Analysis of Figure 1 gives the following information.
Figure 1 Electrochemical equilibrium diagram of water
(298K, =0.29atm)
First, the thermodynamic stability of water
The area between line a and line b in Figure 1 is the thermodynamically stable zone of water in which water does not decompose to produce hydrogen and oxygen. However, if the pressure of oxygen or hydrogen is increased, the system is saturated with gaseous oxygen or gaseous hydrogen. In this case, the system can still be oxidized by oxygen or reduced by hydrogen, or the upper and lower limits of the thermodynamically stable region of water will follow oxygen or hydrogen. The pressure increases and expands outward.
Second, the redox of water
The area outside the a-line and the b-line in Figure 1 is the thermodynamically unstable region of water in which water may decompose. Below the a-line, the water tends to undergo a reduction reaction (1), and the hydrogen is separated and the acidity of the solution is lowered. This area is of little significance for mineralogy, and the Eh value below the a-line is unlikely to be maintained within the time span of geology. But for a much shorter time span hydrometallurgical process, it is possible to maintain the Eh value of a line, it referred to in this area might hydrogen overpotential region. When a reducing agent (such as Zn) having an electrode potential lower than the potential of the hydrogen electrode exists in the water, the hydrogen ion is reduced under acidic conditions to release hydrogen (such as Zn+2H + →Zn 2 + +H 2 ) until the reducing agent electrode potential The potential is increased and the potential of the hydrogen electrode is lowered by the decrease in the acidity of the medium, and the potentials of the two electrodes are finally equal.
(1)
Below the line b, water tends to undergo an oxidation reaction (2), which resolves oxygen and increases the acidity of the solution. Since the known natural minerals have not been found to be capable of oxidizing water, this area has little significance for mineralogy. In hydrometallurgical systems, various oxidants may be encountered, such as Cl 2 , ClO - , ClO 3 - , MnO 4 - , S 2 O 8 2 - ; SO 5 2 - , MnO 2 and PbO 2 , etc. Both can oxidize water. Until the oxidant electrode potential decreases with the decrease in activity, the potential of the oxygen electrode rises due to an increase in the acidity of the medium, and the potentials of the two electrodes are finally equal. However, in some areas above the b-line, the oxidation rate of water is extremely slow, and does not affect its use as a solvent, which can be called an oxygen overpotential zone.
(2)
Third, the acidity and alkalinity of water
According to the acid-base theory, the hydrogen ion activity and the oxygen ion activity are defined as the neutral conditions of the aqueous solution. At 25 ° C, the pH = 7.00 vertical line in Figure 1 is the neutral line of the solution. The left side of the line is an acidic solution and the right side is an alkaline solution. In the usual pH-Eh diagram, the neutral line has been omitted.
Fourth, the absolute neutrality of water
The absolute neutrality of water refers to the neutral condition in which water conforms to both acidity and redox. The acid-alkaline neutral condition with water is similar to the hydrogen ion activity and the hydroxide ion activity. The redox neutrality of water is equal to the potential of the two half-cell reactions of oxidation and reduction of water, ie Ea=Eb . Where Ea corresponds to the potential of reaction (1) and Eb corresponds to the potential of reaction (2).
Ea=―0.0592pH-0.0296lg
Eb=1.229-0.0592pH+0.0148lg
Let rO=-lg ,rH=-lg The above two formulas can be written as
Ea=―0.0592pH+0.0296rH
Eb=1.229-0.0592pH-0.0148rO
On the Eh-pH map, for each rH or rO value, the above equations are each a diagonal line.
It can also be considered according to the electrolysis reaction of water 2H 2 O=2H 2 +O 2 , the redox neutral condition of water =2 Thus, when redox neutral,
rH=27.56
rO=27.86
According to the above conditions, the electrochemical equilibrium diagram of water can be divided into four zones, as shown in FIG. The lower left and lower right areas are respectively a reducing and acidic zone and a reducing and alkaline zone, and the upper left zone and the upper right zone are oxidizing and acidic zones and oxidizing and alkaline zones, respectively.
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