[antst]

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.

[Corribus]

It's hard to say too much without knowing better what you're trying to do, but basically all your charges have to add up to neutral. If you look at your ions, assuming the stoichiometry is 1:1:1:1:1:1:1, your charges do not balance to neutral (you have more positive charge than negative charge). So either you're missing an ion in your solution or you have more of the amount of some of the negatively charged species than you have indicated (say, three times the amount of chlorine ion as the other types of ions).

[Big-Daddy]

Presuming the pH published is correct, it just means that their list of ions in incomplete. e.g. if the article has no relation to acid-base chemistry, then they may not bother to mention that, say, along with "HCO₃^- entered originally into the solution" (e.g. NaHCO₃) there will also be the products of acid-base reactions involving the hydrogencarbonate ion, i.e. H₂CO₃ and the CO₃^2- ion. With the SO₄^2- ion, there will also be HSO₄^-.