[Big-Daddy]

One of the conditions in which we do not use a charge balance expression is when the [H₃O⁺] value (pH) is kept constant. What other conditions would make it impossible to determine a charge balance?

[Borek]

Please elaborate. Charge balance holds always, we may simply ignore it when we know how to solve the problem in an easier way.

[Big-Daddy]

Please elaborate. Charge balance holds always, we may simply ignore it when we know how to solve the problem in an easier way.

Have a look at this problem:

http://answers.yahoo.com/question/index?qid=20100205031358AAjcvnD

The person who answers it writes that "Charge balance cannot be determined because pH is fixed (buffered)." Neither do I understand why this means we cannot use charge balance nor under what circumstances we choose not to use it.

Why can't we write 2[Cu²⁺]+[H₃O⁺]=[HS^-]+2[S^2-] as the charge balance? Not that we necessarily need it for this solution.

[Borek]

OK, what they mean is that we don't know concentrations of other ions used to buffer the system, so we can't write equation for charge balance.

[Big-Daddy]

OK, what they mean is that we don't know concentrations of other ions used to buffer the system, so we can't write equation for charge balance.

I see. So whenever I read "H₃O⁺  (or pH) is kept constant, at (a certain value)" I assume charge balance is out of the question then? What other cases would I realize that charge balance equations cannot be written?

[Borek]

It is not a matter of the problem wording, it is a matter of data available.

[Big-Daddy]

It is not a matter of the problem wording, it is a matter of data available.

OK, so in any case where new ions are introduced without being specified (as in a buffering system), charge balance cannot be calculated.

Under what circumstances in this common? When [H₃O⁺] is kept constant, evidently. When any factor is kept constant without specifying how it is kept constant (when there are equilibria that involve it, e.g. pH when Ka equilibria are there), is it impossible to write charge balance equations?

I'm basically asking how to determine when not to use charge balance.

[Borek]

Honestly, I have no idea about other examples, and I don't think it makes sense to list such cases, as the list could probably go forever. Whenever there are additional unknown equilibria in the solution, you can't assume your system (which is just a part of the whole system) in electrically neutral (which is what the charge balance reflects).