I am researching for an extended experimental investigation into methods of water softening. One such method is the use of ion exchange resin. I have contacted the supplier of the resin for its exchange capacity, but I'm not sure how their answer relates to the removal of magnesium and calcium ions from water.

They say the exchange capacity is "1.0 mEq/mL min of 99% in H form."

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    $\begingroup$ $\ce{H+}$ has charge of +1 but $\ce{Ca^{2+}}$ and $\ce{Mg^{2+}}$ both have a charge of +2. So the ion exchange bed capacity would be about 0.5 millimoles/ml for those cations. // Remember that random packing of spheres only occupies about 66% of volume. So a tube of 100 ml volume will only have about 66 ml of resin. $\endgroup$
    – MaxW
    Commented Feb 11, 2017 at 4:20

1 Answer 1


In the H form there are one plus charged ions ($\ce{H+}$) being exchanged in the water softening process. But the species being exchanged for the $\ce{H+}$ ions are doubly charged ($\ce{Mg^2+}$ and $\ce{Ca^2+}$), so two $\ce{H+}$ ions must be exchanged per $\ce{Mg^2+}$ or $\ce{Ca^2+}$ ion.

So, "1.0 mEq/mL min of 99% in H form" could equivalently be read as:

The exchange resin has the ability to exchange $\pu{0.5mmol}$ of $\ce{Mg^2+}$ or $\ce{Ca^2+}$ ions per $\pu{mL}$ of resin.


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