Specifications

the pH value based on the temperature of the solution. Manual
Temperature Compensation requires the user to enter the temperature
of the solution in order to correct pH readings for temperature. ATC is
considered to be more practical for most pH applications.
Most Eutech meters oer ATC capabilities. Models with this feature
include the pHTestr™10, 20, 30 and all the EcoScan, CyberScan
handheld and bench pH meters.
More on pH Electrodes
The electrode system consists of two half cells: a pH indicating electrode,
which is primarily responsive to the acidity (the hydronium ion
concentration) of a solution, and a reference electrode, which provides
a constant voltage and completes the electrical circuit.
pH indicating electrodes traditionally use a tip made of pH sensitive
glass. Inside this electrode is a buer solution with a xed pH and ionic
strength. A silver wire coated with silver chloride is immersed inside this
internal solution, and establishes electrical contact between the solution
and the meter. The voltage associated with this wire and the voltage
associated with the pH of the internal solution and the inside wall of the
pH sensitive glass tip remain constant. Therefore, changes in voltage from
this electrode result from the voltage developed between the solution
and the outside of the glass tip.
Reference electrodes typically consist of three elements: an internal
reference electrode, a lling solution, and a reference junction through
which the lling solution can “ow”, and provide electrical contact with
the sample and the internal reference electrode. The most convenient
internal reference electrodes are made from a metal (such as silver or
mercury) and its sparingly soluble salt (silver chloride or mercurous
chloride). The lling solution is most often a concentrated solution
of potassium chloride. Most problems with reference electrodes are
associated with the interruption or blockage of ow of the potassium
chloride ll solution through the reference junction.
Single and Double Junction Electrodes
For many applications, a single junction reference electrode is satisfactory.
However, if samples contain proteins, suldes, 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 aords 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 as 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. In most process applications, it is
recommended to use double junction electrodes.
Eutech’s new range of large screen pocket testers pHTestr™ series
feature double junction electrodes that extend useful life and
provide long term cost savings for users.
Normal Aging
As electrodes are used or stored for long periods they will experience
some deterioration in performance. Offsets will change and slope
error will increase. By using the calibration controls these errors can
be corrected. If an electrode is able to be calibrated and is stable and
responsive, it is still a functional electrode and may be used in service
even though it no longer meets “new” electrode specications.
In practice, it is unrealistic to achieve zero oset in electrodes.
The Eutech range of advanced micro -processor based
meters provides offset calibration abilities for consistent and
reliable measurements.
Slope in pH Electrode
As explained earlier the pH electrode produces different mV in
dierent solution.
The slope of the electrode is dened as
Slope = mV/pH unit
A perfect pH electrode, at 25 °C, produces a slope of 59.16 mV per pH
unit. For example, for an electrode with 0 mV oset, when placed in
a solution other than pH 7.00, say pH 4.01, it should read mV value of
177.48 mV. Therefore the slope is calculated to be (177.48 mV - 0 mV) /
3 pH = 59.16 mV/pH. The dierence between this perfect slope reading
and the electrode’s actual reading is called the slope error.
These theoretical values are not always achieved, even with brand
new electrodes. New pH electrode slope should be between 92 % and
102 % of 59.16 mV. If the slope falls below 92 %, cleaning of the electrode
may be needed.
Temperature Compensation
In a perfect pH electrode – one that is zeroed at exactly pH 7 – there
is no temperature eect on the electrode sensitivity at pH 7 no matter
how much the temperature changes. Most pH electrodes are not perfect,
but the errors from changes in temperature are still very minute when
near pH 7, plus or minus one-tenths of a pH, and can be disregarded.
However, the further from pH 7 the solution is and the greater the
temperature changes, the greater the measurement error due to
changes in the electrode’s sensitivity. For most electrodes, the error
will be 0.003 pH/°C/pH away from pH 7.
For example, if a pH meter is calibrated at room temperature (25 °C) and
is measuring a sample around pH 4 at around 5 °C.
Temperature dierence: 25 °C - 5 °C = 20 °C
pH away from neutral: 7 pH - 4 pH = 3 pH
Total error: 0.003 x 20 x 3 = 0.18 pH
To overcome this error, pH meters require some form of temperature
compensation to ensure standardized pH values. Meters and controllers
with Automatic Temperature Compensation (ATC) receive a continuous
signal from a temperature sensing element and automatically correct
pH/ORP | About pH/ORP Measurement
page | 11
The Ideal Electrode
An Ideal Electrode in Theory has
the Following Properties:
Oset = 0 mV
Slope = 59.16 mV/pH @ 25 °C
▼
▼
mV
pH
0
7
ISO Potential Point