Temperature of solutions and mixing

[TUB]

Hello there,
I'd like to mix several solutions. Every solution has a different amount of water and a different temperature.
For a mix that contained only several 20?C solutions and one very little amount of a warm solution, I noticed that the temperature calculation seemingly is not taking the amount of water into consideration, while the concentration calculation is. The example shows my problem: The resulting temperature should be 20. something and not 26?C. Is there a way to get temperature calculation with the water amount taken into consideration or am I making a stupid mistake?

Thanks in advance

SOLUTION 1
    temp      50
    pH        7 charge
    pe        4
    redox     pe
    units     mol/kgw
    density   1
    K         0.1
    F         0
    Cl        0
    Si        0
    Ntg       1 Ntg(g)     -0.107
    Oxg       1 Oxg(g)     -0.677
       -water    10 # kg

SAVE SOLUTION 1

END

SOLUTION 2
    temp      20
    pH        7 charge
    pe        4
    redox     pe
    units     mol/kgw
    density   1
    K         0.2
    F         0
    Cl        0
    Si        0
    Ntg       1 Ntg(g)     -0.107
    Oxg       1 Oxg(g)     -0.677
       -water    200 # kg

SAVE SOLUTION 2

END

SOLUTION 3
    temp      20
    pH        7 charge
    pe        4
    redox     pe
    units     mol/kgw
    density   1
    K         0.3
    F         0
    Cl        0
    Si        0
    Ntg       1 Ntg(g)     -0.107
    Oxg       1 Oxg(g)     -0.677
       -water    200 # kg

SAVE SOLUTION 3

END

SOLUTION 4
    temp      20
    pH        7 charge
    pe        4
    redox     pe
    units     mol/kgw
    density   1
    K         0.4
    F         0
    Cl        0
    Si        0
    Ntg       1 Ntg(g)     -0.107
    Oxg       1 Oxg(g)     -0.677
       -water    200

SAVE SOLUTION 4

END

SOLUTION 5
    temp      20
    pH        7 charge
    pe        4
    redox     pe
    units     mol/kgw
    density   1
    K         0.5
    F         0
    Cl        0
    Si        0
    Ntg       1 Ntg(g)     -0.107
    Oxg       1 Oxg(g)     -0.677
       -water    200

SAVE SOLUTION 5

END

MIX
1 1
2 1
3 1
4 1
5 1

END
   

[dlparkhurst]

You may have an older version of PHREEQC. The current version is 3.8.7, and a bug with mixing has been corrected. You will get correct results with the older version if you use SOLUTION_MIX instead of MIX.

Note that the mixing is done by mass of water, not by the volume calculation because most databases do not have the molar volume data to calculate solution volume. So, I think the temperature will be (200*4*20+10*50)/810 = 20.37.