CHEMISTRY GLOSSARY

calomel electrode    →   kalomel elektroda

Calomel electrode is a type of half cell in which the electrode is mercury coated with calomel (Hg₂Cl₂) and the electrolyte is a solution of potassium chloride and saturated calomel. In the calomel half cell the overall reaction is

Table: Dependence of potential of calomel electrode upon temperature and concentration of KCl according to standard hydrogen electrode

electrode of the second kind    →   elektroda drugog reda

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.

referent electrode    →   referentna elektroda

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

electrode    →   elektroda

Electrode is a conductor that emits or collects electrons in a electrolytic cell

conditional electrode potential    →   uvjetni elektrodni potencijal

Conditional or formal electrode potential (E°’) is equal to electrode potential (E) when overall concentrations of oxidised and reduced form in all its forms in a solution are equal to one. Conditional electrode potential includes all effects made by reactions that do not take part in the electron exchange, but lead to change of ion power, changes of pH, hydrolysis, complexing, precipitating, etc.

At 298 K (25 °C) and by converting natural (Napierian) logarithms into decimal (common, or Briggian) logarithms, Nernst’s equation for electrode potential can be written as follows:

dropping mercury electrode    →   kapajuća živina elektroda

Dropping mercury electrode (DME) is a working electrode arrangement for polarography in which mercury continuously drops from a reservoir through a capillary tube (internal diameter 0.03 - 0.05 mm) into the solution. The optimum interval between drops for most analyses is between 2 and 5 s. The unique advantage to the use of the DME is that the constant renewal of the electrode surface, exposed to the test solution, eliminates the effects of electrode poisoning.

electrode of the first kind    →   elektroda prvog reda

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).

electrode of the third kind    →   elektroda trećeg reda

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.

electrode potential    →   elektrodni potencijal

Electrode potential is defined as the potential of a cell consisting of the electrode in question acting as a cathode and the standard hydrogen electrode acting as an anode. Reduction always takes place at the cathode, and oxidation at the anode. According to the IUPAC convention, the term electrode potential is reserved exclusively to describe half-reactions written as reductions. The sign of the half-cell in question determines the sign of an electrode potential when it is coupled to a standard hydrogen electrode.

Electrode potential is defined by measuring the potential relative to a standard hydrogen half cell

The convention is to designate the cell so that the oxidised form is written first. For example

The e.m.f. of this cell is

By convention, at p(H₂) = 101325 Pa and a(H⁺) = 1.00, the potential of the standard hydrogen electrode is 0.000 V at all temperatures. As a consequence of this definition, any potential developed in a galvanic cell consisting of a standard hydrogen electrode and some other electrode is attributed entirely to the other electrode

indicator electrode    →   indikatorska elektroda

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.