RedOx is something else, involving a change in oxidation state of at least two and possibly more atoms or groups. When water is purified with de-ionisation, the positive ions (cations) are replaced with H+. This is done by presaturating a cationic resin with an acid such as HCl. When the water is then contacted with it, the sodium, calcium and other positive ions replace the H+ in the resin, and the H+ goes into solution. The opposite is also done, by presaturating an anionic resin with an alkali such as like sodium hydroxide. The OH- anions in the resin are then displaced by the anions such chloride Cl-, sulphate SO4-- etc. As you probably already know, H+ and OH-, when they come together, produce H2O. So the net result is pure water. Ion exchange is not a redox process, as the charge on the ion doesn't change and its composition doesn't change. It is only its dancing partner that changes.
Originally Posted by bernard_L
Reverse-osmosis is essentially filtration at a molecular level. Water is forced at very high pressure through a membrane with pores sufficiently large to let water permeate, but not large enough to allow most ions to permeate. Because of their charge, ions have a larger diameter and may be repulsed by selective membranes. The permeate is pure, and the retentate contains a higher concentration of dissolved substances. A practical setup for homes is to use the pipe pressure to do the work, and collect the permeate at a slow rate. The retentate remains in the supply line and will still be fine for bathing, dishwashing etc. The rate at which RO works at these lower pressures allows only a few gallons a day to be purified, but that is usually more than enough for photographic and other home use. The guys who breed tropical fish in 200L tanks might need a larger system. Incidentally, that is also a good place to look for an affordable RO kit.
All the processes to purify water are in one way or another energy, waste and/or cost intensive. In ion exchange, the two waste streams of acid and alkali used to condition the resins can be extensive. In RO, a lot of pumping energy is required to get the water through the membrane, and the membranes are expensive. In distillation, the latent heat of evaporation must be expended in the form of heating or vapour recompression, the latter being more energy (or rather, exergy) efficient. RO is best at removing everything including organics. Distillation is best at producing pathogen-free water, but cannot remove all organic volatiles. So for photographic use, I'd say RO is your best bet. Where I live, we pay ZAR 0,80 per liter. That is about $0,45 per 5 liters. It makes sense to use it for all photographic use other than washing, so I do not really ponder it much. I keep a 20 liter tank with a tap, and top it up with water from our local "fountain" where they fill 5L bottles for ZAR 4 a piece. Our local tap water is dodgy at times, and of course produces very significant drying marks. So the final rinse is always in purified water.
Last edited by dorff; 05-09-2013 at 05:14 AM. Click to view previous post history.