Electrodes of the First Kind
These are the "typical" metal electrodes treated in the preceding sections, from which the metal emerges in the form of dissolved ions and remains in the electrode in accordance with the oxidation number z of the metal ions according to Me → Mez+ + z e-.
The equilibrium potential can be determined using the Nernst equation. The metal is available in two phases: Elemental in the electrode and dissolved in electrolytes.
Electrodes of the Second Kind
With electrodes of the second kind, the dissolved ions do not (or only to a small extent) dissolve in solution, but form with (usually) anions from the electrolyte a poorly soluble compound which deposits almost completely. The metal of the electrode of the second type exists in three different phases: Metallic, ionized in solution and deposited as a salt.
The equilibrium potential can also be calculated here from the Nernst equation, whereby the concentration of the free ions from the solubility product of the precipitating compound in the electrolyte must be determined.
An example of an electrode of the second kind is a silver electrode in a solution of potassium chloride, producing poorly soluble AgCl.
The silver is elemental in equilibrium (as electrode), deposited AgCl and to a small extent dissolved in the electrolyte (free Ag+ ions).
Electrodes of the Third Kind
If more than one substance of the electrolyte reacts with the electrode, a plurality of redox systems are present in parallel which, when both systems share the same anions, are coupled to one another via their solubility products. An example for electrodes of the third type is a silver electrode dipped in AgS and PbS.
Redox electrodes do not go into solution, but only take part as electron donor or acceptor and if necessary, as a catalyst for the electro-chemical redox reactions. The platinum electrodes used in the electrolysis of diluted hydrochloric acid to form H2 and Cl2 are such redox electrodes. Redox electrodes are also used as measuring probes, for example, for pH measurement, where the electrode must not dissolve so that it does not contaminate the medium to be measured, on the one hand, and on the other hand, for stable measurements, where no surface and shape change may occur.
A gas electrode is a metal electrode bathed by a certain gas which is ideally adsorbed as a continuous monolayer on the metal surface. An example of a gas electrode is the standard hydrogen electrode as a reference electrode for determining the standard potential of a substance. This is a platinum electrode bathed by hydrogen, on the catalytically active surface of which H2 is split to form hydrogen atoms, which are deposited on the platinum.
Neutral, ionized and dissolved substances
The theoretical maximum amount of a substance (in grams) deposited on the electrode per ampere hour is defined by the formula AE = M/(z F), where M is the molar mass, F is the Faraday constant, and z is the oxidation number of the substance.
Typical values for metals are between 0.3 g/Ah for the light Al with the oxidation number +3, and 7.3 for the heavy gold with the oxidation number +1.
The dissociation of a compound into anions and cations in a solvent, e.g. saline in water according to NaCl → Na+ + Cl- or hydrochloric acid in water via HCl + H2O → H3O+ + Cl-. The degree of dissociation of a substance indicates the proportion of this substance which is dissociated under the given conditions (concentration, pressure, temperature, ...).
Adsorption of water molecules to dissolved ions via ion/dipole interactions, or to neutral molecules via hydrogen bonds or/and interactions between static (H2O molecule) and induced dipoles. The associated hydration energy as well as the change in the entropy (due to a stronger localization of the water molecules) has a decisive contribution to the free enthalpy of the solution of a substance and thus helps determine its solubility.
The energy necessary to ionize an atom or molecule. Within a period of the periodic table, the first ionization energy (removal of an electron from the neutral atom) increases due to the increasing atomic number (e.g. Li ... Ne: 5.4 ... 21.6 eV), in order to rapidly fall back again to the next period due to the now next higher shell occupied by the atomic nucleus. Already n-fold ionized substances have a significantly higher n ionization energy (e.g. iron: 7.9 eV, 16.2 eV and 30.6 eV for the first, second and third ionization).
The maximum concentration cmax of a substance in another where the mixture is at equilibrium in single phase. In the case of substances with an exothermic solution reaction (e.g. NaOH), the solubility decreases with increasing temperature; for substances with an endothermic solution reaction (e.g. KNO3 or NH4NO3) the solubility increases with temperature. The size of cmax defines whether a substance is, for example, easily or poorly soluble.
The normal or standard potential of a substance is the potential that exists between this substance and a hydrogen electrode when both are dipped in a 1 molar solution of the substance under standard state (25°C, 101.3 kPa). The more positive the standard potential, the more "noble" the substance or the metal (e.g. lithium = -3.02 V, gold = +1.7 V).
