Calculation force pH and EH

Publish: 2021-04-22 18:20:39

1. No matter what the reaction form is, the so-called oxidation means the loss of electrons, and the so-called rection means the gain of electrons, which must be accompanied by the process of electron giving and receiving. When the platinum electrode is inserted into the reversible redox system ah2a 2E 2h, electrons will be given to the electrode and become a half cell with the potential corresponding to the redox capacity of the system. The redox potential of the system is measured by combining it with a standard hydrogen electrode. The redox potential EH is determined by the free energy (or equilibrium constant) of the redox H2, pH, the ratio of redox to redox [ox] / [red], and the following formula is obtained:

(R is the gas constant, t is the absolute temperature, F is the Faraday constant, and N is the number of electrons related to the redox of the system). E 'is the EH of oxidation type and rection type. When pH is f, it is called standard potential, which is an indicator of the redox ability of the system. When the rection rate corresponding to EH is plotted, the S-shaped curve with E0 as symmetry point can be obtained. The system with higher eh can oxidize the system with lower eh, and the equilibrium can be reached when the EH of the two systems are equal. But it's just a thermodynamic phenomenon. In fact, especially for most biological systems, recognizable reactions will not occur without the addition of enzymes and electron transporters. The redox potential can be determined directly by using redox indicator according to the calculation of equilibrium constant. Generally, the electron transfer in organism is from low redox potential to high redox potential, for example, nad → flavinase → cytochrome c system → O2. However, e to the specificity of enzyme and its inhibition, it is not carried out in this way. Due to the concentration of reactive components, it is possible that the system with low standard potential will oxidize the system with high standard potential. In the redox system of organisms, polyphenols and cytochrome c and a are around 200-300 MV, cytochrome b and flavinase are at 0-100 MV, nad is at - 330 MV and iron redox protein is at - 420 MV. In living cells, the potential of aerobic cells is high, while that of anaerobic cells is low. The activity of enzymes, the ability of cell assimilation and the growth and development of microorganisms are also affected by redox potential.

2. PH is pH, EH is the redox potential of platinum electrode relative to standard hydrogen electrode, both of which are comprehensive indicators of environmental physical and chemical properties of media (including water, soil, etc.). The pH value reflects the acidity and basicity of the medium, and the EH value indicates the relative degree of the oxidation or rection of the medium. In marine sediments, eh and pH are two good comprehensive indexes reflecting sedimentary environment

3. There is no formal principle for this naming. What you are talking about is the original name of JST, which is the common name of the instry. For the same proct, many alternative (imitation) manufacturers refer to the instry leaders. You can see these series of procts you mentioned on the JST website

4. These are the distances between terminal pins (spacing, pitch and pH), and several commonly used distances are supplemented:
fh0.5mm
sh1.0mm
gh1.25mm
zh1.5mm
ph2.0mm
eh / xh2.5/2.54mm (the two are actually the same)
vh3.96mm

5. There are many equilibrium phases in soil. In the solid-liquid equilibrium, some equilibria are affected by pH and eh values. Taking pH and eh values as independent variables (coordinates), the diagram that reflects the equilibrium relationship between pH and eh values and the number of phases in the system, the composition of each phase and the concentration of components is called pH eh phase diagram
⊿ eh / ⊿ pH is the change of ion concentration corresponding to the acidity between two points on the curve.

From the previous discussion, we know that the value of PE reflects the abundance of electrons in the environment. A large number of available electrons will give a recing environment, while the lack of available electrons will give an oxidizing environment. We can also think that the pH value reflects the abundance of protons in the environment. A large number of available protons will give an acidic environment, while the lack of available protons will give an alkaline environment. Because protons and electrons have opposite charges, it can be predicted that in a given environment, the oxidizing environment (high PE value) will tend to be acidic (low pH value), while the recing environment (low PE value) will tend to be alkaline (high pH value). In other words, one is rich and the other is necessarily poor. In Figure 2-4, the lines (a) and (b) are inclined to the right, and the reaction lines drawn on the PE pH line are inclined to the right. Because when the pH value increases, the reaction will have a lower PE value

with the above theoretical knowledge, we can use the PE pH diagram to explain the balance between some minerals and solutions in nature, and the existence of some dissolved species in water. For example, H 2 0 - < inline media object > < imageobject > < imagedata role = 3 "fileref = image / 046_ 01. JPG "> < title > < / Title >

