Breakthrough and Exhaustion:

For gain further insight into the processes which occur in a column of mixed bed resin as it removes an impurity, that is worthwhile to construct a series of curves like as those displays in Figure.  These curves describes the behavior of hydrogen form resin as it erased NH₄⁺ ions from solution (the behavior of an anion resin is analogous).  There are two rows of curves schematically represent the concentrations of H⁺ and NH₄ ⁺ on the resin and in the effluent (exiting solution).

In the bottom row, concentration is plotted against volume of solution that has passed through the column. which is, the plots represent the concentration of the denoted ion in the effluent solution after a volume of solution has passed by the resin.  It is supposed in which the concentration of NH₄⁺ in the influent is constant.  NH₄⁺ ions are more tightly attracted to the resin than are H⁺ ions.  Therefore, NH ⁺ ions readily exchange for H⁺ ions on the resin.

Figure: Behavior of Hydrogen Form Cation Exchange Bed as it Removes Ammonium Ions From Solution

As solution passes through the resin column, a associated amounts of ammonium and hydrogen ions on the resin change.  The fact exchange process occurs primarily in a related narrow band of the column rather than over the whole length.  This band is known as the exchange zone. Supposing the column is vertical and which solution flows from top to bottom, a resin above the exchange zone is depleted; that is, practically all the exchange capacity has been used.  Below the exchange zone, fundamentally none of the resin's exchange capacity has been used.  As more and more solution flows by the column, an exchange zone gradually moves downward as more of the resin is depleted. Finally, as the exchange zone approaches the end of the column and small amounts of NH₄⁺ starts to appear in the effluent. A point at that this occurs is known as breakthrough. The concentration of NH₄⁺ in the effluent increases until it is the same as the concentration in the influent if more solution passes through the resin. This condition is known as exhaustion and denotes which essentially all the exchange capacity of the resin has been used.  (Since, of the equilibrium nature of the exchange procedure, a small amount of the resin might remain in the hydrogen form, but not enough to remove any more ionic impurities.)  Remember that because the exchange zone in that case was narrow, an associatively small volume of solution takes the resin from breakthrough to exhaustion.