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Keywords
EXCHANGE
This keyword is used to define the amount and composition of an assemblage of exchangers. The initial composition of the exchange assemblage can be defined in two ways, (1) explicitly by listing the composition of each exchanger or (2) implicitly, by specifying that each exchanger is in equilibrium with a solution of fixed composition. The exchange master species, stoichiometries, and log K's for the exchange reactions are defined with the keywords EXCHANGE_MASTER_SPECIES and EXCHANGE_SPECIES.
Example 1
Line 0: EXCHANGE 1 Measured exchange composition
Line 1a: CaX2 0.3
Line 1b: MgX2 0.2
Line 1c: NaX 0.5
Explanation 1
Line 0: EXCHANGE [number] [description]
- EXCHANGE is the keyword for the data block.
- number--positive number to designate this exchange 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 exchanger.
Line 1: chemical formula, amount
- chemical formula--component of the exchanger.
- amount--quantity of component, in moles.
Notes 1
Line 1 may be repeated to define the entire composition of each exchanger. Although this example only defines one exchanger, X, other exchangers could be included in the exchange assemblage. In the example, the total number of exchange sites of X is 1.5 mol and the total concentrations of calcium, magnesium, and sodium on the exchanger are 0.3, 0.2, and 0.5 mol, respectively.
Example 2
Line 0: EXCHANGE 1 Exchanger in equilibrium with solution 1
Line 1: -equilibrate with solution 1
Line 2a: X 1.0
Line 2b: Xa 0.5
Explanation 2
Line 0: EXCHANGE [number] [description]
- As in example 1.
Line 1: -equilibrate number
- -equilibrate--This string at the beginning of the line indicates that the exchange assemblage is defined to be in equilibrium with a given solution composition. Optionally, equil, equilibrate, -e[quilibrate].
- number--solution number with which the exchange assemblage is to be in equilibrium. Any alphabetic characters following the identifier and preceding an integer ("with solution" in line 1) are ignored.
Line 2: exchanger name, amount
- exchanger name--name of an exchanger that is defined to the program.
- amount--quantity of exchanger, in moles.
Notes 2
The order of lines 1 and 2 is not important. Line 1 should occur only once within the data block. Line 2 may be repeated to define the amounts of other exchangers, if more than one exchanger is present in the assemblage. Example 2 requires the program to make a calculation to determine the composition of the exchange assemblage. The calculation will be performed before the any reaction calculations to determine the concentrations of each exchange component [such as CaX2, MgX2, or NaX (from the default database) provided calcium, magnesium, and sodium are present in the solution] that would exist in equilibrium with the specified solution (solution 1 in this example). The composition of the solution will not change during this calculation.
When an exchange assemblage (defined as in example 1 or example 2) is placed in contact with a solution during a reaction calculation, both the exchange composition and the solution composition will adjust to reach a new equilibrium. After a reaction has been simulated, it is possible to save the resulting exchange assemblage composition with the SAVE keyword. If the new composition is not saved, the exchange assemblage composition will remain the same as it was before the reaction calculation. After it has been defined or saved, the exchange assemblage composition may be used in subsequent simulations through the USE keyword.
Example problems
The keyword EXCHANGE is used in example problems 9 and 10.
Related keywords
EXCHANGE_MASTER_SPECIES, EXCHANGE_SPECIES, SAVE exchange, and USE exchange.
- Example 1
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- Explanation 1
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- Notes 1
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- Example 2
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- Explanation 2
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- Notes 2
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- Example problems
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- Related keywords
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User's Guide to PHREEQC - 07 MAY 96
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