< / picdesc > < / imagedata > < / imageobject > < / inlinemediaobject > -- H 2 s system, as shown in Figure 2-6

Fig. 2-6 PE pH diagram of sulfur system (25 ℃, 101325pa) (according to Shen Zhaoli et al. < hydrogeochemical basis)

If only h + participates in the reaction

hydrogeochemical basis

the free energy of this formula changes to

hydrogeochemical basis

when [H 2 s] = [HS - ], Therefore, on the PE pH diagram, there is a vertical line at pH = 7, which distinguishes the stability of H 2 s and HS - . When [H 2 s] greatly exceeds [HS - ], the pH value is less than 7. When [H 2 s] < HS - ], the pH value was more than 7

If there are electrons and protons in the solution participating in the reaction at the same time, such as:

hydrogeochemical basis

the change of free energy of this formula is:

hydrogeochemical basis

when < inline media object > < imageobject > < imagedata role = "3" fileref = "image / 047"_ 3. When JPG "> < title > < / Title >

< / picdesc > < / imagedata > < / imageobject > < / inlinemediaobject >, PE = 4.25-1.125ph, that is to say, in Figure 2-6, < inlinemediaobject > < imageobject > < imagedata role =" 3 "fileref =" image / 047_ 4. JPG "> < title > < / Title >

< / picdesc > < / imagedata > < / imageobject > < / inlinemediaobject > and HS - are separated from each other

the dissolution reaction between solid and liquid with protons and electrons. For example:

basis of hydrogeochemistry_ When 6. JPG "> < title > < / Title >

< / picdesc > < / imagedata > < / imageobject > < / inlinemediaobject > is 10 - 1 mol · l - 1 ,

PE = 5.87-1.33ph

this is solid sulfur and < inlinemediaobject > < imageobject > < imagedata role =" 3 "fileref =" image / 047_ 7. JPG "> < title > < / Title >

< / picdesc > < / imagedata > < / imageobject > < / inlinemediaobject > the dividing line of stable field, in water < inlinemediaobject > < imageobject > < imagedata role =" 3 "fileref =" image / 047_ 8. JPG "> < title > < / Title >

< / picdesc > < / imagedata > < / imageobject > < / inlinemediaobject > has different values. We will get the equations of different concentrations and the parallel lines of the above lines. Other reactions are shown in table 2-3

Table 2-3 transformation and equilibrium equation of sulfur system. If we give the total activity of sulfur ES = 10 - 1 mol · l - 1 , it can be considered that the activity of dominant species in any dominant field is very close to that of dissolved total sulfur, so we can draw the PE pH diagram by substituting the activity of total sulfur into the ionic charged molecular matter with sulfur in table 2-3 (Fig. 2-6)

iron is the most abundant and widely distributed metal element in the oxidation-rection reaction of groundwater, so it is very important to study the law of iron. The PE pH dominant fields of different iron compounds are shown in Fig. 2-7. The redox equilibrium constant of iron system is shown in table 2-4

Fig. 2-7fe-h 2 o-co 2 stable field of the system (at 25 ℃, 101325pa, {Fe t (total) } = 10 - 6 0mol, Σ CO 2 = 10 - 2.3 mol (according to Shen Zhaoli et al.

Table 2-4 iron system oxidation-rection equilibrium constant table (according to Shen Zhaoli et al. < hydrogeochemical basis)

it can be seen from figure 2-7 that in the oxidation zone, the activity of Fe 3 + does not reach the solubility proct of Fe (OH) 3 ), Among the possible forms of iron, the one with the largest stable field is < inline media object > < imageobject > < imagedata role = 3 "fileref = image / 049_ 1.jpg">

When pH < 5, Fe (OH) 2 + and < inlinemediaobject > < imageobject > < imagedata role = 3 "fileref = image / 049_ 2. The stable field of JPG "> < title > < / Title >

< / picdesc > < / imagedata > < / imageobject > < / inlinemediaobject > is dominant. In the rection environment, only Fe 2 + exists when pH value is less than 8, and FeCo 3 precipitates when pH value is more than 8, followed by < inline media object > < imageobject > < imagedata role = 3 "fileref = image / 049_ 3. Advantage field of JPG "> < title > < / Title >

