Electrode of the first kind is a simple metal electrode immersed in a solution containing its own ion (e.g., silver immersed in a silver nitrate solution). The equilibrium potential of this electrode is a function of the concentration (more correctly of activity) of the cation of the electrode metal in the solution (see Nernst’s electrode potential equation).
Electrodes of the second kind are metal electrodes assembly with the equilibrium potential being a function of the concentration of an anion in the solution. Typical examples are the silver/silver-chloride electrode and the calomel electrode. The potential of the metal is controlled by the concentration of its cation in the solution, but this, in turn, is controlled by the anion concentration in the solution through the solubility product of the slightly soluble metal salt. Contrast with electrode of the first kind and electrode of the third kind.
Electrode of the third kind is a metal electrode assembly with the equilibrium potential being a function of the concentration of a cation, other than the cation of the electrode metal, in the solution. The assembly consists of a metal in contact with two slightly soluble salts (one containing the cation of the solid metal, the other the cation to be determined, with both salts having a common anion) immersed in a solution containing a salt of the second metal (e.g., zinc metal--zinc oxalate--calcium oxalate--calcium salt solution). The potential of the metal is controlled by the concentration of its cation in the solution, but this is controlled by the anion concentration in the solution through the solubility product of the slightly soluble metal salt, which, in turn is controlled by the concentration of the cation of the second slightly soluble salt. These electrodes are very sluggish and unstable due to a series of equilibria to be established to produce a stable potential.
Glass electrode is a hydrogen-ion responsive electrode usually consisting of a bulb, or other suitable form, of special glass attached to a stem of high resistance glass complete with internal reference electrode and internal filling solution system. Glass electrode is also available for the measurement of sodium ions.
The glass electrode, which consists of a thin wall glass bulb, has an extremely high electrical resistance. The membrane of a typical glass electrode (with a thickness of 0.03 mm to 0.1 mm) has an electrical resistance of 30 MΩ to 600 MΩ. The surface of a glass membrane must be hydrated before it will function as a pH electrode. When a glass surface is immersed in an aqueous solution then a thin solvated layer (gel layer) is formed on the glass surface in which the glass structure is softer. This applies to both the outside and inside of the glass membrane.
The simplest explanation for the working of the thin glass electrode is that the glass acts as a weak acid (Glass-H).
The hydrogen ion activity of the internal solution is held constant. When a solution of different pH from the inside comes in contact with the outside of the glass membrane, the glass is either deprotonated or protonated relative to the inside of the glass. The difference in pH between solutions inside and outside the thin glass membrane creates electromotive force in proportion to this difference in pH.
Indicator electrode is working in one of the electrodes in some classical two-electrode cells, e.g., in a potentiometric electroanalytical setup where the potential of the measuring electrode (against a reference electrode) is a measure of the concentration (more accurately activity) of a species in the solution.
Inert electrode is an electrode that serves only as a source or sink for electrons without playing a chemical role in the electrode reaction. Precious metals, mercury, and carbon are typically used as inert electrodes. The inert nature of the electrode can sometimes be questioned. While the electrode may not take part in the reaction as a reactant or product, it still can act as an electrocatalyst.
Ion selective electrode (ISE) is an electrode or electrode assembly with a potential that is dependent on the concentration of an ionic species in the test solution and is used for electroanalysis. Ion-selective electrodes are often membrane type electrodes.
Referent electrode is an electrode whose potential is known and completely independent of analyte concentration. Mostly used referent electrodes are calomel and silver/silver chloride electrode.
Table: Dependence of referent electrodes potentials on KCl concentration
Potential vs. SHE / V | |||||
calomel electrode | Ag/AgCl electrode | ||||
t / °C | 0.1 mol dm-3 | 3.5 mol dm-3 | sat. solution | 3.5 mol dm-3 | sat. solution |
15 | 0.3362 | 0.254 | 0.2511 | 0.212 | 0.209 |
20 | 0.3359 | 0.252 | 0.2479 | 0.208 | 0.204 |
25 | 0.3356 | 0.250 | 0.2444 | 0.205 | 0.199 |
30 | 0.3351 | 0.248 | 0.2411 | 0.201 | 0.194 |
35 | 0.3344 | 0.246 | 0.2376 | 0.197 | 0.189 |
Standards are materials containing a known concentration of an analyte. They provide a reference to determine unknown concentrations or to calibrate analytical instruments.
The accuracy of an analytical measurement is how close a result comes to the true value. Determining the accuracy of a measurement usually requires calibration of the analytical method with a known standard. This is often done with standards of several concentrations to make a calibration or working curve.
A primary standard is a reagent that is extremely pure, stable, has no waters of hydration, and has a high molecular weight.
A secondary standard is a standard that is prepared in the laboratory for a specific analysis. It is usually standardised against a primary standard.
Standard deviation (σ) is a measure of the dispersion of a set of data from its mean. Standard deviation is a statistical term that measures the amount of variability or dispersion around an average
Suppose there are many measurements of a quantity presumed to be similar, like the size of peas in a pod. If the number of readings for each size were plotted, a bell-shaped curve would probably result, with a few small and large peas and most clustered around the average size. Around two-thirds of all measurements fall in the range spanned by the standard deviation, a measure of the spread.
Generalic, Eni. "Standard hidrogen electrode." Croatian-English Chemistry Dictionary & Glossary. 29 June 2022. KTF-Split. {Date of access}. <https://glossary.periodni.com>.
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