Specifications
Magnitude of error = 0.003 pH/°C/pH unit from pH 7
Note: The temperature compensation here refers to electrode related
temperature variation and not solution related variations.
Selection Criteria
Eutech combination electrodes oer the convenience of having the
reference and measuring electrodes combined in a single housing. They
are oered in a variety of congurations to suit most laboratory and eld
application needs.
Electrode Construction
Single Vs Double Junction
Electrodes | About pH Electrodes
Single Junction
Ideal for general purpose applications.
Ag
+
ions are in contact with junction
and this can cause chemical interaction
with sulphur. Not suitable for biological
samples or tris buers.
Glass Body
Glass withstands high temperature of
100 °C or more.
Resistant to corrosive materials
and solvents.
Brittle.
Ideal for laboratory use and is easy
to clean.
Double Junction
Prevents interference between the
inner ll solution and sample.
Electrolyte is free of Ag
+
ions. Suitable
for use with biological samples.
Can be used in place of calomel
reference electrodes.
page | 82
Plastic Body
Not recommended for usage at
temperature above 80 °C.
Moderate resistance to highly corrosive
materials and solvents.
Durable and withstands rough handling.
Ideal for eld use.
About pH Electrodes
Basic Theory and Application of pH Measurement
pH refers to the power or exponent of hydrogen where ‘p’ stands for
power and ‘H’ is the symbol of the element Hydrogen.
pH is dened as the negative logarithm of the molar concentration of the
active hydrogen ions, pH = -log H
+
.
pH provides a convenient way to compare the relative acidity or alkalinity
of a sample at a given temperature. For example, pure water has a neutral
pH of 7, where the activities of hydrogen and hydroxide ions are equal.
If the activity of hydrogen ion is greater than that of hydroxide ion the
sample is described as acidic. In general, as the level of hydrogen ion
activity increases, the pH decreases. A pH below 7 is known as acidic.
On the contrary, as the level of hydrogen ion activity decreases, the pH
increases. A pH above 7 is known as alkaline or basic.
Use of Electrodes for pH Measurement
pH measurement is usually done with the use of a combination electrode.
The combination electrode is an electrode system formed by a glass sensing
half-cell and an internal reference half-cell. As the reference junction acts
as the medium of conductor between the reference electrolyte and the
sample to be measured, it must allow free movement of electrons through
the junction and into the sample. A potential develops on the membrane
surface when a pH electrode comes into contact with a sample and
its value varies with the pH of the sample. This variation in potential is
measured in mV by a meter and is converted to direct pH values.
Slope
The ‘slope’ is the voltage produced per pH. In theory, the value is 59.16 mV
per pH at 25 °C. Practically, the value ranges between 50 and 58 mV.
Inuence of Temperature on pH Measurement
Temperature variations can aect pH. However at a certain pH, usually 7,
temperature will not have an eect on the potential of the system. This
is known as the ‘isopotential point’.
If automatic compensation is not practical, the following equation can
be used to determine error: