I would like to calculate pH value of an acetate buffer at 200 C. I prepared the pH 4.0 acetate pH buffer at room temperature based on the recipe 0.1658 mol/kg acetic acid and 0.0358 mol/kg sodium acetate using MilliQ water. The measured pH is 3.90 which is very close to 4.0. Now I would like to use SpecE8 to estimate the pH at 200 C. There are two problems:
1. The outcome is charge imbalanced. The input represents the actual recipe so I am reluctant to add other ions (e.g., Cl-) for charge balance. Why it is not balanced?
2. The results showed that acetate and acetic acid were decomposed to other hydrocarbon species (e.g., Methane(aq) CO2(aq) Formate NaHCO3(aq) Ethane(aq)), and hence the pH buffer was not a buffer anymore and the calculated pH is 6.39. Even I set the temperature to 25 C, the result is the same and the calculated pH is 5.57, very different to the measured value 3.90. This is not correct. The acetic acid and sodium acetate are very stable in the temperature range and I would like to enforce these two species as stable species so they do not decompose during calculation in GWB. What I want is the pH value of this buffer recipe at different temperatures. How this can be done? I know I can suppress the hydrocarbon species, but there are so many to suppress. Every time I suppressed the major hydrocarbon species, and run again, other hydrocarbon species showed up. Is there a command to make specific species (in this case acetate and acetic acid) stable in calculation?
The script is attached.
By the way, I have the same problem for citric pH buffer.
Acetic pH 4 buffer.sp8