< / picdesc > < / imagedata > < / imageobject > < / inlinemediaobject >. In general groundwater, < inlinemediaobject > < imageobject > < imagedata role = "3" fileref = "image / 049"_ 4. JPG "> < title > < / Title >

< / picdesc > < / imagedata > < / imageobject > < / inlinemediaobject > and < inlinemediaobject > < imageobject > < imagedata role =" 3 "fileref =" image / 049_ 5. JPG "> < title > < / Title >

< / picdesc > < / imagedata > < / imageobject > < / inlinemediaobject > does not exist

We know that the value of PE reflects the abundance of electrons in the system. A large number of available electrons will give the system a recing environment, while the lack of available electrons will give the system an oxidizing environment. The pH value reflects the abundance of protons in the system. A large number of available protons will give the system an acidic environment, while the lack of available protons will give the system an alkaline environment. Because protons and electrons have opposite charges, it can be predicted that in a given environment, the two must have a growth and decline relationship, that is, one is rich, the other is poor. That is to say, the oxidizing environment (high PE value) will tend to be acidic (low pH value), while the recing environment (low PE value) will tend to be alkaline (high pH value). If pH is taken as abscissa and PE (EH) as ordinate, the relationship between PE (EH) value and pH value of all electrodes in a given system is drawn under certain conditions, that is, the PE (EH) - pH diagram of the system, then the direction of redox reaction and the stable state of each component in the system can be determined according to this diagram, so this kind of diagram is also called stable field diagram. PE (EH) - pH diagram has important application in the study of redox equilibrium of natural water and metal corrosivity. Taking Fe-H 2 O-O 2 and S-H 2 O-O 2 systems as examples, the drawing method of PE (EH) - pH diagram is explained

The EH pH diagram of

2.4.3.1 Fe-H 2 O-O 2 system is shown in table 2-4-1

table 2-4-1 chemical reactions of Fe-H 2 O-O 2 system

for reaction 1 in table 2-4-1, its electrode potential is:

hydrogeochemistry

assuming [Fe 2 + ] = [Fe], equation (2-4-31) becomes:

hydrogeochemistry

Draw equation (2-4-32) into eh pH diagram to obtain the straight line ① in figure 2-4-3. Obviously, the point of [Fe 2 + ] = [Fe] falls on the straight line, the point of [Fe 2 + ] > should fall on the top of the straight line, and the point of [Fe 2 + ] < should fall on the bottom of the straight line

for reaction 2 in table 2-4-1, its electrode potential is:

hydrogeochemistry

assuming that [Fe 3 + ] = [Fe 2 + ], equation (2-4-32) becomes:

hydrogeochemistry

Draw equation (2-4-34) into eh pH diagram to obtain straight line ② in figure 2-4-3. In this way, the point of [Fe 3 + ] = [Fe 2 + ], the point of [Fe 3 + ], the point of [Fe 2 + ], and the point of [Fe 3 + ], the point of [Fe 2 + ], the point of [Fe 2 + ], are just above the line

for reactions 3 and 4 in table 2-4-1, the equilibrium constants are respectively:

hydrogeochemistry

it is assumed that [Fe 2 + ] = [Fe (OH) 2 ], [Fe 3 + ] = [Fe (OH) 3 ], Then (2-4-37) and (2-4-38) have:

hydrogeochemistry

Draw formulas (2-4-39) and (2-4-40) into eh pH relationship diagram, and straight lines (3) and (4) in figure 2-4-3 can be obtained respectively. In the figure, the point of [Fe 2 + ] = [Fe (OH) 2 ] just falls on the line ③, the point of [Fe 2 + ] > [Fe (OH) 2 ] falls on the left side of line ③, and the point of [Fe 2 + ] < Fe (OH) 2 ] falls on the right side of line ③; Similarly, the point of [Fe 3 + ] = [Fe (OH) 3 ], the point of [Fe 3 + ], the point of [Fe (OH) 3 ], and the point of [Fe 3 + ], the point of [Fe (OH) 3 ] fall on the left side of line 4, and the point of [Fe 3 + ], the point of [Fe (OH) 3 ] fall on the right side of line 4

