How to deal with charge imbalance in a chemical system?

[bzz77]

I'm using a program for calculating chemical reactions, but I'm not a chemist. I will use the results for a different type of calculation. Sorry, this is probably a silly question.

As input for the program, I am using a chemical system that someone published. it contains:

SO4(2-), Ca2+, Na+, K+, Mg2+, Cl-, HCO3-. It has a specified pH. Each ion has a concentration value.

The problem is that when I put all this into the program, I get an error saying that I have a charge imbalance. I'm very confused because this is a chemical system that has apparently been used many times before. I'm not sure if this is important, but the method my program is based on is Gibbs free energy minimization. The method that was previously used on this system was the Law of Mass Action.

My program can add or subtract charge to make things balance, but this seems dangerous to me. Wouldn't this change the kinds of reactions that can happen? Also, when I do this, I get a crazy pH that is very different that the one published.

Does anyone have any insight into dealing with this? Or understanding it? Thank you very much.

 

[DrClaude]

I'm sure Borek will chime in with more pertinent comments, but I for one would like to see the reaction you're considering.

 

[bzz77]

Thanks for your reply, Dr. Claude. I'm not familiar with Borek (?). I'm not considering a specific reaction. I input the ions listed above with their published values, then the program uses Gibbs free energy minimization to calculate amounts of mineral and aqueous species.

I am woefully ignorant about chemistry, but I don't understand how the people who published about this system (they used the LMA method to calculate amounts of mineral and aqueous species--maybe the LMA method doesn't depend on charge?) didn't have this charge balance problem. Also, I don't understand the significance of the charge balance problem or how to fix it. My program adds a charge to make things balance (or I can change my species amounts), but then the calculated pH is very different than theirs. This doesn't seem right. I would like to reproduce their results.

 

[DrClaude]

Thanks for your reply, Dr. Claude. I'm not familiar with Borek (?). I'm not considering a specific reaction. I input the ions listed above with their published values, then the program uses Gibbs free energy minimization to calculate amounts of mineral and aqueous species.

He's one of the admins of PF, and very active in the Chemistry subforum.

I am woefully ignorant about chemistry, but I don't understand how the people who published about this system (they used the LMA method to calculate amounts of mineral and aqueous species--maybe the LMA method doesn't depend on charge?) didn't have this charge balance problem. Also, I don't understand the significance of the charge balance problem or how to fix it. My program adds a charge to make things balance (or I can change my species amounts), but then the calculated pH is very different than theirs. This doesn't seem right. I would like to reproduce their results.

Can you give references then? I get no idea of what you are doing and what you want to achieve.

 

[HeavyMetal]

Yeah, link us to the paper. It seems to me that if you're following a prep, you can just use the exact numbers that they're using there. Or are you trying to change it to something else?

 

[Chestermiller]

Are your initial input concentrations to the model such that the charges are balanced to begin with? When you say that it has a specified pH, does this mean that the initial H+ and OH- concentrations are also specified initially?

Chet