337
Micronutrients in the Oceans
M 2+ + H 2 PO 4
– → MH 2 PO 4
+
(8.7)
M 2+ + HPO 4
2– → MHPO 4
0
(8.8)
M 2+ + PO 4
3– → MPO 4
–
(8.9)
The association constants K i * for the formation of Mg 2+ and Ca 2+ phosphate ion pairs at
I = 0.7 and 25°C are given in Table 8.3.
K 1 * = [MH 2 PO 4
+ ]/[M 2+ ][H 2 PO 4
– ]
(8.10)
K 2 * = [MHPO 4
o ]/[M 2+ ][HPO 4
2– ]
(8.11)
K 3 * = [MPO 4
– ]/[M 2+ ][PO 4
3– ]
(8.12)
The fractions of the various forms of phosphate in a given medium can be calculated from
α(H 3 PO 4 ) = {1 + K 1 /[H + ] + K 1 K 2 /[H + ] 2 + K 1 K 2 K 3 /[H + ] 3 }
(8.13)
α(H 2 PO 4
– ) = {1 + [H + ]/K 1 + K 2 /[H + ] + K 3 /[H + ]}
(8.14)
α(HPO 4
2– ) = {1 + [H + ]/K 2 + [H + ] 2 /K 1 K 2 + K 2 K 3 /[H + ] 3 }
(8.15)
α(PO 4
3– ) = {1 + [H + ]/K 3 + [H + ]2/K 2 K 3 + [H + ] 3 /K 1 K 2 K 3 }
(8.16)
The dissociation constants for phosphoric acid are given by
ln K 1 = 115.54 – 4576.752/T – 18.453 ln T + (0.069171 – 106.736/T) S 0.5
+ (–0.01844 – 0.65643/T) S
(8.17)
Table 8.3
Values of Log K MX for the Formation
of Mg 2+ and Ca 2+ Phosphate Ion Pairs
Species (X)
log β MgX
log β CaX
H 2 PO 4
–
0.14
–0.15
HPO 4
2–
1.23
0.97
PO 4
3–
3.36
4.51
Table 8.2
Values of the pK i for H 3 PO 4 in Water, NaCl,
and Seawater
Media
pK 1
pK 2
pK 3
H 2 O
2.15
7.20
12.34
NaCl (0.7)
1.73
6.38
11.13
Seawater (S = 35)
1.57
5.86
8.69