The resin's active sites have different attractive strength for different ions, and this selective attraction can serve as a means for ion separation and quantitative analysis. In the resin used in this experiment, sulfonic acid groups bind the cations.
Sulfonic acids are close relatives of the more familiar sulfuric acid.
Schematic pictures of the atomic arrangements of these groups are given below. Like the hydrogens bound to the oxygens of sulfuric acid or methanesulfonic acid, the hydrogens on the sulfonic acid groups of the cation exchanger are acidic and can be readily lost through ionization. When pure water is passed through a cation exchange column in the hydrogen form, only a minute fraction of the acidic hydrogens are lost from the active groups. The imbalance of positive and negative charges is never large enough to be a significant factor in the resin's stoichiometric composition. The charge imbalance remains small because any build-up of negative charge on the surface of the resin attracts and binds the positive hydrogen ions more and more strongly.