electrode technology hasn't changed much in the past 50 to 60
years, yet pH electrode manufacturing remains an art. The special
glass body of the electrode is blown to its configuration by glass
blowers. This is neither a terribly advanced nor "high tech" process
but a very critical and important step in the electrode manufacturing.
In fact, the thickness of the glass determines its resistance
and affects its output.
electrodes are constructed from a special composition glass which
senses the hydrogen ion concentration. This glass is typically
composed of alkali metal ions. The alkali metal ions of the glass
and the hydrogen ions in solution undergo an ion exchange reaction,
generating a potential difference. In a combination pH electrode,
the most widely used variety, there are actually two electrodes
in one body. One portion is called the measuring electrode, the
other the reference electrode. The potential generated at the
junction site of the measuring portion is due to the free hydrogen
ions present in solution.
The potential of the reference portion is produced by the internal
element in contact with the reference fill solution. This potential
is always constant. In summary, the measuring electrode delivers
a varying voltage and the reference electrode delivers a constant
voltage to the meter. The voltage signal produced by the pH electrode
is a very small, high impedance signal. The input impedance requires
that it be interfaced only with equipment with high impedance
circuits. The input impedance required is greater than 1013 ohms.
This is the reason pH electrodes do not interface directly with
pH electrodes are available in a variety of styles for both laboratory
and industrial applications. All are composed of glass and are
therefore subject to breakage. Electrodes are designed to measure
mostly aqueous media. They are not designed to be used in solvents,
such as CCI4, which does not have any free hydrogen ions.
pH electrode, due to the nature of its construction, needs to
be kept moist at all times. In order to operate properly, glass
needs to be hydrated. Hydration is required for the ion exchange
process to occur. If an electrode should become dry, it is best
to place it in some tap water for a half hour to condition the
electrodes are like batteries; they run down with time and use.
As an electrode ages, its glass changes resistance. This resistance
change alters the electrode potential. For this reason, electrodes
need to be calibrated on a regular basis. Calibration in pH buffer
solution corrects for this change. Calibration of any pH equipment
should always begin with buffer 7.0 as this is the "zero point."
The pH scale has an equivalent mV scale. The mV scale ranges from
+420 to -420 mV. At a pH of 7.0 the mV value is 0. Each pH change
corresponds to a change of ±60 mV. As pH values become more acidic
the mV values become greater. For example, a pH of 4.0 corresponds
to a value of 180 mV. As pH values become more basic the mV values
become more negative; pH=9 corresponds to -120 mV. Dual calibration
using buffers 4.0 or 10.0 provides greater system accuracy.
electrodes have junctions which allow the internal fill solution
of the measuring electrode to leak out into the solution being
measured. This junction can become clogged by particulates in
the solution and can also facilitate poisoning by metal ions present
in the solution. If a clogged junction is suspected it is best
to soak the electrode in some warm tap water to dissolve the material
and clear the junction. pH electrodes should always be stored
in a moistened condition. When not in use it is best to store
the electrode in either buffer 4.0 or buffer 7.0. Never store
an electrode in distilled or deionized water as this will cause
migration of the fill solution from the electrode. pH electrodes
have a finite lifespan due to their inherent properties. How long
a pH electrode will last will depend on how it is cared for and
the solutions it is used to measure. Typically, a gel-filled combination
pH electrode will last six months to 1 year depending on the care
and application. Even if an electrode is not used it still ages.
On the shelf, the electrode should last approximately a year if
kept in a moistened condition. Electrode demise can usually be
characterized by a sluggish response, erratic readings or a reading
which will not change. When this occurs an electrode can no longer
be calibrated. pH electrodes are fragile and have a limited lifespan.
How long an electrode will last is determined by how well the
probe is maintained and the pH application. The harsher the system,
the shorter the lifespan. For this reason it is always a good
idea to have a back-up electrode on hand to avoid any system down
time. Calibration is also an important part of electrode maintenance.
This assures not only that the electrode is behaving properly
but that the system is operating correctly.