You can assume that the proper reference electrode is being used

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 25^oC

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)

E_ind = E^o -

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 - E_cell) / 0.0592

pAg = (0.800V-0.244V - E_cell) / 0.0592 E_cell = 0.450 V

pAg = (0.800V-0.244V - 0.450V) / 0.0592 = 1.791

[Ag+] = 1.618 x 10^-2 M

[Cl^-] K_SPAgCl

[Cl^-] K_SPAgCl

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^-, SO₄^2-

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