60
Chemical Oceanography, 4th Edition
relative densities of the same samples. These results were then compared to the calculated
values obtained using conductivity and chlorinity- derived salinities. The differences had
a mean of ±2 × 10 –6 g cm –3 (2 ppm) and showed no measurable differences in using S or Cl.
Thus, the myth that conductivity salinities are better than chlorinity- derived salinities for
determining density does not hold true for seawater collected in the North Atlantic over a
period of 38 years.
One sample had densities 35 to 40 × 10 –6 g cm –3 that was higher than the other samples.
This was due to the dissolution of SiO 2 that occurred during closing of the sample or during storage. In general, these results indicate that standard seawater can be used as a conductivity and density standard to ±0.002 in S and ±2 × 10 –6 g cm –3 in density.
In 1975, the JPOTS committee that had been considering a new equation of state of seawater suggested that a background paper be prepared on the salinity method (Lewis, 1978).
It was concluded that a revision was needed in the definition of salinity, and the committee recommended the practical salinity scale of 1978. This new scale breaks the Cl–S
relationship in favor of a salinity–conductivity ratio relationship. All waters with the same
conductivity ratio have the same salinity (even though the composition may differ). Since
salinity is normally used to determine a physical property like density, this was thought
to be the best method for determining the effect of changes in ionic composition. This is
not always the case since nonelectrolytes like SiO 2 are not detected by conductivity. More
is said about this in further discussion.
A standard seawater of practical salinity of 35.000 (no units or ‰ are needed) has, by
definition, a conductivity ratio of 1.0 at 15°C with a KCl solution containing a mass of
32.4356 g of KCl in a mass of 1 kg of solution. This value was determined as an average of three independent laboratory studies. The salinity dependence of the conductivity
ratio was determined by measuring the conductivity (C) ratio at various temperatures of
S = 35.000 seawater weight evaporated or diluted with water. The final equation is given by
S a a R
a R
a R
a R
a R
T
T
T
T
T
/
/
= +
+
+
+
+
0
1
1 2
2
3
3 2
4
2
5
5 5 2
/
+ ∆S
(2.6)
where
∆S
t
k t
b
b R
b R
T
T
= −
+
−
+
+
+
[(
)/(
(
))](
/
15 1
15
0
1
1 2
2
b b R
b R
b R
T
T
T
3
3 2
4
2
5
5 2
/
/
)
+
+
(2.7)
a 0 = 0.0080
b 0 = 0.0005
a 1 = –0.1692
b 1 = –0.0056 k = 0.0162
a 2 = 25.3851
b 2 = –0.0066
a 3 = 14.0941
b 3 = –0.0375
a 4 = –7.0261
b 4 = 0.0636
a 5 = 2.7081
b 5 = –0.0144
∑a i = 35.000
∑b i = 0.0000
and R T = C(S, t, 0)/C(35, t, 0) at atmospheric pressure (p = 0).
Chemical Oceanography, 4th Edition
relative densities of the same samples. These results were then compared to the calculated
values obtained using conductivity and chlorinity- derived salinities. The differences had
a mean of ±2 × 10 –6 g cm –3 (2 ppm) and showed no measurable differences in using S or Cl.
Thus, the myth that conductivity salinities are better than chlorinity- derived salinities for
determining density does not hold true for seawater collected in the North Atlantic over a
period of 38 years.
One sample had densities 35 to 40 × 10 –6 g cm –3 that was higher than the other samples.
This was due to the dissolution of SiO 2 that occurred during closing of the sample or during storage. In general, these results indicate that standard seawater can be used as a conductivity and density standard to ±0.002 in S and ±2 × 10 –6 g cm –3 in density.
In 1975, the JPOTS committee that had been considering a new equation of state of seawater suggested that a background paper be prepared on the salinity method (Lewis, 1978).
It was concluded that a revision was needed in the definition of salinity, and the committee recommended the practical salinity scale of 1978. This new scale breaks the Cl–S
relationship in favor of a salinity–conductivity ratio relationship. All waters with the same
conductivity ratio have the same salinity (even though the composition may differ). Since
salinity is normally used to determine a physical property like density, this was thought
to be the best method for determining the effect of changes in ionic composition. This is
not always the case since nonelectrolytes like SiO 2 are not detected by conductivity. More
is said about this in further discussion.
A standard seawater of practical salinity of 35.000 (no units or ‰ are needed) has, by
definition, a conductivity ratio of 1.0 at 15°C with a KCl solution containing a mass of
32.4356 g of KCl in a mass of 1 kg of solution. This value was determined as an average of three independent laboratory studies. The salinity dependence of the conductivity
ratio was determined by measuring the conductivity (C) ratio at various temperatures of
S = 35.000 seawater weight evaporated or diluted with water. The final equation is given by
S a a R
a R
a R
a R
a R
T
T
T
T
T
/
/
= +
+
+
+
+
0
1
1 2
2
3
3 2
4
2
5
5 5 2
/
+ ∆S
(2.6)
where
∆S
t
k t
b
b R
b R
T
T
= −
+
−
+
+
+
[(
)/(
(
))](
/
15 1
15
0
1
1 2
2
b b R
b R
b R
T
T
T
3
3 2
4
2
5
5 2
/
/
)
+
+
(2.7)
a 0 = 0.0080
b 0 = 0.0005
a 1 = –0.1692
b 1 = –0.0056 k = 0.0162
a 2 = 25.3851
b 2 = –0.0066
a 3 = 14.0941
b 3 = –0.0375
a 4 = –7.0261
b 4 = 0.0636
a 5 = 2.7081
b 5 = –0.0144
∑a i = 35.000
∑b i = 0.0000
and R T = C(S, t, 0)/C(35, t, 0) at atmospheric pressure (p = 0).