Reference electrodes

Reference Electrodes

Reference ElectrodesBasic function of a reference electrode is to

maintain a constant electrical potential against which deviations may be measured.

The desirable characteristics that a reference electrode should passes are

1. It should be easy to construct and

2. It should develop potentials which are reproducible even if small currents are passed.

Reference ElectrodesA reference is an electrode that has the half-cell potential known,

constant, and completely insensitive to the composition of the solution under study. In conjunction with this reference is the indicator or working electrode, whose response depends upon the analyte concentration.


Ideal Reference Electrode: Is reversible and obeys the Nernst equation Exhibits a potential that is constant with time Returns to its original potential after being

subjected to small currents Exhibits little hysteresis with temperature

cycling


It’s a primary reference electrode. Its potential is considered to be zero.

Electrode reaction: half cell: pt, H2 / H+ (1N) Eo = zero d-Limitation1. It is difficult to be used and

to keep H2 gas at one atmosphere during all determinations.

2. It needs periodical replating of Pt. Sheet with Pt. Black

Standard Hydrogen Electrode


Calomel Electrodes:


Silver/Silver Chloride Electrodes:

Ag/AgClDisadvantage of silver-silver chloride

electrode1. It is more difficult to prepare than SCE.2. AgCI in the electrode has large solubility in

saturated KCl

Advantage of Ag-AgCI electrodes over SCE.1. It has better thermal stability.2. Less toxicity and environmental problems with

consequent cleanup and disposal difficulties.

Indicator electrode Ecell=Eindicator-Ereference

It must be: (a) give a rapid response and(b) its response must be reproducible. Metallic electrodes: where the redox reaction

takes place at the electrode surface.Membrane (specific or ion selective)

electrodes: where charge exchange takes place at a specific surfaces and as a result a potential is developed.

Metallic Indicator Electrodes

There are four types of metallic indicator electrodes:

1. Electrodes of the first kind.

2. Electrodes of the second kind.

3. Electrodes of the third kind.

4. Redox electrodes.

Metallic Indicator Electrodes

Membrane Indicator Electrodes Properties: Minimal solubility. A necessary property of an ion-selective

medium is that its solubility in analyte solutions approaches zero.

Electrical conductivity. A membrane must exhibit some electrical conductivity. Generally, this conduction takes the form of migration of singly charged ions within the membrane.

Selective reactivity with the analyte. A membrane or some species contained within the membrane matrix must be capable of selectively binding the analyte ion. Three types of binding:

Ion-exchangeCrystallizationComplexation

Membrane Indicator Electrodes

Glass Electrodes:


Glass Electrodes: PotentialThe boundary potential.The potential of the internal Ag/AgCl

reference electrode.A small asymmetry potential.


Liquid Membrane Electrodes:Cation exchangersAnion exchangersNeutral macrocyclic compounds, which

selectively complex certain cations


Ion-Selective Field-Effect Transistors (ISFETs)

Molecular-Selective Electrode Systems Gas-Sensing Probes: Microporous materials- manufactured from

hydrophobic polymers that have a porosity of about 70% and a pore size of less than 1m, and are about 0.1mm thick.

Homogeneous films- solid polymeric substances through which the analyte gas passes by dissolving in the membrane, diffusing, and then desolvating into the internal solution.

They are usually thinner than microporous in order to hasten the transfer of gas and thus the rate of response of the system.

Molecular-Selective Electrode Systems

Gas-Sensing Probes:

Instruments for Measuring Cell Potentials Direct-Reading InstrumentsCommercial Instruments

Utility General-purpose Expanded-scale Research

Documents

Reference electrodes