Section: 14 | Properties of Seawater |

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PROPERTIES OF SEAWATER

In addition to the dependence on temperature and pressure, the physical properties of seawater vary with the concentration of the dissolved constituents. A convenient parameter for describing the composition is the salinity, S, which is defined in terms of the electrical conductivity of the seawater sample. The defining equation for the practical salinity is:

S = a₀ + a₁K^1/2 + a₂K + a₃K^3/2 + a₄K² + a₅K^5/2

where K is the ratio of the conductivity of the seawater sample at 15 °C and atmospheric pressure to the conductivity of a potassium chloride solution in which the mass fraction of KCl is 0.0324356, at the same temperature and pressure. The values of the coefficients are:

a₀ = 0.0080
a₁ = –0.1692
a₂= 25.3851
a₃ = 14.0941
a₄ = –7.0261
a₅ = 2.7081

Σa_i = 35.0000

Thus, when K = 1, S = 35 exactly (S is normally quoted in units of ‰, i.e., parts per thousand). The value of S can be roughly equated with the mass of dissolved material in grams per kilogram of seawater. Salinity values in the open oceans at midlatitudes typically fall between 34 and 36.

It is customary in oceanography to define the pressure at a given point as the pressure due to the column of water between that point and the surface. Thus, by convention P = 0 at the sea surface. To a good approximation the pressure in decibars (dbar) can be equated to the depth in meters. Thus at 45° latitude the pressure is 5000 dbar at 4902 m, 10000 dbar at 9700 m.

Table 1 gives several properties of seawater as a function of temperature for a salinity of 35. Tables 2 and 3 give density and electrical conductivity, respectively, as a function of salinity at several temperatures, and Table 4 lists typical concentrations of the main constituents of seawater as a function of salinity. Table 5 gives the freezing point as a function of salinity and pressure.

References

The Practical Salinity Scale 1978 and the International Equation of State of Seawater 1980, Unesco Technical Papers in Marine Science No. 36, Unesco, Paris, 1981; sections No. 37, 38, 39, and 40 in this series give background papers and detailed tables.
Kennish, M. J., CRC Practical Handbook of Marine Science, CRC Press, Boca Raton, FL, 1989.
Poisson, A. IEEE J. Ocean. Eng. OE-5, 50, 1981. [https://doi.org/10.1109/JOE.1980.1145442]
Webster, F., in AIP Physics Desk Reference, E. R. Cohen, D. R. Lide, and G. L. Trigg, Eds., Springer-Verlag, New York, 2003.

Column definitions for Table 1 are as follows.

Column heading	Definition
t	Temperature, in °C
ρ	Density, in g cm^-3
β	(1/ρ) (dρ/dS) = Fractional change in density per unit change in salinity (S)
α	–(1/ρ) (dρ/dt) = Fractional change in density per unit change in temperature (°C^–1)
κ	Electrical conductivity in S cm^-1 (siemens per cm)
η	Viscosity, in mPa s (equal to cP)
c_p	Specific heat, in units J kg^-1 °C^-1
v	Speed of sound, in m s^-1

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TABLE 1. Properties of Seawater as a Function of Temperature and Normal Atmospheric Pressure
All Values at Salinity S = 35 g Salt per kg Seawater

Name	t/°C	ρ/g cm^–³	10⁷·β	10⁷·α/°C^–¹	κ/S cm^–¹	η/mPa s	c_p/J kg^–¹ °C^–¹	v/m s^–¹
Seawater	0	1.028106	7854	526	0.029048	1.892	3986.5	1449.1
Seawater	5	1.027675	7717	1136	0.033468	1.610
Seawater	10	1.026952	7606	1668	0.038103	1.388	3986.3	1489.8
Seawater	15	1.025973	7516	2141	0.042933	1.221
Seawater	20	1.024763	7444	2572	0.047934	1.085	3993.9	1521.5
Seawater	25	1.023343	7385	2970	0.053088	0.966
Seawater	30	1.021729	7338	3341	0.058373	0.871	4000.7	1545.6
Seawater	35	1.019934	7300	3687
Seawater	40		7270	4004			4003.5	1563.2

Tables 2 to 5 have data on the density ρ (Table 2), electrical conductivity λ (Table 3), composition (Table 4), and freezing point t_fp (Table 5) of seawater as a function of temperature and salinity.

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TABLE 2. Density of Surface Seawater as a Function of Temperature and Salinity
Salinity Values Are in Units of g Salt per kg Seawater

Name	t/°C	ρ(S = 0)/g cm^-3	ρ(S = 5)/g cm^-3	ρ(S = 10)/g cm^-3	ρ(S = 15)/g cm^-3	ρ(S = 20)/g cm^-3	ρ(S = 25)/g cm^-3	ρ(S = 30)/g cm^-3	ρ(S = 35)/g cm^-3	ρ(S = 40)/g cm^-3
Seawater	0	0.999843	1.003913	1.007955	1.011986	1.016014	1.020041	1.024072	1.028106	1.032147
Seawater	5	0.999967	1.003949	1.007907	1.011858	1.015807	1.019758	1.023714	1.027675	1.031645
Seawater	10	0.999702	1.003612	1.007501	1.011385	1.015269	1.019157	1.023051	1.026952	1.030862
Seawater	15	0.999102	1.002952	1.006784	1.010613	1.014443	1.018279	1.022122	1.025973	1.029834
Seawater	20	0.998206	1.002008	1.005793	1.009576	1.013362	1.017154	1.020954	1.024763	1.028583
Seawater	25	0.997048	1.000809	1.004556	1.008301	1.012050	1.015806	1.019569	1.023343	1.027128
Seawater	30	0.995651	0.999380	1.003095	1.006809	1.010527	1.014252	1.017985	1.021729	1.025483
Seawater	35	0.994036	0.997740	1.001429	1.005118	1.008810	1.012509	1.016217	1.019934	1.023662
Seawater	40	0.992220	0.995906	0.999575	1.003244	1.006915	1.010593	1.014278	1.017973	1.021679

