【How-to】How do you find the charge balance equation

Water must be electrically neutral (cation charge must equal anion charge). So, in theory, if you use the equation ([C]-[A])/([C]+[A])*100 (concentrations in meq/L, where meq/L = mmol/L*absolute value of ionic charge), the result is in percent error. A perfect analysis will lead to a charge balance error of 0%.