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The previous cell would be difficult to use for
many systems.
We would like something that can be placedin the solution we wish to measure.
The electrodes we’ll be looking at have that
goal in mind but still represent a complete
electrochemical cell when used.
H2
1 M HCl
Pt black
plate
asbestos fiber
Hg2Cl2/KCl
Hg
KCl solution
fiber 25oC
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saturated AgCl/KCl
Ag
wire
AgCl
Several approaches have been taken.
Simple metal Solid state
Glass membrane Enzyme
Liquid membrane Gas sensing
We’ll review representative examples of each.
You can assume that the proper referenceelectrode is being used.
A bare metal in contact with a solution of it’s
cation.
Mn+ + ne = M(s)
Eind = Eo -
indicating
electrode
0.0592
n
1
[ Mn+]log
1
[Ag+]
A silver wire is dipped into a silver nitrate
solution. A potential of 0.450 V was measuredvs. SCE.
What is [Ag+] for the unknown solution?
pAg = (0.800V-0.244V - Ecell) / 0.0592
pAg = (0.800V-0.244V - Ecell
) / 0.0592
Ecell = 0.450 V
pAg = (0.800V-0.244V - 0.450V) / 0.0592
= 1.791
[Ag+] = 1.618 x 10-2 M
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[Cl-]
KSP AgCl
[Cl-]
KSP AgCl
For some metals, a good electrode can’t be
made or no metal is involved - just ions.
An inert indicating electrode like Pt can be
used. This type of electrode only measuresthe ratios of the ions.
No quantitation but suitable for titrations.
Ag wire
0.1N HCl
AgCl
thin
glass
wall
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H3O+ partially populates
both the inner and outerSiO2 surfaces.
The concentration
difference results in apotential across the
glass membrane.
A special glass is used:
22% Na2O, 6% CaO, 72%
SiO2
H3O+
Si
O
Si
O
Si
O
Si
O
Si
O
Si
Si
O
Si
O
Si
O
Si
H3O+
H3O+
H3O+
H3O+
•
Similar to a pH electrode except the
membrane is an organic polymersaturated with a liquid ion exchanger.
• Interaction of this exchanger with target
ions results is a potential across themembrane that can be measured.
• The Ca2+ electrode is one of the best
examples.
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Ag/AgCl
reference
electrode
internal
reference
solutionion exchange
reservoir
porous
membranesensing
area
The reservoir forces exchanger into the
membrane. The exchanger formscomplexes with the species of interest.
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
The results in aconcentration
difference and a
resulting !V thatwe can measure.
Concentration Major
Ion Range, M Interferences
Ca2+ 100 - 5 x10-7 Pb2+, Fe2+, Ni2+, Hg2+, Sr 2+
Cl- 100 - 5 x10-6 I-, OH-, SO42-
NO3- 100 - 7 x10-6 I-, ClO4
-, ClO3-, Br -, CN-
ClO4- 100 - 7 x10-6 I-, ClO3
-, Br -, CN-
K+ 100 - 1 x10-7 Cs+, NH4+, Tl+
Ag wire
Ag2S/AgCl
pellet
Inert body
Membrane
Primary
absorbed
ions
The
primaryabsorbed
ions resultin a [ ]
gradient
and !Vbeing
produced.
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In this example, a normal pH electrode
is coated with a urease impregnated gel.
Urea will permeate the gel where the
enzyme will attack it, resulting inthe formation of ammonium.
The resulting change in pH can be
measured.
Here, an indicating electrode is placed
into a specific solution.
On the opposite side, there is
a permeable membrane.
Permeation of the target
analyte results in an
equilibrium change thatwe can measure.
indicating
electrode
membrane
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E
log(conc.)
standard concentration
unknown
concentration
