Advantage zone map
Eh and pH are reflected in the potential of the metal - and mineral water - two thermodynamic properties of the most important parameters of the water system, so predominance area diagram in the variable as pH and Eh, and pH shown some potential conditions at a specified temperature Stable state or equilibrium state of the system within the range. If the system involves a gas such as CO 2 participating in the reaction or dissolved in water, the gas partial pressure and pH can also be used as variables.
Considering the relationship between system potential and pH, the chemical reactions occurring in aqueous solution can be divided into four types: (1) reactions involving electrons without hydrogen ions; (2) reactions involving no electrons with hydrogen ions; a reaction involving both electrons and hydrogen ions; (4) reactions involving neither electrons nor hydrogen ions. The first three types of reactions can represent the thermodynamically most stable chemical components of the reaction system under given Eh-pH conditions in the form of a dominant zone map. Corresponding to these three types of reactions, there are three forms of equilibrium lines on the dominant area map. The corresponding type (1) reaction is a set of equilibrium lines parallel to the axis of abscissa (pH axis), and each line corresponds to a value of activity. For a given ion activity, when the potential is higher than the corresponding equilibrium line, the electrode reaction proceeds from the reducing body to the oxidant, that is, an oxidation reaction occurs, so that the oxidant side of the electrode reaction is stable. Conversely, when the potential is lower than the corresponding equilibrium line, the side of the reducing body of the electrode reaction is stable. The corresponding type (2) reaction is a set of equilibrium lines parallel to the ordinate axis (Eh axis), each line corresponding to a pH value. When the pH of the solution is greater than the corresponding equilibrium line, the reaction will be consumed to produce H + or OH - in the direction. For a direction - the pH of the solution is less than the corresponding equilibrium line, the reaction will be consumed to generate H + or OH. The corresponding type (3) reaction is a diagonal line.
The redox reaction is characterized by the participation of electrons in the reaction. Although free protons and electrons are not present in the aqueous solution, relative proton and electron activity can still be determined. In the equilibrium equation, the treatment of e and H + is the same, so similar to pH=-lg|H + |, a scale of reaction redox intensity can also be defined.
Pe=-lg|e| (1)
Pe is the (imaginary) electron activity in equilibrium, which measures the relative tendency of a solution to accept or migrate electrons. In a strongly reducing solution, the imaginary "electron pressure" or electron activity is large, and the tendency is to give electrons. Just as the hypothetical proton activity is very low at high pH values, the hypothetical electron activity at low pe values ​​is also low, indicating a relatively high oxidation tendency.
Consider redox balance
Ox+ze=Red
The equilibrium constant can be written to include the electronic activity
then
(2)
The potential E is determined by the ratio of the activity of the two states of the reduced state and the oxidized state, as follows
(3)
The standard potential E Θ corresponds to the value when the two ion activities are equal.
The potential E value can be calculated from the relationship - ΔG = zEF from the change in free energy occurring in the reaction, thus
Substituting equation (2) into the above formula
(4)
Equation (4) reflects the relationship between E and pe, and E = 0.05917 pe at 25 °C.
The parameter pe provides a dimensionless scale like pH. And E is used to measuring with volts. Here are a few chemical reaction equations written in pe, and also give the corresponding reaction equation written in E.
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