for reactions 5, 6 and 7 in table 2-4-1, the electrode potentials are as follows:

hydrogeochemistry

assuming that [Fe (OH) 3 ] = [Fe 2 + ], [Fe (OH) 2 ] = [Fe] and [Fe (OH) 3 ] = [Fe (OH) 2 ], Then we can get:

hydrogeochemistry

draw the equation (2-4-44 ~ 46) into the EH pH relationship diagram, and then we can get the straight lines in figure 2-4-3. Obviously, the point of [Fe (OH) 3 ] = [Fe 2 + ] just falls on the straight line, the point of [Fe (OH) 3 ] > [Fe 2 + ] falls on the top of the straight line, and the point of [Fe (OH) 3 ] < Fe 2 + ] falls on the bottom of the straight line; Similarly, the point of [Fe (OH) 2 ] = [Fe] falls on the straight line 6, the point of [Fe (OH) 2 ] > falls on the top of the straight line 6, and the point of [Fe (OH) 2 ] < falls on the bottom of the straight line 6 The point of Fe (OH) 3 ] = [Fe (OH) 2 ], the point of [Fe (OH) 3 ] > the point of [Fe (OH) 2 ], and the point of [Fe (OH) 3 ] the point of [Fe (OH) 2 ], the point of [Fe (OH) 2 ] are just above the straight line, and the point of [Fe (OH) 2 ] is below the straight line

from figure 2-4-3, we can find the following rule:

(1) high oxidation substances are generally located above the corresponding low oxidation substances. For example: Fe 3 + above Fe 2 + , Fe (OH) 3 above Fe (OH) 2 , Fe 2 + above Fe, etc

(2) the solid component is generally located on the right side of the corresponding liquid component. For example, Fe (OH) 3 is located on the right side of Fe 3 + and Fe (OH) 2 is located on the right side of Fe 2 +

Fig. 2-4-3 eh pH structure diagram of Fe-H 2 O-O 2 system. There are five types of components in the figure, which are: Fe 3 + , Fe 2 + , Fe (OH) 3 , Fe (OH) 2 , Fe. Each component has a dominant region, in which the concentration of this component is higher than the other four components. Our task is to determine the dominant region of these five components. The known condition here is that the boundary line of the dominant region of each component is composed of seven straight lines in EH pH structural line diagram and part or all of the boundary lines of the graph. For example, the dominant region of Fe 2 + is the polygon ABCDE in figure 2-4-3, because: from the line ② in the figure, the region under the line, [Fe 2 + ] > [Fe 3 + ]; It can be seen from line 5 that in the lower left part of the line, [Fe 2 + ] > [Fe (OH) 3 ]; At the same time, it can be seen from line 3 that on the left side of the line, [Fe 2 + ] > [Fe (OH) 2 ]; It can be seen from the line ① that in the upper part of the line, [Fe 2 + ] > [Fe]. For example, line 6 is the boundary between Fe (OH) 2 and Fe, which indicates that above the line, [Fe (OH) 2 ] > [Fe], below the line, [Fe] > [Fe (OH) 2 ]; But because the upper part of the straight line, [Fe 2 + ] > [Fe] has been determined by the straight line ①, and the left side of the straight line, [Fe 2 + ] > [Fe (OH) 2 ] has been determined by the straight line ③, so in the area above the straight line ① and on the left side of the straight line ③, [Fe 2 + ] is not only greater than [Fe (OH) 2 ], but also greater than [Fe]. Similarly, the dominant regions of the other four components can be determined. Thus, the stable field diagram of Fe-H 2 O-O 2 system is obtained as shown in Fig. 2-4-4

Fig. 2-4-4 stable field diagram of Fe-H 2 O-O 2 system

2.4.3.2 PE pH diagram of S-H 2 O-O 2 system

It is similar to Fe-H 2 O-O 2 system. The following is the order in table 2-4-2 to draw the relationship curve of PE pH for each reaction

table 2-4-2 electrode reactions of S-H 2 o 2 system, The equilibrium constant of the reaction is:

hydrogeochemistry

taking logarithm of both sides at the same time:

hydrogeochemistry

namely

hydrogeochemistry

make [HS - ] = [H 2 s], Equation (2-4-48) becomes:

hydrogeochemistry

Draw equation (2-4-49) into the PE pH diagram to obtain the straight line (1) in Figure 245. Obviously, the point of [HS - ] = [H 2 s] just falls on the straight line ①, the point of [HS - ] > [H 2 s] should fall on the right side of the straight line ①, and the point of [HS - ] < [H 2 s] should fall on the left side of the straight line ①

