Cations are ions with a positive valence state (they are willing to accept electrons) and have the ability to react with anions which are ions with a negative valence state (they have extra electrons to share). The sharing of electrons creates electroneutrality. For example, the calcium ion is a divalent cation and will combine with two monovalent chloride ions to form the electrically neutral salt known as calcium chloride. A balanced water analysis will have the same concentration of cations as anions when reported as "ppm as calcium carbonate" or as meq/l. Silica, a very weak anion, is not used to calculate the ionic balance of cations and anions (though it is used in the calculation of TDS).
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