pH Measurement
The measurement of pH
using a pH Electrode is an age old method that centres around
the perception of a substance as acidic or alkaline and is dependent on the concentration
of the hydrogen Ion (H+) within the substance.
the perception of a substance as acidic or alkaline and is dependent on the concentration
of the hydrogen Ion (H+) within the substance.
The use of a pH electrode allows this measurement to be expressed in a meaningful way.
Derived from the
Sorensen Equation the pH value is defined as a negative logarithm of the H+
concentration in a given solution.
A high H+ Concentration equals: 1mol/L = 10o pH = 0
(ACIDIC)
A low H+ Concentration equals: 10-14 mol/L pH=14
(ALKALINE)
It is therefore simple to measure the
pH of a substance and compare it with other substances. pH 0 is extremely
acidic, pH 14 is extremely alkaline and pH 7 neutral.
Measuring pH Values
Measuring the pH of a
substance requires the use of a pH Electrode and a Reference Electrode. The pH
Glass at the end of the electrode acts as the sensing part of the circuit.
The second part of
the circuit is the reference electrode. This is a stable point that has a
defined potential and is independent of the solution to be measured. The
reference electrode fig is made up of a reference element that is commonly a
Silver/Silver Chloride wire encased in a known electrolyte. The reference
electrode then has a junction, which is the contact between the stable internal
reference electrode and the solution to be measured. This is commonly a porous
ceramic pin.
The evolution of pH
electrodes came with the joining of the two separate electrodes to produce the
COMBINATION pH ELECTRODE. The formation of the combination electrode can be
seen below. The formation of the combination electrode still has the pH Glass
membrane acting in the same way. The reference electrode is continually encased
around the pH electrode. As the pH Glass comes into contact with an aqueous
substance to measure, a gel layer forms on the membrane. This also happens on
the inside of the glass layer. The pH value of the aqueous solution will either
force Hydrogen Ions out of the Gel layer or into this layer. The Internal
buffer in the glass electrode has a constant pH value and this keeps the
potential at the inner surface of the membrane constant. The membrane potential
is therefore the difference between the inner and outer charge. If you then
factor in the reference electrode with its stable potential you have a
combination pH electrode that encapsulates the measuring electrode and
reference electrode.
P&R Labpak are
able to offer a huge range of electrodes and through it’s electrode supplier
Sentek can offer their equivalents which are less expensive and yet are the
same or better quality. Contact us on
0870 034 2055 or e-mail us at sales@prlabs.co.uk with your electrode
enquiries.
How the pH Electrode Works
As the pH Glass comes
into contact with an aqueous substance to measure, a gel layer forms on the
membrane.
This also happens on the inside of the glass layer.
This also happens on the inside of the glass layer.
The pH value of the
aqueous solution will either force Hydrogen Ions out of the Gel layer or into
this layer.
The Internal buffer in the glass electrode has a constant pH value and this keeps the potential at the inner
surface of the membrane constant.
The Internal buffer in the glass electrode has a constant pH value and this keeps the potential at the inner
surface of the membrane constant.
The membrane potential is therefore the difference between the inner and outer charge.
If you then factor in the reference electrode with its stable potential you have a combination
pH electrode that encapsulates the measuring electrode and reference electrode.
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