For reactions 2 and 3 in table 2-4-2, the equilibrium constants of reactions are as follows:

hydrogeochemistry

logarithm is taken on both sides of equations (2-4-50) and (2-4-51) and sorted out:

hydrogeochemistry

let [s 2 - ] = [HS - ], (2-4-52),

hydrogeochemistry

respectively (2-4-53) are as follows:

hydrogeochemistry

Draw formulas (2-4-54) and (2-4-55) into the relationship between PE and pH to obtain straight lines (2) and (3) in Figure 245 respectively. In the figure, the point of [s 2 - ] = [HS - ] just falls on the straight line ②, the point of [s 2 - ] > [HS - ] falls on the right side of the straight line ②, and the point of [s 2 - ] < [HS - ] falls on the left side of the straight line ②; Similarly, the point of = falls on line 3, the point of = falls on the right side of line 3, and the point of = falls on the left side of line 3

for reaction 4 in table 2-4-2, its electronic activity is:

hydrogeochemistry

order = [S], equation (2-4-56) changes to:

hydrogeochemistry

Draw equation (2-4-57) into the relationship between PE and pH, and the straight line in figure 2-4-5 can be obtained. Obviously, the point of = [S] just falls on line 4, the point of ＞ [S] should fall on the upper right side of line 4, and the point of ＜ [S] should fall on the lower left side of line 4

for reactions 5, 6, 7, 8 and 9 in table 2-4-2, their electronic activities are as follows:

hydrogeochemistry

respectively = [S], [S] = [H 2 s], = [HS - ], = [s 2 - ], [S] = [HS - ], From (2-4-58) ～ (2-4-62) have:

hydrogeochemistry

draw the formula (2-4-63) ～ (2-4-67) to the relationship between PE and pH, we can get the straight lines in Figure 2-4-5, such as ⑤, ⑥, ⑦, Ⅷ and Ⅸ. In this way, we can get the PE pH structure line diagram of S-H 2 O-O 2 system (Fig. 2-4-5). In the diagram, the line 10 is the upper limit of water stability field, and the line 9322; Is the lower limit of water stable field

in s-h

EH pH diagram takes eh as the ordinate and pH as the abscissa, which represents the diagram of stable field of various dissolved components and solid components within a certain eh value and pH value range, so it is also called stable field diagram. It should be noted that the EH pH diagram only shows the stable field of various dissolved components and solids in the aqueous solution when the redox reaction reaches the equilibrium state under the standard state, while the natural groundwater system usually deviates from the equilibrium state. EH pH diagram can predict the possible dissolved components and solid species within a certain eh pH range, but it can not predict the reaction rate

in order to better understand the EH pH map and apply it to explain some hydrogeochemical problems, it is necessary to understand the methods and proceres for drawing eh pH map. Therefore, a simple eh pH diagram of Fe-H 2 O-O 2 system is introced

because we are concerned about the equilibrium of the reaction between dissolved substances and minerals in the water environment, we must first study the stable field of water itself when we study any eh pH diagram

The upper limit of water stability field is determined by the reaction between water and oxygen. The reaction is as follows:

hydrogeochemical basis

in the standard state, according to the formula (1.95), then

hydrogeochemical basis

because h 2 O activity is 1, (1.109) formula becomes:

hydrogeochemical basis

in the near surface water environment, the partial pressure of oxygen Po 2 will not be greater than 1, Therefore, Po 2 = 1 is selected as the upper limit, and because pH = - LG [H + ], equation (1.110) becomes

hydrogeochemical basis

according to equation (1.97), e 0 , and first get & # 8710; G R value

o 2 + 4H + + 4E = 2H 2 o

&; G_f002 ×－ 237.2kJ/mol

∆ G_r=2 ×－ 237.2) - (0 + 0) = - 474.4kj/mol

take the obtained & # 8710; If G R value is substituted into (1.97), then

e 0 = 1.23V

and e 0 value is substituted into (1.111), then

hydrogeochemical basis

(1.112) is the EH pH equation of upper limit of H 2 o stable field, which is shown in line a of Fig. 1.4

