Effects of temperature on pH

[mouseatwork18]

Hello,

I am hoping somebody can help me understand why the initial temperature is important when setting up a solution. In the code below I get very different answers (pH, pe) for the two solutions. The first is set-up at 20C and then subject to REACTION_TEMPERATURE 20 70 105. The second is set-up at 105C and then subject to the same REACTION_TEMPERATURE 20 70 105. I would have expected that if this is a closed system at equilibrium, the states at 70C and 105C should be the same, regardless of what the starting temperature. What am I missing?

Many thanks for your help!



Solution 1 NaCl
-temperature 20
Na 500
Cl 500
REACTION_TEMPERATURE 1
20.0 70.0 105.0
END

Solution 2 NaCl
-temperature 105
Na 500
Cl 500
REACTION_TEMPERATURE 2
20.0 70.0 105.0
END

[dlparkhurst]

I think the difference is caused by a difference in charge imbalance between the two initial solutions. You are specifying a pH of 7 for both, but the log Ks for hydrolysis reactions are different, which causes a difference in the calculated charge imbalance. Each of these charge imbalances is preserved in the subsequent temperature calculations resulting in different pH values.

You can think of it as having two different amounts of an unknown element in the two initial solutions that account for the charge imbalances.

If you add "pH 7 charge" to both initial solutions, then I think it will provide a better comparison between the two sets of calculations.