Engineering Mechanics Chapter 5 Equilibrium of a Rigid Body
pH electrodes
2. ▪ The pH meter is
designed to measure the
concentration of
hydrogen ions in a
solution.
▪ Three parameters are
involved in the
measurement: the actual
molar concentration of
hydrogen ions, the
dissociation constant of
the acid (pKa), and
temperature
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▪ pH is defined as the negative of the logarithm to the
base 10 of the hydrogen ion concentration
(pH = –log10 [H+])
3. 3
▪ Around 1928 the first commercial
pH meter was produced by the
Cambridge Instrument Company
5. Scale of pH: 0 to
14pH
Acid solution: Less
than 7pH
Basic solution:
Greater than 7pH
Neutral solution:
7pH
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6. When the indicator electrode and the reference
electrode are immersed in a solution
containing hydrogen ions, the small potential
difference between the two half-cells is
measured by a very sensitive voltmeter.
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8. Potentiometric Measurements
▪When current passes from
metal to electrolyte, or from
electrolyte to metal, the type of
carrier usually changes
suddenly, and whenever there is
an interface between the metal
and ions of that metal in a
solution then an electric
potential is produced.
Potentiometric methods are
based on the quantitative
relationship between the
potential of a cell as given
by the following distribution
of potential:
Ecell = Ereference +
Eindicator +Ejunction
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9. Variations of pH
with E is directly
recorded with
potentiometer scale
graduated to read
pH
▪ A saturated
potassium
chloride
solution is
normally
used as a salt
bridge in the
reference
electrode
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10. ▪ The standard hydrogen electrode is the
international standard but is seldom used for
routine work
▪ The saturated calomel electrode
is a widely used reference electrode that
contains an inert element (Eg platinum) in
contact with mercury, mercurous chloride
(calomel) and a solution of potassium
chloride of known concentration
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11. Hydrogen electrode consists of an inert but catalytically active metal surface
(Platinum)
Hydrogen electrode is based on the redox half cell:
2 H+
(aq) + 2 e−
→ H2(g)
The electrode is dipped in an acidic solution and pure hydrogen gas is bubbled
through it. The concentration of both the reduced form and oxidised form is
maintained at unity. That implies that the pressure of hydrogen gas is 1 bar
(100 kPa) and the activity of hydrogen ions in the solution is unity.
Electrode life is 7-20 days before its response become sluggish
Activity of hydrogen electrode is governed by Nernst Equation
Hydrogen electrode-Redox system- affected by oxidising and reducing agents
Limitations in applications
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12. Glass electrodes are commonly used for pH
measurements.
There are also specialized ion sensitive glass electrodes
used for determination of concentration of lithium,
sodium, ammonium, and other ions.
Glass electrodes have been utilized in a wide range of
applications — from pure research, control of industrial
processes, to analyze foods, cosmetics and comparison
of indicators of the environment
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13. Thin membrane of glass when interposed between two solutions , a
potential difference is observed dependent on the ions present in the
solution
The membrane thickness of pH glass electrode is of the order of 0.05-
0.15mm
Bulbs of the order of 10 mm in diameter
Low melting point
High conductivity
Temperature range 60ºC
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15. Measuring solution containing particulate matter can damage
glass membrane
Glass membrane easily broken
Limited pH range , long response, antimony electrode used
as an alternate electrode (forms thin oxide layer sensitive to
pH)
Care and Maintenance – faster response, long lifetime and
reliable measurements
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18. Calomel electrode (SCE) is a reference electrode
based on the reaction between elemental mercury
and mercury(I) chloride.
(Hg2Cl2, "calomel") is a saturated solution of
potassium chloride in water.
Electrolyte solution forms a conductive salt bridge
between metallic element and sample solution
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20. 20
Concentration of KCL 3.8M,3.5M or 0.1M
Temperature below 70ºC
Ag/Agcl Electrodes for high temperature 10ºC - to
110ºC
Hg-Hgcl sulphate electrode with a salt bridge solution
of potassium sulphate is used in test solutions (without
chloride ions)
LiCl is used for organic solvents than KCl
Trapped air bubbles and precipitate of AgCl (Salt
bridge) may clog the porous pin causes malfunction
24. Filling solution – 3M NaCl saturated with AgCl
This gel appears to be cloudy and semi solid
AgAgCl are easily dried and spoiled so it is advised to keep the tips wet and
store in AgCl
Usually last for 3-6months
Double Junction – these electrodes have an additional salt bridge to prevent
reactions between the electrode fill solution and your sample which would
otherwise cause damage to the electrode junction. They are required to test
samples that contain proteins, heavy metals or sulphides.
Single Junction – these are for general purpose applications for samples that
won’t block up the junction
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27. Modern pH electrodes are usually of the "combination"
type, meaning that a single cylinder contains both the
reference electrode, and a glass membrane electrode
Internal reference electrode is identical to external
reference electrode (Ag/AgCl)
These are protected against light by ruby red glass which
absorbs light
Temperature ranges from -10 to 100ºC
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