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Thread: distilled vs. reverse osmosis

  1. #1

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    distilled vs. reverse osmosis

    I tested my reverse osmosis systems water the other day since i had access to a TDS meter (total dissolved solids) my regular city water tested at 200ppm. my RO water tested exacxtly the same as a gallon of distilled...0ppm. My question is...If this tests so pure would there be any problem using RO water in place of distilled when mixing chemicals?

  2. #2
    the Docter is in Arne Croell's Avatar
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    distilled vs. reverse osmosis

    There is no problem using it. Just make sure you exchange the filter cartridges regularly. Actually most labs use RO systems now instead of distillation since the energy consumption is much lower and a good RO system gives you an even better quality. 18 MegaOhms resistance is a typical value for good RO water for semiconductor use ( which would be total overkill for your purpose, btw).

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    distilled vs. reverse osmosis

    I once tried using reverse osmosis water for ABC pyro stock solutions, with disastarous results. I remade the solutions w/ distilled water and, with all other factors the same, the results were fine. I doubt many other applications would be this critical, but this is a case I found it really mattered.
    Brook

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    distilled vs. reverse osmosis

    Hi Robert,

    It probably would not be a problem to use water that has been filtered by reverse osmosis. We refer to it as "purified" drinking water here in Irving, TX. Our drinking water, straight out of the tap, tastes like it's loaded with sanitation chemicals. The only way I can make it palatible, is to filter it through the filter on the ice maker and water dispenser that is built into the door of my refrigerator. My wife drinks bottled spring water, most of the time.

    My only concern is when I am making up working solutions of developer. I use distilled water, although the developers that I use contain a sequestering agent. I still want to eliminate any possibility of introducing chlorine compounds. Perhaps I'm being too picky, but so far, it's been working just fine.

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    distilled vs. reverse osmosis

    Thanks for the response. As I said I tested the water with a TDS meter. I have talked to the people at kinetico ( I have the kinetico GX deluxe system with a 20 gallon holding tank) Now granted these are the people that sold me the system but they claim that it filters out chlorine compounds. Reverse osmosis, as I understand it works very similar to a dialysis system. it runs through two prefilters and then through some type of bladder for final purification. As I said it reads 0ppm just as a bottle of distilled reads. The tap water reads 200ppm. Shouldn't chlorine compounds read on a TDS meter? This would be very convenient as I have a tap right at the darkroom sink.

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    distilled vs. reverse osmosis

    brook, That is what has me asking questions. I have mixed developer with both and can't see any difference. The people at Kinetico will tell you if you are trying to treat well water the results will vary depending on the contents of your well naturally. But I'm running treated city water through it and with a reading of 0 parts per million. The same reading i get with distilled.

  7. #7

    distilled vs. reverse osmosis

    Sorry to burst your bubble Robert, but there is no way reverse osmosis water can have 0 TDS. I worked in the ultrapurification industry for many years and this is just not possible unless you are doing 3 or 4 passes. Typically a reverse osmosis membrane (depending on the kind you are using) will have from 95 to 98 % rejection rate. That means that if your city water is comming in at 200 ppm TDS, your RO water would have anywhere from 10 to 2 ppm TDS. Home units are much less efficient than industrial units and I am sure your home unit most likely has a rejection rate of 85 to 90%.
    Most likely your TDS meter is not sensitive enough or calibrated correctly. If you have a dual meter that also reads conductivity and/or resistance if you use the resistansa setting you will see that RO water has less than 18.7 mmhos of resistance. 18.7 is the number obtainced for water with no salts present. Salts are what lend water the necessary bridge to conduct electricity.

    Chlorine will not show up as TDS, you have to have a specific kit for chlorine and free chlorine present in water.

    Having said this, RO is the typical method used to purify drinking water, and it is pure enough to use for developing, there is no need to use destilled water. Heck, at 200 ppm TDS I would say your tap water is good enough for developing, mine has 300 and it is what I use for

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    distilled vs. reverse osmosis

    Any idea if RO/DI water is also suitable? I seem to remember something about it altering the ph of water.

  9. #9
    the Docter is in Arne Croell's Avatar
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    distilled vs. reverse osmosis

    My comment above was more related to my experience with lab systems (Millipore) than household ones. A TDS meter also measures resistance/condunctivity, just converts the conductivity to ppm assuming a certain type of salt (usually sodium or potassium chloride). What it does not measure are organic contaminants, but those should have been eliminated by the water treatment for tap water. Its important to keep the system clean preventing growth of algae etc. to avoid downstream organic contamination.

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    distilled vs. reverse osmosis

    Robert,

    Just to add a little to the mix. Both Distillation and Reverse Osmosis are processes that are excellent for removing particulates and bacteria, may be good or excellent for dissolved solids, but are not good for dissolved ionized gases. When looking at the effectiveness of the RO system, you also have to look at the quality of the RO unit you're using. I would suspect that some of those home type kitchen sink systems are not all that good and that could be the source of problems that Brook encountered. If you have a source for good quality RO water, then it could work just fine, but only testing will really tell.

    About Distilled Water - if you do as I do and buy it by the two gallon jug at the local grocery store, you need to be concerned about how long you keep the water after you open it, and how long you store the bottles. Once the bottle is opened, it is exposed to bacteria and contamination can begin. Whether this will affect the chemical quality or not, I don't know. Perhaps the chemicals themselves can kill any bacteria in the water anyway. Regarding storage of Distilled water, if the storage bottle/container is not made out of inert materials, Ions or plasticizers will leach out into the water over time. I would think that those plastic jugs are not really the best materials to use. Again, I don't know if this will affect the quality of your developer or not, but it is something to consider. I make a practice not to use "old" distilled water for my developer just to play it safe.

    One other note, Arne mentioned the the resistance of water. Controlling the resistivity of water is a different purification process called Deionization that removes most of the ions from the water. That is a process that I don't think will improve the quality of our photography chemicals and is also a pretty expensive process (resin bed cartridge replacement cost $$$). I haven't heard of anyone using Deionized water for photographic chemical processing.

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