Answer: Yes, you have been told correctly. DI tanks have both cationic and anionic resins that have been regenerated with an acid such as hydrochloric, and a base, such as sodium hydroxide. The cationic resin attaches the hydrogen (H) atom, and the anionic resin attaches the hydroxide (OH) molecule. A water softener uses sodium chloride (NaCl) to regenerate the cationic resin and attaches a sodium ion (Na) to the resin, and one would think that it would be good to precede the DI with a softener. But the softener, when it's in service, releases sodium ions, and the sodium ions are closer in affinity to hydrogen than are calcium, magnesium or other ions. Thus, use of a softener in front of the DI, will introduce sodium ions and interfere with the hydrogen form resin’s ability to attach other cations.

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