**Charge balance equations** are derived using the concept of electroneutrality, where the sum of positive charges equals to the sum of negative charges in a solution. Such equations are useful for analysing acid-base equilibria and formulating complex acid-base titration equations.

Consider a solution containing water, a strong acid of concentration *C _{a }*and a strong base of concentration

*C*with the following equilibria:

_{b }With reference to charge balance of the above equilibria, the sum of the number of moles of cations *H ^{+}* and

*B*must equal to that of anions

^{+}*A*and

^{–}*OH*. As the volume is of the solution is common to all ions,

^{–}When formulating charge balance equations for aqueous compounds with multiple equilibria, we need to account for every charged species on the LHS of a particular equilibrium, which can be complicated. To avoid mistakes, select equilibria where species on the LHS are neutral. For example, equilibria for a diprotic acid can be presented in the following ways:

Select the first and third equilibria, i.e. we can imagine part of the initial number of moles of *H _{2}A* dissociating into

*H*and

_{a}^{+}*HA*with the remaining part of the initial number of moles of

^{–}*H*dissociating into

_{2}A*2H*and

_{b}^{+}*A*. For the first equilibria, the charge balance is

^{2-}For the third equilibria, the charge balance is

Combining eq31 and eq32,

###### Question

Write the charge balance equation for a triprotic acid.

###### Answer

Since,

we have,