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Re: bernd 050902

I am pretty sure that PHREEQC is doing the calculation correctly in all
cases. The problems you are solving are not equivalent. I just looked at
case 1 and case 3 for comparison. Initially solutions 0, 1, 2, and 3 have
volumes of 1 L. In case 1, you are taking .1 x 1 L of 0, .15 x 1 L of 1,
.25 x 1 L of 2, and .5 x 1 L of 3 to make 1 L of 1001. However, in case 3,
you are taking .072 x 1 L of 0, .108 x 1 L of 1, .180 x 1 L of 2, and .360
x 1 L of 3 to make .72 L of 1001. If you look at the results, the solution
compositions of the mixture are the same in terms of molality.

However, the next step causes a difference because solutions 0, 1, and 2
have 1 L of solution for both cases, but solution 1001 has 1 L for case 1
and .72 L for case 3. For the second mix in case 1, you add .1 x 1 L of
solution 0, .2 x 1 L of solution 1, .3 x 1L of solution 2, and .4 x 1 L of
solution 1001 to make 1 L of 1002.  For case 3, you add .072 x 1 L of
solution 0, .144 x 1 L of solution 1, .216 x 1 L of solution 2, and (here
is the difference) .288 x .72 L of solution 1001 to make .6394 L of
solution 1002. The proportion of 1001 in 1002 is different between the two
cases .4x1/1 = .4 for case 1 and .288x.72/.6394 = .32 for case 3.

I don't know which you want. Case 3 will give a constantly diminishing
volume of water; case 2 will eventually give a huge volume of water; so I
think you probably want to scale things correctly to keep a constant volume
of water.


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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