CHEMISTRY GLOSSARY

redox    →   redoks

Redox is a short form of the term for oxidation-reduction.

redox potential    →   redoks potencijal

Redox potential is the potential of a reversible oxidation-reduction electrode measured with respect to a reference electrode, corrected to the hydrogen electrode, in a given electrolyte.

redox reaction    →   redoks-reakcija

Redox reaction is an oxidation-reduction reaction is a reaction in which one or more electrons are transferred. When an atom, ion, or molecule loses one or more electrons, it is oxidised. When an atom, ion, or molecule gains one or more electrons, it is reduced.

redox titration    →   redoks-titracija

Redox titration (oxidation-reduction titration) is a titration based on a redox reaction. For example, iron in water can be determined by converting dissolved iron to Fe²⁺ and titrating the solution with potassium permanganate (KMnO₄), a powerful oxidising agent.

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    →   indikator

Indicator is a substance used to show the presence of a chemical substance or ion by its colour. Acid-base indicators are compounds, such as phenolphtaleine and methyl orange, which change colour reversibly, depending on whether the solution is acidic or basic. Oxidation-reduction indicators are substances that show a reversible colour change between oxidised and reduced forms.

oxidation    →   oksidacija

The term oxidation originally meant a reaction in which oxygen combines chemically with another substance. More generally, oxidation is a part of a chemical reaction in which a reactant loses electrons (increases oxidation number). Simultaneous reduction of a different reactant must occur (redox reaction).

galvanic celll    →   galvanski članak

Galvanic cell (voltaic cell) is a simple device with which chemical energy is converted into electrical energy. Galvanic cells consist of two separate compartments called half cells containing electrolyte solutions and electrodes that can be connected in a circuit. Two dissimilar metals (e.g., copper and zinc) are immersed in an electrolyte. If the metals are connected by an external circuit, one metal is reduced (i.e., gains electrons) while the other metal is oxidized (i.e., loses electrons).

In the example above, copper is reduced and zinc is oxidized. The difference in the oxidation potentials of the two metals provides the electric power of the cell.

A voltaic cell can be diagrammed using some simple symbols. In the diagram the electrodes are on the outer side of the diagram and a vertical line (|) is used to separate the electrode from the electrolyte solution found in the compartment. A double vertical line (||) is used to separate the cell compartments and is symbolic of the salt bridge. Usually in a diagram the species oxidized is written to the left of the double slash. Here is an example of the Daniell cell:

The names refer to the 18th-century Italian scientists Alessandro Volta (1745-1827) and Luigi Galvani (1737-1798).

Nernst’s electrode potential equation    →   Nernstova jednadžba za elektrodni potencijal

For general reaction of some redox system

dependence of electrode potential of redox system upon activity of oxidised and reduced form in solution is described in Nernst’s equation for electrode potential:

where E = to electrode potential of redox system

E° = standard electrode potential of redox system

R = universal gas constant

T = thermodymical temperature

F = Faraday’s constant

z = number of electrons exchanged in redox reaction

a_O = activity of oxidised form

a_R = activity of reduced form

n = stechiometrical coefficient of oxidised form

m = stechiometrical coefficient of reduced form

potentiometric titration    →   potenciometrijska titracija

Potentiometric titration is a volumetric method in which the potential between two electrodes is measured (referent and indicator electrode) as a function of the added reagent volume. Types of potentiometric titrations for the determination of analytes in photoprocessing solutions include acid-base, redox, precipitation, and complexometric.

Potentiometric titrations are preferred to manual titrations, since they are more accurate and precise. They are also more easily adapted to automation, where automated titration systems can process larger volumes of samples with minimal analyst involvement.

A titration curve has a characteristic sigmoid curve. The part of the curve that has the maximum change marks the equivalence point of the titration. The first derivative, ΔE/ΔV, is the slope of the curve, and the endpoint occurs at the volume, V', where ΔE/ΔV has the maximum value.