2. Uses of the Glass Electrode
• pH measurement
• analyze foods
• Cosmetics
• environmental regulations
• soil acidity
It is fast responding and functions well in physiological systems
(Biological Systems).
3. Construction of pH Glass Electrode
+ -
Ag/AgCl
reference
electrode
Glass pH
electrode
5. The Gel Layer
• The glass has a lithium
silicate skeleton that forms
a thin hydrated layer on
both sides of the
membrane.
• Ions can penetrate this thin
layer and alter the
electrochemical potential.
• Without the hydrated layer
no pH measurements would
be possible
• The structure of the glass
has been optimised so that
virtually only H+ ions can
enter the gel layer
6. Principle
• H+ ion selective electrode
• Nernst Equation
log( aH + )
2.303RT
E=E −0
nF
• the electrode measures
the activity of hydrogen
ions and that pH is the E = E 0 + 0.0591 pH @ 25o C
negative log of the
activity of hydrogen ions
• The Nernst equation
predicts that for every
decade change in H+ ion
activity (1 unit change in
pH) the potential will
change by about 60 mV
It is advisable to keep a usage book and record the mV and pH every time the electrode is calibrated. This will provide a history of performance and deterioration of the electrode
The glass electrode is the best ion selective electrode invented, it can measure down to 10 -13 M H + ions, ug/L is ppb. Li ions not H ions are responsible for carrying the electrical charge through the glass electrode. We know this from the fact that the internal filling solution, usually 0.1M HCl does not interfere with the measurements even at pH12.
