The apparent molal heat capacity, enthalpy, and free energy of seawater fit to the Pitzer equations

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The thermochemical properties (osmotic coefficient, enthalpy and heat capacity) of seawater from S=0 to 120 and 0 to 200 °C have been fit to the Pitzer equations. The osmotic coefficient ( ϕ), apparent molal heat capacity ( ϕ Cp) and relative enthalpy ( ϕ L) have been fit to equations of the form ϕ SS − 1 = ( 1 / m ) { − A ϕ I 1.5 / ( 1 + 1.2 I 0.5 ) + m ( B SS ϕ + m C SS ϕ ) } Cp ϕ SS − C ¯ ⁢ p ss 0 = A J { I / ( 1.2 ⁢ m ) } ln ( 1 + 1.2 I 0.5 ) + 2 ⁢ R ⁢ T ⁢ m ( B SS J + m C SS J ) L ϕ SS = ( A H I / 1.2 ⁢ m ) ln ( 1 + 1.2 I 0.5 ) − 2 ⁢ R ⁢ T ⁢ m ( B SS L + m C SS L ) where I is the ionic strength, m is the molality of sea salt ( m=∑ m i , i is an ion); A X ( X= ϕ, J and H) is the Debye–Hückel slope, and C¯p ss 0 is the apparent molal heat capacity of sea salt in water. The Pitzer parameters B SS X and C SS X for sea salt have been fit to functions of temperature in the form Y SS X = ∑ a i ⁢ j ( T − T R ) i where a ij are adjustable parameters, Y SS X is the Pitzer coefficient, T is the temperature in K, and T R=273.15 K). The standard deviations of the seawater fits were 0.0006 J g −1 K −1 for the heat capacity from 0 to 40 °C and 0.0038 J g −1 K −1 from 0 to 200 °C. The extrapolated values of C¯p ss 0 from 0 to 200 °C were in reasonable agreement with the values determined from the major sea salts. The combined c p data have been fit to equations that are consistent with the theoretical values of C¯p ss 0. By appropriate integration of ϕ Cp SS, the apparent molal enthalpy at 298 K and osmotic coefficients at 273 K, equations were determined for all the thermochemical properties as a function of ionic strength and temperature. The thermochemical properties determined in this manner are in good agreement with direct measurements at different salinities and temperatures. The equations can be used to determine all the thermochemical properties of seawater from 0 to 200 °C and I=0 to 2.4 ( S=120).