Download/Embed scientific diagram | Diagrama de Pourbaix del hierro  y voltaje en función del pH (línea azul). from publication: Síntesis de nanopartículas. Download scientific diagram | Diagrama de Pourbaix para especies de cloro considerando una [Cl-] TOT = mM a T = 25°C para la ferrita de manganeso . ESPECIE. ΔG° (cal/mol). Pb. 0. PbO. , Pb3O4. , Pb2O3. , PbO2. , Pb2+. -5, Pb4+. 72, H2O. , REACCIONES DE.
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Predominant ion boundaries are represented by lines. As such a Pourbaix diagram can be read much like a standard phase diagram with a different set of axes.
Similarly to phase diagrams, they do not allow for reaction rate or kinetic effects.
Diagrama de Pourbaix – Wikipédia, a enciclopédia livre
The diagrams are named after Marcel Pourbaix —the Russian -born, Belgian chemist who invented them. Pourbaix diagrams are also known as E H -pH diagrams due to the labeling of the two axes.
The vertical axis is labeled E H for the voltage potential with respect to the standard hydrogen electrode SHE as calculated by the Nernst equation. The “H” stands for hydrogen, although other standards may be used, and they are for room temperature only. The lines in the Pourbaix diagram show the equilibrium conditions, that is, where the activities are pournaix, for the species on each side of that eiagramas.
On either side of the line, one form of the species will instead be said to be predominant. In order to draw the position of the lines with the Nernst equation, the activity of the chemical species at equilibrium must be defined. Usually, the activity of a species is approximated as equal to the concentration for soluble species or partial ve for gases.
The same values should be used for diagrajas species present in the system. Sometimes additional lines are drawn for other concentrations. While such diagrams can be drawn for any chemical system, it is important to note that the addition of a metal binding agent ligand will often modify the diagram.
For instance, carbonate has a great effect upon the diagram for uranium. See diagrams at right.
Diagrama de Pourbaix – Wikipedia, la enciclopedia libre
The presence of trace amounts of certain species such as chloride ions can also greatly affect the stability of certain species by destroying passivating layers. In addition, changes in temperature and concentration of solvated ions in solution will shift the equilibrium lines in accordance with the Nernst equation.
The diagrams also do not take kinetic effects into account, meaning that species shown as unstable might not react to any significant degree in practice. A simplified Pourbaix diagram indicates regions of “Immunity”, “Corrosion” and “Passivity”, instead of the stable species. They thus give a guide to the stability of a particular metal in a specific environment. Immunity means that the metal is not attacked, while corrosion shows that general attack will occur.
Passivation occurs when the metal forms a stable coating of an oxide or other salt on its surface, the best example being the relative stability of aluminium because of the alumina layer formed on daigramas surface when exposed to air. Poutbaix the simple case of a thermodynamic system consisting of a metal M and water, various reaction equations may be written having the form:.
The equation must be balanced for M, H, O and charge. A base-balanced equation can be converted to an acid-balanced equation using the equilibrium constant for the self-ionization of waterand only acid-balanced equations are considered below. There are three types of line boundaries in a Pourbaix diagram: Vertical, horizontal, and sloped.
The reaction equation may be written:. The reaction equation is:. The reaction equation is written:. As a function of temperature.
This will vary with temperature. Using the expressions for the free energy in terms of potentials, the energy ce is given by a Nernst equation:. The reaction equation is found to be:.
In many cases, the possible conditions in ed system are limited by the stability region of water. In the Pourbaix diagram for uranium, the limits of stability of water are marked by the two dashed green lines, and the stability region for water falls between these lines.
Below this line, water will be reduced to hydrogen, and it will usually not be possible to pass beyond this line as long as there is still water present to be reduced.
Diagrama de Pourbaix
Using the Nernst equation as above, but with an E 0 of 1. Above this line, water will be oxidized to form oxygen gas, and it will usually not be possible to pass beyond this line as long as there is still water present to be oxidized.
Pourbaix diagrams have several uses, for example in corrosion studies, geoscience and in environmental studies. Using the Pourbaix diagram correctly will help shed light not only on the nature of the species present in the solution or sample but may also help to understand the reaction mechanism.
Pourbaix diagrams are widely used to describe the chemical behaviour of chemical species in the hydrosphere. In these cases, pE is used instead of E H.
From Wikipedia, the free encyclopedia. Archived from the original on Environmental Chemistry – a global perspective 3rd ed. Geological Survey water-Supply Paper United States Government Printing Pourbakx.
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