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This keyword data block is used to define a name, chemical reaction, log K , and temperature dependence of log K for each gas component and mineral that can be used for speciation, batch-reaction, transport, or inverse-modeling calculations. In addition, molar volumes can be defined for solids, and the critical temperature and pressure and the acentric factor can be defined for gases. Normally, this data block is included in the database file and only additions and modifications are included in the input file.
Dissolution reaction for phase to aqueous species. Any aqueous species, including e - , may be used in the dissolution reaction. The chemical formula for the defined phase must be the first chemical formula on the left-hand side of the equation. The dissolution reaction must precede any identifiers related to the phase. The stoichiometric coefficient for the phase in the chemical reaction must be 1.0.
units --Units may be calories, kilocalories, joules, or kilojoules per mole. Only the energy unit is needed (per mole is implied) and abbreviations of these units are acceptable. Explicit definition of units for all enthalpy values is recommended. The enthalpy of reaction is used in the Van’t Hoff equation to determine the temperature dependence of the equilibrium constant. Internally, all enthalpy calculations are performed in the units of kJ/mol. Default units are kJ/mol.
-analytical_expression --Identifier for coefficients for an analytical expression for the temperature dependence of log K . If defined, the analytical expression takes precedence over log_k and the Van’t Hoff equation to determine the temperature dependence of the equilibrium constant. Optionally, analytical_expression , a_e , ae , -a [ nalytical_expression ], -a [ _e ], -a [ e ].
A 1 , A 2 , A 3 , A 4 , A 5 , A 6 --Six values defining log K as a function of temperature in the expression , where T is kelvin. Coefficients are defined in order from A 1 to A 6 ; if less than six parameters are defined, the undefined parameters are set to zero.
molar_volume , the molecular weight divided by the density of the solid at 25 °C. In the example for gypsum 172.18 (g/mol, gram per mole) / 2.33 (g/cm 3 , gram per cubic centimeter) = 73.9 cm 3 /mol (cubic centimeter per gram). Default is 0 cm 3 /mol.
The set of Lines 1 and 2 must be entered in order, and either Line 3 ( log_k ) or Line 5 ( -analytical_expression ) should be entered for each phase (default log K is 0.0). The analytical expression ( -analytical_expression ) takes precedence over log_k and the Van’t Hoff equation ( delta_H ) to determine the temperature dependence of the equilibrium constant. Lines 3 to 5, Line 6 for a solid, and Lines 7 to 9 for a gas may be entered as needed in any order. The equations for the phases may be written in terms of any aqueous chemical species, including e - .
where P is pressure (atm), Δ V r is the volume change of the reaction (cm 3 /mol), R is the gas constant (82.06 atm cm 3 mol -1 K -1 , atmosphere cubic centimeter per mole per kelvin), and T is the temperature (K).
where V m is the molar volume of the gas (cm 3 /mol), b is the minimal volume of the gas (cm 3 /mol), a is the Van der Waals attraction factor (atm cm 3 mol -2 , atmosphere cubic centimeter per mole squared) and α is a dimensionless function of reduced temperature and acentric factor. With P and Vm known, the fugacity coefficient of a gas can be calculated as:
The identifier -no_check can be used to disable checking charge and elemental balances (see SOLUTION_SPECIES). The use of -no_check is not recommended, except in cases where the phase is only to be used for inverse modeling. Even in this case, equations defining phases should be charge balanced. The identifier also can be used to define the mineral formula for an exchanger with an explicit charge imbalance (see explanation under EXCHANGE).
Dissolution reaction for phase. In implementing isotope calculations, the dissolution reaction was generalized to allow, in addition to any aqueous species, solid and gas species. To distinguish solids and gases (defined in the PHASES data block) from aqueous species, “(s)” and “(g)” are appended to the species in the equation. For clarity, “H2O(l)” can be used in an equation to designate liquid water, but it is equivalent to using “H2O”. The chemical formula for the defined phase must be the first chemical formula on the left-hand side of the equation. The stoichiometric coefficient for the defined phase in the chemical reaction must be 1.0. The dissolution reaction must precede any identifiers related to the phase.
-add_logk --The value of the named_expression is multiplied by the coefficient and added to the log K for the phase. This identifier may be used multiple times in the definition of the equilibrium constant for the phase. Optionally, add_logk , add_log_k , -ad [ d_logk ] or -ad [ d_log_k ].
named_expression --Name of an expression defined in a MIX_EQUILIBRIUM_PHASES data block.
Example data block 2 demonstrates capabilities that were added during the implementation of isotopic calculations as described by Thorstenson and Parkhurst (2000, 2004). First, to simplify the definition of equilibrium constants for the isotopic variants of a mineral or gas, the dissociation reaction was generalized to allow solids and gases in the equation. Solids are identified by an appended “(s)” and gases are identified by an appended “(g)”. The original definition of the solid or gas may or may not have the appended string in its name; PHREEQC attempts to find the name in the list of phases with and without the appended string. Note in this example that “Gypsum(s)” is used in Line 2a, but “Gypsum” is defined as the name of the phase in Line 1b.
When specifying individual equilibrium constants for isotopic phases, a single fractionation factor may appear in the expressions of the equilibrium constants for multiple isotopic forms of a phase. To avoid many manipulations with analytical expressions in the definition of equilibrium constants, the MIX_EQUILIBRIUM_PHASES data block allows assigning a name to the expression for a fractionation factor. This named expression can then be used in calculating the equilibrium constants for phases defined in the PHASES data block by use of the -add_logk identifier. In the definition of CH3D(g) in the Example data block 2, the fractionation factor for deuterium between methane and liquid water is added to an equilibrium constant derived from the symmetry of the molecule to define the equilibrium constant for the dissociation reaction.
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