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I recently installed a water softener and having a fair amount of clear iron in our water, I've been using Morton Rust Remover pellets which are basically NaCl with citric acid added as an additive. I'm guessing the iron removal function has something to do with the citric acid since it's the only notable difference between this salt and regular water softener salt.

At some point I'd like to stop using NaCl and switch the KCl since it seems to be more environmentally friendly. However, I really notice a difference using the iron removal pellets and I've been unable to find an equivalent product for sale which uses potassium.

Can I simply mix some citric acid in with the potassium in the water softener to achieve the same result? Or do the citric acid and the sodium have some special relationship which helps with the iron removal?

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Your ion-exchange column removes $\ce{Fe^{3+}}$ ions from your water by exchanging them for $\ce{Na^+}$.

When you back-flush you use concentrated solutions of $\ce{Na^+}$ ions and citric acid to remove the iron out of your ion-exchange column. The flush water is waste of course which must be disposed.

The exact chemistry mechanism of all of this depends on a number of factors:

  • NaCl has a solubility of 36 g/L at 20 C which is 0.62 molar but KCl has a solubility of 34.2 g/L at 20 C which is 0.46 molar.
  • I'd expect potassium citrate to be less soluble than sodium citrate but I didn't find any data to support this notion in a quick search.
  • I'd expect that you column has a better attraction to $\ce{Na^+}$ ions than to $\ce{K^+}$ ions.
  • If you have some sort of automatic monitor then it may not function properly with $\ce{K^+}$ ions. I'd guess that the monitor just looks to determine if the water is yellow, so I'd guess that $\ce{K^+}$ ions would work fine in this regard.
  • Not sure what pH you'd want. I'd guess maybe half trialkali citrate with half dialkali hydrogen citrate to keep the back-flush solution slightly acidic.

All in all my guess would be that potassium/potassium citrate flush solution would work. I'd expect the column to need flushing more often. Maybe doing a flush once in a while with the sodium/citric acid mixture would give the ion-exchange column a good cleaning.

The other consideration here is cost. Since the sodium/citric acid mixture is the common back-flushing salt, I'd expect a potassium/citric mixture to be more expensive.

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