158
Chemical Oceanography, 4th Edition
where a i is the activity of i, and pE = –log a Ox , the log of the activity of an electron. Rearranging
this equation, we have
pE = pE 0 + (1/n)log a Ox / a Red
(4.44)
where pE 0 = (1/n) log K. Since pE = Eh/(2.303RT/ F), we have the more familiar form
Eh = Eh 0 + (2.303RT/ nF) log a Ox / a Red
(4.45)
where Eh 0 = (2.303RT/ nF) log K = (0.0591/n) log K at 25°C.
The upper theoretical limit of the pE or Eh of oxygenated water is controlled by the
reaction
1/2 O 2 (g) + 2H+ + 2e – = H 2 O(aq)
(4.46)
Using log K = 41.6, log aH 2 O = –0.01, and pH = 8.1 at 25°C, the pE is given by
pE = (log P + 50.7)/4
(4.47)
at log P = –0.69, pE = 12.5, or Eh = 0.73 V. The experimentally measured values (about 0.5 to
0.6 V) for the Eh of open surface seawater are lower than this theoretical value. By using
the reaction
O 2 + 2H + + 2e – = H 2 O 2
(4.48)
one obtains a lower theoretical pE = 6.3 or Eh = 0.4 V (taking [H 2 O 2 ] = 10 –7 M), which is
closer to the experimentally determined values.
For anoxic conditions, the negative pE or Eh is thought to be controlled by the reactions
SO 4
2– + 9H + + 8e – = HS – + 4H 2 O
(4.49)
SO 4
2– + 8H + + 6e – = S o (s) + 4H 2 O
(4.50)
Using log K = 34.0 and 36.6, respectively, for Reactions 4.49 and 4.50, the pE is given by
pE = [–log(HS – ) – 41.4]/8
(4.51)
pE = [–log(SO 4
2– ) – 41.4]/8
(4.52)
at (HS – ) = 10 –3 to 10 –6 , pE = –4.8 to –4.4 or Eh = –0.28 to –0.26 V.
The pH and Eh environments that one encounters in marine waters are shown in
Figure 4.27. The upper and lower limits are determined by the properties of water. The
cross- textured and dotted bands represent the stability band for oxygen (4 to 260 M) and
sulfide (10 –3 to 10 –6 M) concentrations as given by Reactions 4.49 and 4.50. Most ocean
waters have pH values between 7.6 and 8.3 and Eh values greater than 0.2 V. A sample calculation of the oxidation state of a metal in seawater is made for the iron system examined
by Kester, Byrne, and Liang (1975). The two oxidation states of iron are related by
Fe 3+ + e – = Fe 2+
(4.53)
Chemical Oceanography, 4th Edition
where a i is the activity of i, and pE = –log a Ox , the log of the activity of an electron. Rearranging
this equation, we have
pE = pE 0 + (1/n)log a Ox / a Red
(4.44)
where pE 0 = (1/n) log K. Since pE = Eh/(2.303RT/ F), we have the more familiar form
Eh = Eh 0 + (2.303RT/ nF) log a Ox / a Red
(4.45)
where Eh 0 = (2.303RT/ nF) log K = (0.0591/n) log K at 25°C.
The upper theoretical limit of the pE or Eh of oxygenated water is controlled by the
reaction
1/2 O 2 (g) + 2H+ + 2e – = H 2 O(aq)
(4.46)
Using log K = 41.6, log aH 2 O = –0.01, and pH = 8.1 at 25°C, the pE is given by
pE = (log P + 50.7)/4
(4.47)
at log P = –0.69, pE = 12.5, or Eh = 0.73 V. The experimentally measured values (about 0.5 to
0.6 V) for the Eh of open surface seawater are lower than this theoretical value. By using
the reaction
O 2 + 2H + + 2e – = H 2 O 2
(4.48)
one obtains a lower theoretical pE = 6.3 or Eh = 0.4 V (taking [H 2 O 2 ] = 10 –7 M), which is
closer to the experimentally determined values.
For anoxic conditions, the negative pE or Eh is thought to be controlled by the reactions
SO 4
2– + 9H + + 8e – = HS – + 4H 2 O
(4.49)
SO 4
2– + 8H + + 6e – = S o (s) + 4H 2 O
(4.50)
Using log K = 34.0 and 36.6, respectively, for Reactions 4.49 and 4.50, the pE is given by
pE = [–log(HS – ) – 41.4]/8
(4.51)
pE = [–log(SO 4
2– ) – 41.4]/8
(4.52)
at (HS – ) = 10 –3 to 10 –6 , pE = –4.8 to –4.4 or Eh = –0.28 to –0.26 V.
The pH and Eh environments that one encounters in marine waters are shown in
Figure 4.27. The upper and lower limits are determined by the properties of water. The
cross- textured and dotted bands represent the stability band for oxygen (4 to 260 M) and
sulfide (10 –3 to 10 –6 M) concentrations as given by Reactions 4.49 and 4.50. Most ocean
waters have pH values between 7.6 and 8.3 and Eh values greater than 0.2 V. A sample calculation of the oxidation state of a metal in seawater is made for the iron system examined
by Kester, Byrne, and Liang (1975). The two oxidation states of iron are related by
Fe 3+ + e – = Fe 2+
(4.53)