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Chemical Oceanography, 4th Edition
The fractions of these forms are controlled by the pH and the composition of the waters.
The ionization constants for the three- step dissociation are defined by
K 1 = [H + ][H 2 PO 4
– ]/[H 3 PO 4 ]
(8.4)
K 2 = [H + ][HPO 4
2– ]/[H 2 PO 4
– ]
(8.5)
K 3 = [H + ][PO 4
3– ]/[HPO 4
2– ]
(8.6)
The pK’s (–log K) for H 3 PO 4 in water, NaCl (0.7m), and seawater (salinity [S] = 35) at 25°C
are given in Table 8.2. The increase in the ionization as one goes from NaCl to seawater
is related to the increase in ionic strength (H 2 O → NaCl) and the formation of strong ion
pairs (NaCl → seawater) of Ca 2+ and Mg 2+ with phosphate ions. For M 2+ = Mg 2+ or Ca 2+
we have
Table 8.1
Distribution of Elements in Organisms (N) and
Seawater (A): A Measure of Availability to Need
Element
N (g/100 g)
A (g/ m 3 )
A/ N
H
7
—
—
Na
3
10,750
3600
K
1
390
390
Mg
0.4
1300
300
Ca
0.5
416
830
C
30
28
1
Si a
0.5
0.50
1
Si b
10
0.50
0.05
N
5
0.30
0.06
P
0.6
0.030
0.05
O (O 2 + O 2 )
47
90
2
S
1
900
900
Cl
4
19,300
4800
Cu
0.005
0.010
2
Zn
0.00125
0.005
4
B
0.002
0.005
2500
V
0.003
0.0003
0.1
As
0.0001
0.015
150
Mn
0.002
0.005
2.5
F
1
1.4
1400
Br
0.0025
66
26,000
Fe a
0.001
0.00005
0.05
Fe b
0.000
0.050
1.3
Co
0.00005
0.0001
2
Al
1
0.120
120
Ti
0.100
—
—
a Phytoplankton.
b Diatoms.