Encyclopedia of Environmental Science and Engineering, Volume I and II

1270 WATER CHEMISTRY

The boundary conditions for the stability of water

(Figure 10(d)) are given at high p values by the oxidation

of water to oxygen:

1/2 H 2 O  1/4 O 2  H^ ^  e (40)

and at low p values by the reduction of water to hydrogen

H 2 O  e  1/2 H 2  OH^ ^. (41)

Water in equilibrium with the atmosphere ( P o 2  0.21 atm.)

at pH  7.0 (25C) has a p  13.6 ( E H  0.8 volt.)

Figure 10(a) gives the relationships among several oxi-

dation states of nitrogen as a function of p. For most of

the aqueous range of p , N 2 gas is the most stable species.

However, at large negative p values ammonia becomes

predominant and for p greater than 12 nitrate dominates

pH  7. The fact that the nitrogen gas of the atmosphere has

not been converted largely into nitrate under the prevailing

aerobic conditions at the land and water surfaces indicates a

lack of efficient biological mediation.

FIGURE 10 Equilibrium concentrations of

biochemically important redox components

as a function of p at a pH of 7.0. These equi-

librium diagrams have been constructed from

equilibrium constants listed in Table 3 and 4

for the following concentrations representa-

tive of natural water systems: pH  7.0; CT

(total carbonate carbon)  10 ^3 M; [H 2 (aq)]

 [H 2 S(aq)]  [HS]  [SO 42 ]  10 ^5 M;

[NO^3 ]  [NO^2 ]  NO^4 ]  10 ^3 M; pN2^2 

0.78 atm. and thus [N 2 (aq)]  0.5 
 10 ^3 M.

Figure from Stumm, W. and J. Morgan,

Aquatic Chemistry, Wiley-Interscience, New

York, 1970, p. 331.

1

2

3

4

+25

+20

+15

+10

+5

0

(^0510) pH
0 0
5
+0.5
+10
10
15
20
25 +15
p (^) p
H 2
H 2 O
H 2 O
O 2
HOCl
OCl–
Cl–
Cl 2 (aq)
EH(v)
FIGURE 9 Stability field diagram for the chlorine
system: p versus pH. The curves labelled 1,2,3
and 4 represent equilibria derived from Eqs. (25)
and (26) of Table 4 and pKa for HOCl: This latter
value is 7.53 at 25C and is represented by the verti-
cal line between HOCl and OCl stability fields. In
dilute solutions Cl 2 (aq) exists only at low pH. Cl 2 ,
OCl, and HOCl are all unstable or metastable in
water since they are all slowly reduced by water as
shown by the position of their stability fields, with
respect to the H 2 O–O 2 equilibrium curve. Figure
from Stumm, W. and J. Morgan, Aquatic Chemistry,
Wiley-Interscience, New York, 1970, p. 320.
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