275
The Carbonate System
The TA of seawater is defined as the concentration of all the bases that can accept H + when
a titration is made with HCl to the carbonic acid end point. The value of TA is given by
TA = [HCO 3
– ] + 2[CO 3
2– ] + [B(OH) 4 –] + [OH – ] – [H + ] +[SiO(OH) 3
– ]
+ [MgOH + ] + 2[HPO 4
2– ] + 3[PO 4
3– ]
(7.88)
The percentage of TA from the various bases is shown in Table 7.6 for ocean waters with
pH = 8[SiO(OH) 3
– ] = 10 –5.25 , and [HPO 4
2– ] = 10 –5.52 . For most waters, [HCO 3
– ], [CO 3
2– ], and
[B(OH) 4
– ] are the most important bases. For anoxic waters, HS – and NH 3 can also contribute to the TA.
The carbonate alkalinity A C is defined by
A C = [HCO 3
– ] + 2 [CO 3
2– ]
(7.89)
and is calculated from
A C = TA – [B] T
(7.90)
where [B] T = [B(OH) 4– ] + …, the sum of all the bases other than HCO 3
– and CO 3
2– . The
[B(OH) 4
– ] concentration, which is the largest source of [B] T , can be calculated from
[B(OH) 4
– ] = K* HB [B] T /(K* HB+ [H+] T )
(7.91)
where [B] T = 1.2 × 10 –5 S (Lee et al., 2010). The contribution from other bases (phosphate,
silicate, ammonium, and hydrogen sulfide) can be determined using the dissociation constants for the appropriate acid (Chapters 8 and 10, Appendix 4).
The TA of seawater is determined by titrating a given amount of seawater with HCl to
the carbonic acid end point. The titration is followed by measuring the EMF of a glass pH
and reference electrode. A typical titration of 234 cm 3 of S = 35 seawater with 0.25N HCl
is shown in Figure 7.8. The titration shows two end points, V 1 and V 2 . The value of A T is
determined from
TA = V 2 N HCl / W
(7.92)
where N HCl is the normality of the HCl solution, and W is the weight of the seawater
titrated (W = Volume × Density). The difference between the first and second end point
can be used to determine the TCO 2 :
Table 7.6
Contribution of Various Components
to the Total Alkalinity of Seawater
Species
Percentage TA
HCO 3
–
89.8
CO 3
2–
6.7
B(OH) 4–
2.9
SiO(OH) 3–
0.2
MgOH +
0.1
OH –
0.1
HPO 4
2–
0.1
The Carbonate System
The TA of seawater is defined as the concentration of all the bases that can accept H + when
a titration is made with HCl to the carbonic acid end point. The value of TA is given by
TA = [HCO 3
– ] + 2[CO 3
2– ] + [B(OH) 4 –] + [OH – ] – [H + ] +[SiO(OH) 3
– ]
+ [MgOH + ] + 2[HPO 4
2– ] + 3[PO 4
3– ]
(7.88)
The percentage of TA from the various bases is shown in Table 7.6 for ocean waters with
pH = 8[SiO(OH) 3
– ] = 10 –5.25 , and [HPO 4
2– ] = 10 –5.52 . For most waters, [HCO 3
– ], [CO 3
2– ], and
[B(OH) 4
– ] are the most important bases. For anoxic waters, HS – and NH 3 can also contribute to the TA.
The carbonate alkalinity A C is defined by
A C = [HCO 3
– ] + 2 [CO 3
2– ]
(7.89)
and is calculated from
A C = TA – [B] T
(7.90)
where [B] T = [B(OH) 4– ] + …, the sum of all the bases other than HCO 3
– and CO 3
2– . The
[B(OH) 4
– ] concentration, which is the largest source of [B] T , can be calculated from
[B(OH) 4
– ] = K* HB [B] T /(K* HB+ [H+] T )
(7.91)
where [B] T = 1.2 × 10 –5 S (Lee et al., 2010). The contribution from other bases (phosphate,
silicate, ammonium, and hydrogen sulfide) can be determined using the dissociation constants for the appropriate acid (Chapters 8 and 10, Appendix 4).
The TA of seawater is determined by titrating a given amount of seawater with HCl to
the carbonic acid end point. The titration is followed by measuring the EMF of a glass pH
and reference electrode. A typical titration of 234 cm 3 of S = 35 seawater with 0.25N HCl
is shown in Figure 7.8. The titration shows two end points, V 1 and V 2 . The value of A T is
determined from
TA = V 2 N HCl / W
(7.92)
where N HCl is the normality of the HCl solution, and W is the weight of the seawater
titrated (W = Volume × Density). The difference between the first and second end point
can be used to determine the TCO 2 :
Table 7.6
Contribution of Various Components
to the Total Alkalinity of Seawater
Species
Percentage TA
HCO 3
–
89.8
CO 3
2–
6.7
B(OH) 4–
2.9
SiO(OH) 3–
0.2
MgOH +
0.1
OH –
0.1
HPO 4
2–
0.1