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Ion replacement on resin in water softening regeneration

Calcium 2+ replaces sodium 1+ on resin to soften water.  I understand it is because calcium 2+ has a greater attraction to the negative sites on the resin than sodium does.  Let's say then that calcium has the power to kick the sodium ions off the resin and take their place.  I get it.

My question is: how is it possible in the regeneration process for sodium which is less attracted to the resin to replace the calcium which has a stronger attraction?  How can the 'rules' simply be reversed?

I am looking for a detailed explanation.  All the answers I see simply state "since there are so many more sodium ions in the brine solution, they just overpower the calcium."  That is a very weak answer.  There must be some answer involving dissociation, equilibrium, dissociation constant values, etc., that more scientifically answer my question.

Thank you very much.  Richard Olsen, Edmond, Oklahoma

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Re: Ion replacement on resin in water softening regeneration

The answer is equilibrium. A highly concentrated sodium chloride solution is used during regeneration and is slowly pushed through the resin bed. The drive toward equilibrium and the difference between calcium concentration in bulk solution and calcium concentration on the resin and likewise the very high sodium concentration in solution and low sodium concentration on the resin assist in replacing the calcium with the sodium. The calcium that comes off the resin is carried away by the movement of the water through the resin bed. 


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