I have used RO water (reverse osmosis water) and it's right at 7. I have tested this with a calibrated meter. RO isn't distilled but I figured I'd chime in. Most in hydro use RO water and not distilled.

--chris

I'm a plumbing contractor in real life. FYI "ALL" bottled drinking water sold in the United States is required to be reverse-osmosis treated.

There's a difference between R/O and "distilled". I just thought I'd throw that into the mix in case someone is trying to compare the two (distilled vs R/O) thinking they are the same; they're not.

I'd like to see where you got the RO requirement, Freeman. I'm not exactly saying you're wrong. I'm definitely saying it isn't enforced, though. Nestle doesn't use RO in their spring water process. Just charcoal filtration and ultraviolet disinfection. Evian uses nothing (and made the news for it last year). There are several ways companies are allowed to purify water and still call it purified. RO is just one. Doesn't matter. Wasn't arguing the safety of my drinking water. I actually do use RO for that.

Anyway, that is beside the point. I wasn't confused on the similarities of RO and distilled. I was at one time confused on why distilled is bad for you because lack of natural minerals is the common claim and rather than think about the science I went with the herd. If distilled is bad because of lack of nutrients, RO would be bad, too. That was the point of that comment. Now I know it's because RO isn't as acidic (in a fresh system, though it is in one where the film is damaged) as distilled even though nearly every gardening book I have and material that came with the test kit states it's neutral. It's not. It's only neutral directly after distillation, but in short order absorbs CO2 and becomes acidic (carbonic acid).

Chris, sorry but I have to be blunt. You're wrong. Most don't use RO. Feel free to do your own searches on this very site to see the general feeling that RO is too wasteful and expensive. You've fallen victim to marketing, perhaps. I was simply stating that many that are new to hydro that can't afford a meter use distilled because books on gardening and some sources on hyrdo toot it as neutral. Actually, I didn't originally mention "newbies", so that's my error. Still, it wasn't an opinion. That was a statement of fact that is based on the multiple mentions of it by newbies over the last year (how long I've been here) on this very forum. Many with experience have in turn told them (and me) that it is unnecessary and to just put money toward a meter or at least drops and leave tap water in a bucket overnight to gas off the chlorine. In fact, Freeman was one of those and even sent me an e-mail on the subject several months back (which I found very informative, btw).

My main point was to ask the questions of the last paragraph of my original post while informing new people that might see the title that distilled isn't neutral contrary to popular claims. Another reason I asked the questions here is because I find the people on this forum much more scientific than many of the regular posters on the soil forum and many here have the equipment like me to test water readily and might be using a different method than kits advise because they have realized the same issue with distilled water. So, can anyone answer the questions even though they aren't about hydro nute testing but soil testing?

I don't personally test my soil. my cooperative extension does it for free. (well you have to pay for shipping) Have you called your local co-op and asked how they do it? I will try to call mine if I remember when I have time for that discussion.

You might get a low PH reading on distilled water because there are no buffers. I bet if you added just a bit of tap water back in, the PH would be much closer to 7 while still being very pure. I think distilled water should be at 7 if it was 100% pure, but without some dissolved solids, a little bit of acid will equate to a much lower PH.

joe.jr
"I'd like to see where you got the RO requirement, Freeman."

I'll research it and get back to you. I have a friend who bottles drinking water in a place called Parumph, Nevada. He's my original source for the ruling that all bottled drinking water must be R/O processed. Then again, that was a few years ago, so maybe things have changed since. I'll check it out.

zanderspice is correct. Distilled water contains no disolved ions - conversely - it has no defined pH. But a "whiff" of CO2 may bring enough ions into the water to swing the pH into the acid range. Even a contaminated pH probe will influence the reading.
Don't use anything to force the pH to read 7.0. That will severely influence the soil pH reading.

I was speaking based on what I've seen recommended from other forums, as you are speaking based on this forum. Obviously both of us are a bit off base and I think both sides are too based on our respective reference points.

But I will say that I know that RO water has many fans in hydro, but again that's from personal experience. It may not be representative of hydro users as a whole. But I'd say this forum doesn't have a large enough user base either to prove/disprove the viability/use of RO water.

