- are sensitivity to direct sunlight. Like all semi-conductor elements, the ISFET chip is sensitive to light (measured value fluctuations).
- will only work with pH meters, which are adapted to the ISFET technology and are not interchangeable with meters for conventional pH glass electrodes.
- should not be used in conjunction with chlorine or other chemicals which will permanently damage the ISFET chip.
The ISFET electrode characteristics can differ between manufactures and new development can reduce earlier disadvantages.
For information on your specific nonglass pH electrode, see the instruction manual that came with your electrode.
Take the opportunity to share your knowledge and experiences with others.
Please, use the form below to publish your contribution about the subject "Non-glass pH electrodes". Before your page is published, it will be reviewed to make sure it applies to the submission guidelines for this website.
Don't forget to read what others have written, maybe you have a comment to add.
Click below to see contributions from other visitors to this page...