MIX
This keyword data block
is used to mix together two or more aqueous solutions. Mixing may be used
alone, in combination with additional reactions, or during advection or
transport calculations. All applications of MIX result in a batch-reaction
calculation that produces aqueous equilibrium, including redox equilibrium.
Example
data block
Line 0: MIX 2 Mixing solutions 5, 6, and 7.
Line 1a: 5 1.1
Line 1b: 6 0.5
Line 1c: 7 0.3
Explanation
Line
0: MIX [ number ] [ description ]
MIX
is the keyword for the data block.
number
--A positive number designates the following mixing parameters. Default is 1.
description
--Optional comment that describes the mixture.
Line
1: solution number, mixing fraction
solution number
--Defines a solution to be part of the mixture.
mixing
fraction --Decimal number that is multiplied
times the moles of each element in the specified solution; the mixture is the
sum of each solution times its mixing fraction. Mixing fractions may be greater
than 1.0.
Notes
In mixing, each
solution is multiplied by its mixing fraction and a new solution is calculated
by summing over all of the fractional solutions. In the Example data block, if
the moles of sodium in solutions 5, 6, and 7 were 0.1, 0.2, and 0.3, the moles
of sodium in the mixture would be 
.
The moles of all elements are multiplied by the mixing fraction of the
solution, including elements hydrogen and oxygen. Thus, the mass of water is
effectively multiplied by the same fractions. In the Example data block, if all
solutions have 1 kg of water, the total mass of water in the mixture is
approximately 
kg, and the
concentration of sodium would be approximately 0.16 mol/kgw
(0.3/1.9). The charge imbalance of each solution is multiplied by the mixing
fraction, and all the imbalances are then summed to calculate the charge
imbalance of the mixture. The temperature of the mixture is approximated by
multiplying each solution temperature by its mixing fraction, summing these
numbers, and dividing by the sum of the mixing fractions. Other intensive
properties of the mixture are calculated in the same way as temperature. This
approach for calculating the temperature of mixtures is an approximation
because enthalpies of reaction are ignored. For example, heat generated by
mixing a strong acid with a strong base is not considered.
This formulation of
mixing can be used to approximate constant volume processes if the sum of the
mixing fractions is 1.0 and all of the solutions have the same mass of water.
The calculations are only approximate in terms of mixing volumes because the summation
is made in terms of moles (or mass) and no consideration is given to the
partial molar volumes of solutes. Similarly, the formulation for mixing can
approximate processes with varying volume; for example, a titration.
Mixing results in a
batch-reaction calculation, which produces aqueous equilibrium, including redox
equilibrium. SOLUTIONs may be
defined with redox disequilibrium by defining concentrations of individual
valence states of elements. When SOLUTIONs
are mixed, all valence states of elements react to redox equilibrium. Thus,
even if a single solution in redox disequilibrium is mixed with a mixing
fraction of 1.0 (which will not change the total concentrations of elements),
redox reactions will occur among the valence states of elements, which in turn
will change the pH and pe of the solution.
When multiple
batch-reaction steps are defined in KINETICS,
REACTION, REACTION_PRESSURE, or REACTION_TEMPERATURE, and if INCREMENTAL_REACTIONS is false
(cumulative reaction steps), then each batch-reaction step uses the same mixing
factors; if INCREMENTAL_REACTIONS
is true (incremental reaction steps), then the mixing fractions are applied
during the first batch-reaction step only.
Related
keywords
INCREMENTAL_REACTIONS, SOLUTION, SAVE solution , USE solution , and USE mix .