Reference electrodes
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Reference electrodes are used to maintain a constant potential against which the potential of an indicator or working electrode can be measured. An ideal reference electrode has a reproducible and stable potential that is not affected by small currents or changes in temperature or solution composition. Common reference electrodes include the standard hydrogen electrode, which defines zero potential, and silver/silver chloride electrodes. Reference electrodes are used along with indicator or working electrodes in electrochemical cells to measure the potential difference between the electrodes, which depends on the analyte concentration.
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Reference electrodes
- 2. Reference Electrodes Basic 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.
- 3. Reference Electrodes A 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.
- 4. Reference Electrodes 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
- 6. It’s a primary reference electrode. Its potential is considered to be zero. Electrode reaction: half cell: pt, H2 / H+ (1N) Eo = zero d-Limitation 1. 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
- 8. Reference Electrodes Silver/Silver Chloride Electrodes:
- 9. Ag/AgCl Disadvantage of silver-silver chloride electrode 1. 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.
- 10. 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.
- 11. 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.
- 13. 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-exchange Crystallization Complexation
- 15. Membrane Indicator Electrodes Glass Electrodes: Potential The boundary potential. The potential of the internal Ag/AgCl reference electrode. A small asymmetry potential.
- 16. Membrane Indicator Electrodes Liquid Membrane Electrodes: Cation exchangers Anion exchangers Neutral macrocyclic compounds, which selectively complex certain cations
- 19. 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 1µm, 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.
- 21. Instruments for Measuring Cell Potentials Direct-Reading Instruments Commercial Instruments Utility General-purpose Expanded-scale Research