The charge number of an atom in a bond assuming that the valence electrons of an atom or molecule are each attributed to the more electro-negative ligand. Atoms in the elementary state always have the oxidation number 0, for monatomic ions the oxidation number corresponds to the ion charge (e.g. Fe3+ has the oxidation number +3).
Electrolytes and Electrodes
The ability of an electrolyte to deposit a close metal layer without holes. Particularly at low average current densities, local fluctuations in the current density can lead to areas on the surface of the workpiece, on which the current density is too low for a sufficient metal deposition.
Conductivity of the Electrolytes
Electrolytes are pure ionic conductors with a certain electrical conductivity. Theoretically, the conductivity can be derived from the mobilities of the (different) ions contained in the electrolyte, the concentration of which is determined by the concentration of the contents as well as their degree of dissociation dependent on temperature and concentration.
In practice, interactions between the ions as well as polarization effects on the electrolyte/electrode interface also play a role, so that the system electrode/electrolyte behaves only in a more or less good approximation like an ohmic resistance.
Pure water contains, via the autoprotolysis H2O + H2O → H3O+ + OH- at room temperature about 10-7 mol H3O+ and OH- per litre, which accordingly
pH = -log10(H3O+)
corresponds to a neutral pH of 7. Because the autoprotolysis is thermally activated, the pH value of 100°C hot water is already approximately 6. Acids as proton donors increase in aqueous solutions via the dissociation of protons (e.g. hydrochloric acid: HCl + H2O → H3O+ + Cl-) the concentration of H3O+ ions, where the pH-value drops. Some examples of pH values:
Due to the geometric shape of the electrodes and the inhomogeneous surface of the electrode (roughness, nanocrystallinity) which is on a microscopic scale, the current density varies across the surface of the electrodes, which has an effect on the electro-chemical reactions occurring on the electrode. A distinction is made here between ...
Brightness Throwing Power
Electrolytes, in which glossy layers can be deposited over a large range of the current density distribution, have a high brightness throwing power
Electrolytes with high macro scattering allow uniform layer growth over a certain range of the current density distribution varying by the geometric electrode shape.
Electrolytes with high micro scattering enable uniform layer growth over a certain range of the current density distribution varying due to the roughness of the electrode surface.
The ratio of the amount of metal actually deposited on the cathode to the theoretical value calculated according to the electro-chemical equivalent. Values < 100% are due to reactions parallel to metal deposition, which also consume electrons (e.g. H2 formation from H3O+ ions in the electrolytes). Values above 100% occur when the anode dissolves not only electrolytically, but also chemically, with released electrons being available for metal deposition.,
The difference between the voltage required for the electrolysis and the theoretical potential difference derived from the electrochemical voltage. The causes of overvoltage are energy-consuming processes in the electro-chemical reaction chain, such as diffusion, adsorption, stripping of the hydration shell, gas formation, etc.
The necessary potential difference between the cathode and the anode calculated theoretically for the electrolysis according to the Nernst equation, plus the overvoltage.
Conductivity of the Electrolytes
The ability of a material not to break during external mechanical (overload) loading but to deform.
A few ductile substances, such as glass at room temperature, are brittle and break easily, while soft metals such as gold with high ductility can be readily formed even at room temperature (e.g. such as gold leaf). The ductility of a substance depends greatly on its crystalline structure and thus the deposition conditions of the layer.
Shine and brightness
Which surface properties of a substance lead to a brilliant impression is not yet fully understood, even if a very smooth, fine-crystalline structure plays an important role. In electro-plating deposition, this generally requires a high nucleation density while suppressing the growth of these nuclei to larger crystallites.
The mechanical resistance where a material opposes the penetration of a foreign body. Hard layers do not easily scratch and wear less due to abrasion. There are various methods of hardness testing, such as the Rockwell hardness test (penetration of a test specimen into the substance), according to Brinell (determination of the area of the impression of a steel ball pressed onto the test specimen) or Mohs: The Mohs hardness is an empirical table of unified values between 1 (talc) and 10 (diamond) in which a substance is capable of scratching another substance which is below this substance on the scale (greater Mohs hardness). The hardness of a substance depends strongly on its (nano)crystalline structure and thus the deposition conditions of the layer.
The ratio of the microscopic void volume (pores) of a substance to its total volume. In material engineering, porosity is classified according to micro-porous (pores < 2 nm), meso-porous (2 ... 50 nm) and macro-porous (> 50 nm). The attained porosity of a secluded metal layer strongly depends on the deposition conditions and can only theoretically reach a value of 1.