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TABLE 3. Electrical Conductivity of Seawater (in S cm^-1) as a Function of Temperature and Salinity
Salinity Values Are in Units of g Salt per kg Seawater; Conductivity Units Are in Units of Siemens per Centimeter (S cm^-1)

Name	t/°C	λ(S = 5)/S cm^-1	λ(S = 10)/S cm^-1	λ(S = 15)/S cm^-1	λ(S = 20)/S cm^-1	λ(S = 25)/S cm^-1	λ(S = 30)/S cm^-1	λ(S = 35)/S cm^-1	λ(S = 40)/S cm^-1
Seawater	0	0.004808	0.009171	0.013357	0.017421	0.021385	0.025257	0.029048	0.032775
Seawater	5	0.005570	0.010616	0.015441	0.020118	0.024674	0.029120	0.033468	0.037734
Seawater	10	0.006370	0.012131	0.017627	0.022947	0.028123	0.033171	0.038103	0.042935
Seawater	15	0.007204	0.013709	0.019905	0.025894	0.031716	0.037391	0.042933	0.048355
Seawater	20	0.008068	0.015346	0.022267	0.028948	0.035438	0.041762	0.047934	0.053968
Seawater	25	0.008960	0.017035	0.024703	0.032097	0.039276	0.046267	0.053088	0.059751
Seawater	30	0.009877	0.018771	0.027204	0.035330	0.043213	0.050888	0.058373	0.065683

Table 4 contains values of the ionic strength and composition of seawater at various salinities. The ionic strength, the three left-most data columns, is expressed in terms of molality, that is mol of constituent per kg of water. Composition, the three right-most data columns, is expressed as mass (in g) per kg of water.

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TABLE 4. Composition of Seawater and Ionic Strength as a Function of Salinity
Salinity Values Are in Units of g Salt per kg Seawater

Constituent	Molality S = 30	Molality S = 35	Molality S = 40	Composition S = 30	Composition S = 35	Composition S = 40
Cl^-	0.482	0.562	0.650	16.58	19.33	22.36
Br^-	0.00074	0.00087	0.00100	0.057	0.067	0.078
F^-		0.00007			0.001
SO₄^2-	0.0104	0.0114	0.0122	0.97	1.06	1.14
HCO₃^-	0.00131	0.00143	0.00100	0.078	0.085	0.059
NaSO₄^-	0.0085	0.0108	0.0139	0.98	1.25	1.60
KSO₄^-	0.00010	0.00012	0.00015	0.013	0.016	0.020
Na⁺	0.405	0.472	0.544	9.03	10.53	12.13
K⁺	0.00892	0.01039	0.01200	0.338	0.394	0.455
Mg²⁺	0.0413	0.0483	0.0561	0.974	1.139	1.323
Ca²⁺	0.00131	0.00143	0.00154	0.051	0.056	0.060
Sr²⁺	0.00008	0.00009	0.00011	0.007	0.008	0.009
MgHCO₃⁺	0.00028	0.00036	0.00045	0.023	0.030	0.037
MgSO₄	0.00498	0.00561	0.00614	0.582	0.655	0.717
CaSO₄	0.00102	0.00115	0.00126	0.135	0.152	0.166
NaHCO₃	0.00015	0.00020	0.00024	0.012	0.016	0.020
H₃BO₃	0.00032	0.00037	0.00042	0.019	0.022	0.025
Ionic strength	0.5736	0.6675	0.7701

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TABLE 5. Freezing Point of Seawater (in °C) as a Function of Salinity and Pressure
Salinity Values Are in Units of g Salt per kg Seawater

Name	P/dbar ^a	t_fp(S = 0)/°C	t_fp(S = 5)/°C	t_fp(S = 10)/°C	t_fp(S = 15)/°C	t_fp(S = 20)/°C	t_fp(S = 25)/°C	t_fp(S = 30)/°C	t_fp(S = 35)/°C	t_fp(S = 40)/°C
Seawater	0	0.000	–0.274	–0.542	–0.812	–1.083	–1.358	–1.638	–1.922	–2.212
Seawater	50	–0.038	–0.311	–0.580	–0.849	–1.121	–1.396	–1.676	–1.960	–2.250
Seawater	100	–0.075	–0.349	–0.618	–0.887	–1.159	–1.434	–1.713	–1.998	–2.287
Seawater	500	–0.377	–0.650	–0.919	–1.188	–1.460	–1.735	–2.014	–2.299	–2.589

^aPressure above atmospheric.

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PROPERTIES OF SEAWATER

References

TABLE 1. Properties of Seawater as a Function of Temperature and Normal Atmospheric Pressure
All Values at Salinity S = 35 g Salt per kg Seawater

TABLE 2. Density of Surface Seawater as a Function of Temperature and Salinity
Salinity Values Are in Units of g Salt per kg Seawater

TABLE 3. Electrical Conductivity of Seawater (in S cm^-1) as a Function of Temperature and Salinity
Salinity Values Are in Units of g Salt per kg Seawater; Conductivity Units Are in Units of Siemens per Centimeter (S cm^-1)

TABLE 4. Composition of Seawater and Ionic Strength as a Function of Salinity
Salinity Values Are in Units of g Salt per kg Seawater