One for the Chemists in our midst: I'm going to need a fair quantity of deionised water in the next week or so to give the initial fill to a (new to me) Fanuc Wire Eroder that followed me home. As I understand it it has it's own small ion exchange resin column to keep the electrolyte de-ionised but the initial fill should be de-ionised water rather than tap water.
Is the resin ion exchange column used for producing de-ionised water the same sort of thing used in commercial water softeners which have such a resin column that can be recharged by passing brine through them? (If so I have several!!!)
Speaking from memory of more years ago than I care to remember - I have not researched this, but FWIW:
Ion exchange resins are either cation exchange or anion exchange. Anion exchange resins have attached + ionic groups with loosely associated anions. When "recharged", this anion is OH-; passage of water containing other anions results in the latter replacing the OH-. Cation exchange resins have attached - groups and associated H+ ions, and the feedwater replaces its + impurities with the H+.
Passage through 2 separate beds, one of each, means the OH- and H+ simply combine to give pure water. The resins are regenerated, separately, with alkali (OH-) or acid (H+) as appropriate.
I believe domestic water softeners work slightly differently. They still use ion-exchange resins, but have a mixture of both types of resin in a single bed, and this means that regeneration with HO- or H+ is clearly impossible (would need a mixture of alkali and acid, which would cancel out). Regeneration with NaCl means the associated anion is Cl- in the anion exchange bits, and the associated cation in the cation exchange bits is Na+; consequently the softened product has the impurity (typically calcium and magnesium salts, sulphate or bicarbonate) replaced with dilute NaCl*. Since Ca++ and Mg++ cause hardness (i.e. react with soap to form a nasty scum) and Na+ does not, this is fine for purpose. However, if the feedwater is for use in a boiler, or some other use which involves evaporation, it will leave behind the NaCl. Since the NaCl can be flushed out, whereas the Ca or Mg salt deposits probably can't, this may be OK; only you can say whether the corrosive NaCl being left behind, at least until you flush, is acceptable. If not - and I doubt it is - you need an industrial/scientific type of two-bed de-ioniser.
Of course, with your background, you may know some or most of this, but I have give a detailed description to make it helpful to others. Also, since I have no practical experience since university of water softener technology things may have moved on since I learned about it. However, the principles are probably still valid. Hope it helps.
*Hence the recommendation that you leave the kitchen cold water supply unsoftened, to reduce your NaCl intake.
The lab deionisers I have see always contain a cation and anion exchange resin which are sometimes in separate beds (or cartridges) or sometimes combined together.
Water softening type resins typically only contain a cation resin (for removing Mg & Ca). Such a cation resin is easily regenerated using brine. An anion exchange resin might be regenerated with sodium hydroxide, but the cation and anion resins are treated separately (one benefit of the two bed type exchanger). The mixed cation/anion single bed units tend to produce purer water though.
You can probably test whether your resin is effective enough by measuring the conductivity of the water. There are useful tables around but from memory about 1MOhm/cm is reasonable and 10MOhm/cm is good.
Water softeners usually use ion-exchange resins which exchange the calcium and magnesium ions which cause make water "hard" for sodium ions, so they wouldn't produce deionised water.
I would suggest that you make friends with the lab technician at your local comprehensive school which will have a deioniser for the chemistry dept. (when I was teaching chemistry we used a still but I think most schools went over to deioniser although distilled water should do fine)
I went over to my local bearing supplier recently, Southwest bearings in Melksham, and they had IIRC 25 litre containers of DI water in front of the counter for what purpose I don't know. I asked the price and it was about £12 + VAT. Last time I was at a local electroplater in Yate they had a 1000 litre IBC container of the stuff and said I could have some if I had a container, I'd just spent some money with them having some items copper plated so they were in a generous frame of mind but I didn't have a container so got some from Halfords instead.
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