The lower limit of stable field of

water is determined by the following semi reaction formula

hydrogeochemical basis

in the standard state, according to formula (1.95), then

hydrogeochemical basis

because in the near surface environment, P H 2 will not be greater than 1, so p H 2 = 1 is taken as the lower limit, Then (1.114) formula becomes:

hydrogeochemical basis

e to the H + and H 2 in (1.113) formula; G F are all zero, so & 8710; G R = 0, (1.115) formula becomes:

hydrogeochemical basis

(1.116) formula is eh pH equation of H 2 o lower limit of stable field, which is shown in line B of (a) in Figure 1.4. Figure 1.4 (a) shows that between a and B lines, H 2 O is stable and unstable. Above line a is oxidized water and below line B is reced water

When drawing eh pH diagram of Fe-H 2 O-O 2 system, the type of iron in the system must be specified first. It is assumed that only the simple free ions Fe 2 + and Fe 3 + are considered for the dissolved components, and their ion pairs are not considered; Only Fe (OH) 2 and Fe (OH) 3 are considered for solid components

The semi reaction equations of

Fe 2 + and Fe 3 + are as follows

hydrogeochemical basis

refer to Appendix 1 for the & 8710of Fe, Fe 2 + and Fe 3 + ; If G F values are 0, - 78.87 and -4.6 (kJ / mol), then (1.117) and (1.118) of the formula are (8710); G R are 78.87 and-74.27 (kJ / mol) respectively. According to the formula (1.97), we can get

hydrogeochemical basis

for the formula (1.117), the Nernst equation is:

hydrogeochemical basis

according to convention, we must specify the activity of Fe 2 + to calculate the corresponding eh value. Assuming [Fe 2 + ] = 10 - 5 mol, the equation (1.121) is as follows:

hydrogeochemical basis

(1.122) is the lower limit of Fe 2 + stable field, which is shown in line C of Fig. 1.4 (b)

For (1.118), the Nernst equation is:

basis of hydrogeochemistry

according to convention, if [Fe 2 + ] = [Fe 3 + ], then (1.123) becomes:

basis of hydrogeochemistry

(1.124) is the upper limit of Fe 2 + stable field, and the lower limit of Fe 3 + stable field, Line D shown in Figure 1.4 (b). It can be seen from the diagram that the reactions of equations (1.117) and (1.118) are independent of pH, and there are two parallel horizontal lines on the EH pH diagram

The hydroxide reaction of iron is as follows:

basis of hydrogeochemistry

refer to Appendix 1, and calculate the formula of (1.125) and (1.126) &8710; G R values were 93.10 and 220.1 (kJ / mol), respectively; Will be; The LGK values of G R are - 16.29 and - 38.52 respectively

According to (1.125), then

hydrogeochemical basis

according to (1.21), then

LG (oh - ) = LGK W - LG [H + ], because pH = - LG [H + ], So

hydrogeochemical basis

substituting (1.128) formula into (1.127) formula, after sorting out

hydrogeochemical basis

according to the above calculation, LGK = -16.29; Suppose [Fe 2 + ] = 10 - 5 mol; According to table 1.6, K W = 10 - 14 at 25 ℃

substituting the above values into formula (1.129), then

basis of hydrogeochemistry

treat formula (1.126) in the same way, then

basis of hydrogeochemistry

express formulas (1.130) and (1.131) in line E and line f in Figure 1.4 (b); The e line is the upper limit of Fe 2 + stable field, and the f line is the upper limit of Fe 3 + stable field. According to Figure 1.4 (b), when pH > 2.76, the reaction of formula (1.126) will not occur, but the following reactions will occur:

hydrogeochemical basis

refer to & 8710 in Appendix 1; G F value, we can get the value of (1.132); G R = - 93.9kj/mol, substituting (1.97) formula, then E 0 = 0.972v The Nernst equation of formula 1.132 is as follows:

