View Full Version : distilled vs. reverse osmosis

21-Mar-2005, 09:20
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?

Arne Croell
21-Mar-2005, 09:30
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).

21-Mar-2005, 09:52
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.

Gem Singer
21-Mar-2005, 09:56
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.

21-Mar-2005, 10:09
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.

21-Mar-2005, 10:19
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.

Jorge Gasteazoro
21-Mar-2005, 10:43
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

Tom Westbrook
21-Mar-2005, 10:47
Any idea if RO/DI water is also suitable? I seem to remember something about it altering the ph of water.

Arne Croell
21-Mar-2005, 10:47
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.

Dan Dozer
21-Mar-2005, 10:56

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.

21-Mar-2005, 11:06
Thanks, I'm sure that is the case Jorge. The meter although brand new is a hand held Milwaukee. It has an accuracy of +or- 2% f.s. ( whatever f.s. is?) The first thing we did was calibrate in the solution provided with the meter. The reading from the RO system is 0 but as you said the meter must not be sensitive enough .It claims to read from 0 - 1990. I left the probe in long enough to reach the same temp as the water even tried moving it around in different areas of the container trying to get a reading of more than 0 and still can not get it to read any higher. I don't know if I'd attempt to reduce platinum or mix ferric oxalate with RO water but I think it will work well enough for mixing developers, toners, and clearing baths.

Sal Santamaura
21-Mar-2005, 11:14
"(whatever f.s. is?)"

Full scale. In this case, +/- 2% of 1990, or +/-39.8

Arne Croell
21-Mar-2005, 11:21
Dan, both ion exchange and RO remove ions, although through different processes, and not all ions (a standard ion exchanger, like that in certain dishwashers, replaces some ions (Fe, Mg, Ca etc.) with sodium ions) . The TDS measurement actually relies on the charge of ions in the water. The lab setup I was referring to actually uses first an ion exchanger and then a multiple stage RO unit to arrive at the ultrapure 18MegOhm water.

21-Mar-2005, 11:22
The RO water I had the trouble with was purchased from a grocrey store with the RO unit on site. If memory can be trusted it was called double reverse osmosis.
That said, I regularly make up other film developers with plain old tap water, and fresh D-23 has proven to be near fool proof ( and I, if anyone, can give fool proof a run for the money). Pyrogallol based developers seem to be espically finicky, ABC being the most prone to problems in my experience. I have made Pyrocat HD stock solutions from tap water with perfectly good results. On a practical level, unless you really have a thing for 1890's pyro formulas, I bet your results will be fine.

21-Mar-2005, 11:35
I just used the RO water to reduce sodium chloroplatinate (NA2) from 20% solution to a 5%solution. Now this may be so minute that I can't tell but there is no difference in the way the solution reacts to the print in comparison to the solution made with the distilled water. But I agree, we're talking drops here so why chance it...use the distilled. One other note....I was at the grocery store and went to grab a bottle of distilled. And lo and behold you have to watch the distilled water. Some jugs if you read the fine print are steam distilled and others are reverse osmosis distilled. And here I thought distilled meant steam....In large print the one reads....Giant Eagle Distilled Water....the fine print reads....bottled at the source: Alpine natural springs process by filtration, reverse osmosis.

21-Mar-2005, 11:48
brook, all i use is pyro developers, wd2d+ and pyrocatHD.....You mean there are other types of developers? (lol)

Calamity Jane
21-Mar-2005, 12:58
But . . . but . . . but . . .

. . . if you don't use distilled water, how do you explain to the revenuers why there's a still bubbling away in your basement : - 0

Steve Bell
21-Mar-2005, 13:27
Dan, you mentioned "I haven't heard of anyone using Deionized water for photographic chemical processing." I've checked all around here, no one sells distilled water anymore. All I can get is deionised water intended for topping up batteries, electric irons etc. As this used to be distilled in the past, I thought it was the same. As we have very hard chalky water here I intended to use it along with a wetting agent for the final wash of 4x5 FP4+. Will this be a worthwhile improvement over tap water?

Dan Dozer
21-Mar-2005, 14:24
Arne - I'm curious what part of the country you are in. I work as a specialty design consultant on high technology laboratory facilities. However, I'm not an expert on the plumbing system designs - I'm only an architect. I'm only asking because the typical system setup on my projects has standard filter cartridges first, then through the RO system to a storage tank, then to the Deionization system and circulated through the building. 18 MegOhms is standard as it leaves the DI unit. Your experience is that the DI and RO units are reversed. I wonder what the pros and cons are for each approach?

Steve - back to the photography stuff. I can still get distilled water around here, but the supply seems to be shrinking. I wasn't aware of the differences that Robert indicates whether it is steam distilled or RO distilled, but I'm now going to pay more attention. I just checked a jug I bought last week and it says steam distilled. I would also think that just about any type of purfied water for film washing purposes would be better than normal tap water. Laboratory users use deionized water when rinsing their glass ware to reduce the possiblity of water spots on the glass when it dries. The same should hold true for film and you might find out that you don't even need the wetting agent at all.

Charlie Skelton
22-Mar-2005, 03:32
As an aside, I remember a joke in New Scientist where someone in conversation dropped in " Everything I know about semi-permeable membranes, I picked up by a form of osmosis".



Kirk Keyes
22-Mar-2005, 09:29
"I'm only asking because the typical system setup on my projects has standard filter cartridges first, then through the RO system to a storage tank, then to the Deionization system and circulated through the building. 18 MegOhms is standard as it leaves the DI unit. Your experience is that the DI and RO units are reversed."

I agree with Dan - this has been my experience too. The RO unit does the bulk of the demineralization, but since it cannot really remove the last traces of ions, the water should go through a deionizing resin bed (for both anionic and cationic) to do the final "polishing". In addition to this, there should be a carbon trap to remove gases and organic compounds, as well as a UV light source to kill any bacteria in the system.

Many larger labs use a circulating system to keep the water constantly moving through the polishing resin beds, the carbon trap, and past the UV light. A large holding tank is used to store a volume of water, as these systems are slow to make deionized water and if there is a large demand, you will need a reservoir somewhere in the system.

Perhaps Brook is mistaking a RO demineralization system with a RO water softener. The softener will remove calcium and iron ions and replace them with sodium. I don't think it does anything with carbonate (someone please clarify this) and so that may be causing the issues seen with certain developers.

Jorge Gasteazoro
22-Mar-2005, 09:39
There are no RO "softening" membranes. The membranes are not ion specific as opposed to the ion exchange resin which can be tailored to either replace the Calcium ion with a Hydrogen ion or a sodium ion. Typically softening is used in home units because it is easier and safer to regenerate the resin with salt.

CArbonate is usually removed by lowering the pH converting it to carbonic acid which subsequently breaks down into carbon dioxide. In the case of home units the carbonate is left in the reject and many times is the cause of scaling and pluggin of the membrane.

Arne Croell
22-Mar-2005, 09:55
Dan, my experience is from Germany and a University lab environment, not a production facility. In our lab all the water goes through the DI system first,; as an example that includes the water for the "dishwasher" for the lab glassware. Just a part goes through the Millipore RO system after that to produce the 18MOhm water for the real high purity applications. It is not a continuous circulation system, and that might be the reason its different from a factory unit.

Gene Crumpler
23-Mar-2005, 19:03
John Sexton told us about Ansel Adams experience with a deionzer in his dark room. AA started having problems with his prints. After much investigation, AA called Ilford tech guys in. As soon as they discovered the deion unit, they told AA to disconnect the unit to his darkroom. This solved the problems. Interesting little tid bit!