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Chemical Oceanography, 4th Edition
TCO 2 = (V 2 – V 1 )N HCl / W
(7.93)
The pH SWS of the initial solution before the addition of HCl can be determined from
pH SWS = –(E – E*)/(2.303RT/ F)
(7.94)
The quantities V 1 , V 2 , and E* are determined from the measured EMF (E) and the volume
of added HCl (V) using an iteration technique. A reasonable approximation of V 1 , V 2 , and
E* can be determined by calculating the [H + ] SWS from
[H + ] SWS = 10[(E – 400)/k]
(7.95)
where k = 2.303 RT/ F = 59.16 mV at 25°C.
Using the EMF data from 150 to 210 mV, the function F 2 is calculated from
F 2 = (V 0 + V)[H+] SWS
(7.96)
where V 0 is the initial volume of seawater. The values of F 2 are then fit to the linear equation
V = a + b F 2
(7.97)
(Figure 7.9). At F 2 = 0, the value of V equals V 2 . The value of V 1 is determined from the initial EMF data (–15 to 50 mV) using the function
F 2 = (V 2 – V)[H + ] SWS
(7.98)
The values of F 1 are fit to the equation
V = a + b F 1
(7.99)
At F 1 = 0, the value of V equals V 1 (Figure 7.10). The value of E* is determined from the
values of E past the second end point (V > V 2 ) using the equation
HCl Volume (cm
3 )
0.0
0.4
0.8
1.2
1.6
2.0
EMF (mV)
100
150
200
250
300
350
400
Figure 7.8
The changes in the EMF of a pH electrode during the titration of seawater with hydrochloric acid.