basis of hydrogeochemistry

substituting the values of e 0 (0.972v) and [Fe 2 + ] (10 - 5 mol) into (1.133), then the formula

basis of hydrogeochemistry

(1.134) is expressed in line g in Figure 1.4 (b), It is the boundary between Fe 2 + and Fe (OH) 2 stable fields

when pH > 8.36, the reactions involving Fe 2 + are replaced by the following reactions related to Fe (OH) 2 :

hydrogeochemical basis

refer to Appendix 1 and calculate the & # 8710 of (1.135) and (1.136); The values of G R are 12.2 and - 27.2 (kJ / mol) respectively. Substituting (1.97) formula, the e 0 values are - 0.o6 and 0.28 (V) respectively

The Nernst equations of

(1.135) and (1.136) are respectively

hydrogeochemical basis

substituting e 0 into the above two equations, then

hydrogeochemical basis

(1.139) and (1.140) are shown as line I and line h respectively (Fig. 1.4 (b)). The I line is the lower limit of Fe (OH) 2 stable field; The H line is the lower limit of Fe (OH) 3 stable field and the upper limit of Fe (OH) 2 stable field

The lines representing all the above reactions are shown in Fig. 1.4 (c). It should be noted that line I is below line B of the lower limit of H 2 o stable field, so line I is meaningless, and Figure 1.4 (c) is omitted. From Fig. 1.4 (c), we can clearly see the stable field ranges of Fe 2 + , Fe 3 + , Fe (OH) 2 (s) and Fe (OH) 3 (s). For example, when [Fe 2 + ] = [Fe 2 + ] = 10 - 5 mol, eh > 0.77v, pH < 2.76, Fe 3 + is stable, beyond this range, Fe (OH) 3 (s), or Fe 2 + may be formed; When pH > 8.36 and eh ＜ - 2.10v, Fe (OH). (s) precipitation may be formed

Figure 1.4 eh pH diagram of Fe-H 2 O-O 2 system (25 ℃, 10 5 PA, [Fe 2 + ] = [Fe 3 + ] = 10 - 5 mol)

(a) stable field of water b) The structural line of Fe-H 1 O-O 2 system e) A complete diagram showing the dissolved components and the stable field of the solid. In fact, there are many insoluble solids containing iron, such as FeCo 3 , FES 2 , Fe 3 o 4 , Fe 2 o 3 in groundwater system. It is rather complicated and tedious to draw up eh pH diagrams of all dissolved components and solids containing iron. Generally speaking, the corresponding eh pH map is compiled according to the research purpose, or the previous research results are used to explain some hydrogeochemical problems

the EH pH diagram of any system indicates the stable field range of dissolved components and solids under certain conditions. Therefore, when using the EH pH diagram in the literature, we must pay attention to its specified conditions. For example, FIG. 1.5 is the EH pH diagram of fe-s-c-h 2 O system, and the specified condition is, < inline media object > < imageobject > < imagedata role = 3 "fileref = image / 047_ 1.jpg">

101325Pa,25℃ If the specified conditions change, the stable field range of the relevant components will also change. Therefore, the EH pH diagram can only be used as a guide to explain the stable dissolved components and solids within a certain EH and pH range; When EH and pH change, what kind of redox reaction may occur

in order to explain how to use eh pH diagram to explain some hydrogeochemical phenomena, Figure 1.5 is used to illustrate. It can be seen from the figure that when [Fe 2 + ] = 10 - 4 mol / L = 5.58mg/l, Fe 2 + is stable in water within the range of pH < 6.2 and eh = - o.08 - + 2.1V; If pH value is stable at 6.2 and eh ＞ + 0.21v, Fe (OH) 3 precipitation may occur; Some groundwater with high content of Fe 2 + is transparent and colorless when it is pumped out, and soon there will be reddish brown suspended solids, which is the result of Fe (OH) 3 precipitation caused by the increase of EH. If eh remains unchanged and pH increases, it is greater than 6.2. It can be seen from figure 2.5 that FeCo 3 precipitation may occur. In the long-term pumping wells with high Fe 2 + content, the filter is often blocked, which is caused by CO 2 escaping, groundwater pH rising and water diversion

9. It can be called soil chemical properties.

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