H   R R   OH Ca SO₄ Ca   R R   SO₄ H OH H   R R   OH Na Cl Na   R R   Cl H OH WATER TREATMENT PROCESSES DOE-HDBK-1015/2-93 Principles of Water Treatment CH-04 Rev. 0 Page 12 Consider a situation in which the entering impurities are calcium sulfate (CaSO  ) and sodium 4 chloride  (NaCl), and the ion exchanger is a mixture of both hydrogen and hydroxyl resins (mixed-bed).   (4-3) (4-4) In the reaction with NaCl, both Na   and Cl  ions are removed from solution and replaced by H + - + and  OH  ions, respectively (the CaSO   reaction has the same result).  If the initial solution - 4 contained only NaCl, then the concentrations of Na   and Cl  ions were equal.  Because both are + - removed  with  100  percent  efficiency,  the  concentrations  of  H    and  OH   ions  added  to  the + - solution are equal; thus, the solution is neutral.  In solution, H   and OH  ions must obey the + - relationship for the ionization of water (refer to Module 1). K   = 10 = [H  ] [OH ] w -14 + - Because of the very small value of the dissociation constant K  , the great majority of H   and w + OH  ions supplied by the resin must combine to form water by the following reaction. - H   + OH      H  O (4-5) + - 2 By this process, the original NaCl solution becomes pure water. The  preceding  examples  involve  hydrogen  and  hydroxyl  resins.    The  use  of  other  resins, especially cation resins, is very common.  For instance, suppose a solution containing Na   ions + is passed through a lithium resin.  Again referring to the relative affinities, Na   is attracted to + the resin more strongly than is Li  ; thus, Na   ions will displace Li   from the resin. + + +