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This keyword is used to define a half reaction and relative log K for each exchange species. Normally, this data block is included in the database file and only additions and modifications are included in the input file.


Line 1a:      X- = X-
Line 2a:           log_k     0.0
Line 1b:      X- + Na+ = NaX
Line 2b:           log_k     0.0
Line 1c:      2X- + Ca+2 = CaX2
Line 2c:           log_k     0.8
Line 1d:      Xa- = Xa-
Line 2d:           log_k     0.0
Line 1e:      X- + Na+ = NaX
Line 2e:           log_k     0.0
Line 1f:      2Xa- + Ca+2 = CaXa2
Line 2f:           log_k     2.0



Keyword for the data block. No other data are input on the keyword line.

Line 1: Association reaction

Association reaction for exchange species. The defined species must be the first species to the right of the equal sign. The association reaction must precede any identifiers related to the exchange species. Master species have an identity reaction (lines 1a and 1d).

Line 2: log_k log K

log_k--Identifier for log K at 25oC. Optionally, -log_k, logk, -l[og_k], or -l[ogk].

log K--Log K at 25oC for the reaction. Default 0.0. Unlike log K for aqueous species, the log K for exchange species is implicitly relative to a single exchange species. In the default database file, sodium (NaX) is used as the reference and the reaction X- + Na+ = NaX is given a log K of 0.0 (line 2b). The log K for the exchange reaction for the reaction given in line 2c is then numerically equal to the log K for the reaction 2NaX + Ca+2 = CaX2 + 2Na+. Master species have log K of 0.0 (lines 2a and 2d); reference species have log K of 0.0 (lines 2b and 2e).


Lines 1 and 2 may be repeated as necessary to define all of the exchange reactions. One identity reaction is needed to define the exchange master species (in example, lines 1a and 2a, 1d and 2d) for each exchanger. The reference half reaction for each exchanger will have a log K of 0.0 (in example, lines 1b and 2b, 1e and 2e); in the default database file the reference half reaction is Na+ + X- = NaX. Multiple exchangers may be defined simply by defining multiple exchange master species and additional half reactions involving these master species, as in this example.

Temperature dependence of log K can be defined with the standard enthalpy of reaction (identifier delta_h) using the van't Hoff equation or with an analytical expression (-analytical_expression). See SOLUTION_SPECIES or PHASES for examples.

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. The equation given for the exchange species (line 1) is used to determine the mass-action equation and the contribution of the species to each mole-balance equation. Alternatively, the contribution of the species to each mole-balance equation can be defined using the -mole_balance identifier. See SOLUTION_SPECIES and SURFACE_SPECIES for an example. If the -no_check identifier is needed, then the -mole_balance identifier is also needed.

Example problems

The keyword EXCHANGE_SPECIES is not used in the example problems. See listing of default database file in Attachment B for examples.

Related keywords


Example problems
Related keywords

User's Guide to PHREEQC - 07 MAY 96
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