ionic nature and yields ionic free water. The usual water
softening by ion-exchange yields soft water with lots of ions.
Reverse Osmosis types can reduce considerably both ionic and
non-ionic contaminants. Distilled water is essentially de-ionised
As for ultra-pure, fractional distillation, and ???. As for myself
and very hard water a small capacity RO unit may do. They
cost little but do need to be plumbed in. Dan
For years I had bottled distilled water delivered and finally got sick of the cost, went to hardware and got two filter cans and put them inline with the smallest micron filters I could find, change them once a year and I've never looked back and couldn't be happier. Use PFlow as per directions, hang my negs from a wire I have running the length of my darkroom, and never need to clean them.
To get water that is free of ions and organics, a mixed bed resin is often used. It combines and anionic resin, a cationic resin and an organic adsorbant such as charcoal or one of the more powerful resins for organics.
Many household fiters will do both.