The previous cell would be difficult to use for many systems.
We would like something that can be placed in 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
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 reference electrode 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 measured vs. 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
[Cl-] KSPAgCl
[Cl-] KSPAgCl
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 measures the ratios of the ions.
No quantitation but suitable for titrations.
Ag wire 0.1N HCl AgCl thin glass wall
H3O+ partially populates both the inner and outer SiO2 surfaces.
The concentration difference results in a potential 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 polymer saturated with a liquid ion exchanger.
• Interaction of this exchanger with target ions results is a potential across the membrane that can be measured.
• The Ca2+ electrode is one of the best examples.
Ag/AgCl reference electrode internal reference solution ion exchange
reservoir
porous membrane sensing
area
The reservoir forces exchanger into the membrane. The exchanger forms complexes with the species of interest.
Ca2+
Ca2+ Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
The results in a concentration difference and a resulting !V that we can measure.
Concentration Major Ion Range, M Interferences
Ca2+ 100 - 5 x10-7 Pb2+, Fe2+, Ni2+, Hg2+, Sr2+
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
primary absorbed ions result in a [ ] gradient and !V being produced.
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 in the 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 that we can measure.
indicating electrode
membrane
E
log(conc.)
standard concentration unknown
concentration
