EQUILIBRIUM

Keywords

EQUILIBRIUM_PHASES

This keyword is used to define the amounts of an assemblage of pure phases that can react reversibly with the aqueous phase. Conceptually, when the phases included in this keyword data block are brought into contact with an aqueous solution, each phase will dissolve or precipitate to achieve equilibrium or will dissolve completely. Pure phases include fixed-composition minerals and gases with fixed partial pressures. Two types of input are available: in one type, the phase itself reacts to equilibrium (or a specified saturation index or gas partial pressure); in the other type, an alternative reaction occurs to the extent necessary to reach equilibrium (or a specified saturation index or gas partial pressure) with the specified pure phase.

Example

Line 0:  EQUILIBRIUM_PHASES 1 Define amounts of phases assemblage.
Line 1a:      Chalcedony  0.0     0.0
Line 1b:      CO2(g)      -3.5    1.0
Line 1c:      Gibbsite(c) 0.0     KAlSiO8  1.0
Line 1d:      Calcite     1.0     Gypsum   1.0
Line 1e:      pH_Fix      -5.0    HCl      10.0

Explanation

Line 0: EQUILIBRIUM_PHASES [number] [description]

: EQUILIBRIUM_PHASES is the keyword for the data block. Optionally, EQUILIBRIUM, EQUILIBRIA, PURE_PHASES, PURE.
: number--positive number to designate this phase assemblage and its composition. Default is 1. A range of numbers may also be given in the form m-n, where m and n are positive integers, m is less than n, and the two numbers are separated by a hyphen without intervening spaces.
: description--is an optional character field that describes the phase assemblage.

Line 1: phase name [saturation index ([alternative formula] or [alternative phase]) [amount]]

: phase name--name of a phase. The phase must be defined with PHASES input, either in the default database file or in the current or previous simulations of the run. The name must be spelled identically to the name used in PHASES input (except for case).
: saturation index--target saturation index for the pure phase in the aqueous phase (line 1a); for gases, this number is the log of the partial pressure (line 1b). Default is 0.0. The target saturation index may not be attained if the amount of the phase in the assemblage is insufficient.
: alternative formula--chemical formula that is added (or removed) to attain the target saturation index. By default, the mineral defined by phase name dissolves or precipitates to attain the target saturation index. If alternative formula or alternative phase is entered, phase name does not react; the stoichiometry of alternative formula or the alternative phase is added or removed from the aqueous phase to attain the target saturation index. Alternative formula must be a legitimate chemical formula composed of elements defined to the program. Line 1c indicates that the stoichiometry given by alternative formula, KAlSi₃O₈ (potassium feldspar), will be added or removed from the aqueous phase until gibbsite equilibrium is attained.
: alternative phase--the chemical formula defined for alternative phase is added (or removed) to attain the target saturation index. By default, the mineral defined by phase name dissolves or precipitates to attain the target saturation index. If alternative phase or alternative formula is entered, phase name does not react; the stoichiometry of the alternative phase or alternative formula is added or removed from the aqueous phase to attain the target saturation index. alternative phase must be defined through PHASES input (either in the database file or in the present or previous simulations). Line 1d indicates that the phase gypsum will be added to or removed from the aqueous phase until calcite equilibrium is attained.
: amount--moles of the phase in the phase assemblage or moles of the alternative reaction. Default is 10.0 moles. This number of moles is the maximum amount of the mineral or gas that can dissolve. It may be possible to dissolve the entire amount without reaching the target saturation index, in which case the solution will have a smaller saturation index for this phase than the target saturation index. If amount is equal to zero, then the phase can not dissolve, but will precipitate if the solution becomes supersaturated with the phase.

Notes

If just one number is included on line 1, it is assumed to be the target saturation index and the amount of the phase defaults to 10.0 mol. If two numbers are included on the line, the first is the target saturation index and the second is the amount of the phase present. Line 1 may be repeated to define all pure phases that are assumed to react reversibly. It is possible to include a pure phase that has an amount of zero (line 1a). In this case, chalcedony can only precipitate if the solution is supersaturated with this phase, either by initial conditions, or through dissolution of pure phases or other specified reactions (mixing or stoichiometric reactions). It is possible to maintain constant pH conditions by proper specification of an alternative formula and a phase (PHASES input). Line 1e would maintain a pH of 5.0 by adding HCl, provided a phase named "pH_Fix" were defined with reaction H⁺ = H⁺ and log K = 0.0 (see example 8). (Note: If the acid, HCl, is specified and, in fact, a base is needed to attain pH 5.0, it is possible the program will fail to find a solution to the algebraic equations.)

After a pure-phase assemblage has reacted with the solution, it is possible to save the resulting assemblage composition (that is, the identity and number of moles of each phase) with the SAVE keyword. If the new composition is not saved, the assemblage composition will remain the same as it was before the reaction calculation. After it has been defined or saved, the assemblage may be used in subsequent simulations by the USE keyword.

Example problems

The keyword EQUILIBRIUM_PHASES is used in example problems 2, 3, 5, 6, 7, 8, and 10.

Related keywords

PHASES, SAVE equilibrium_phases, and USE equilibrium_phases.

Example
Explanation
Notes
Example problems
Related keywords

User's Guide to PHREEQC - 07 MAY 96