-chris

Just a note here about choosing to grow in either tap or treated (R/O or distilled) water :

Many of us, myself included, live in areas where the water is pumped in from a great distance away. In my case (Southern California) that water picks up a lot of calcium on its journey from the Colorado River and points beyond. The calcium content is so great, it reads an EC of 1.2 right out of the tap.

Many plants love calcium, so it turns out to be an added benefit. Treated water removes that calcium content to the best of my knowledge.

Not so fast, greystoke. :) Distilled water does have ions. It has hydronium and hydroxide ions. Most people don't know that water actually dissolves itself. In any volume of water, a certain amount of the water is actually in solution.

Pure water (distilled) also is pH 7.

Here is a link that might be useful: Water dissolves water.

Point taken.
But the concentrations are very low, and knocked off balance for the slightest reason. Declaring the pH "undefined" is not far off the mark.

First: Chris, you're right. This forum isn't big enough to represent the norm. I should have said on this forum as I only know a handful of hydro growers personally.

Second: Hydrorob, distilled water has a pH of 7. . . unless you allow it to have contact with air. It rapidly absorbs carbon dioxide to form a carbonic acid (as indicated in the link below by the EPA, but their range is 5.6-7). Since posting the other day I have done a ton of research. Online and offline. I spoke with a chemist from Eli Lilly while there on a sales call (who thought it was common knowledge that distilled water has a pH of 5.6 to 6.5), the local waste water treatment plant, and both local hydro shops. One hydro shop said that is exactly why you should buy an RO unit (go figure) while the other said I was wrong. I challenged her to test the distilled water she uses to clean her probes and wham, I was right. We used the water that was still in the container, not the used water. The water treatment facility said yesterday that it's one reason drinking water doesn't go through a distillation process. Distilled water readily absorbs anything and would eat away at pipes in no time flat, which should tell people why drinking it is a bad idea.

Grizzman: Yep, called the extension. They said that distilled water has a pH of 7. Of course, when I went through the master gardener program, a lot of bad info was put out so I don't assume they all know what they are talking about. In the program, they had several different teachers and a few completely contradicted each other on various issues. None thought hydroponics was a valid form of gardening and a couple suggested it is "too experimental". That told me right there they were idiots. They don't test soil anymore, but gave me a list of private companies that do. That was before I posted. I originally wanted to have the extension test it. I am only testing it myself because I couldn't get them to and don't want to pay $20 plus shipping for each sample.

Here is a link that might be useful: EPA

I'd always read never to use distilled water in hydroponics. Only RO and then only if the water quality of municipal water was bad is what I'd been taught.

RO is nice, but you end up having to add in more nutrients than you would if your city water is decent. In some places you don't have to add cal/mag to your system at all if you use city water because there's plenty already in it.

I am almost ready to start my first DWC hydroponic garden and is wondering if my tap water is good enough or should I use a Brita filter for the nutrients. The guy at the hydro store recommends it but after reading some of the comments, I'm thinking that maybe I don't have to. Please tell me what would you do.

Here's our city water quality fact sheet:
PH 7.5 - 8.5
Turbidity 0.01 - 0.50 NTU
Conductivity 250 - 400 us
Total dissolved solid 142 - 278 mg/L
Hardiness 91 - 154 mg/L as CaCO3
Chlorine 0.10 = 0.40 mg/L
Nitrates 1.05 - 3.83 mg/L
Volatile and synthetic organic chemicals were not detected

FWIW,

I live in Cincinnati and if I fill a large vat with 20 gallons of tap water, aerate it for 48 hours and test its pH, I get a reading of 6.9. This is at the top of the water and it is possible some things have settled on the bottom, despite being aerated (not a powerful pump!).

Mike

Hello Lolak,

Your water is not terribly bad for hydroponic growth, despite being quite rich in Ca (and probably a little in Mg) it does not have any significant issues. I would definitely recommend you use a solution made by yourself in which you avoid high additions of calcium and work around the chemical analysis of your water.

The levels of chlorine are also quite significant so I would advice you aerate the water for 24 hours before putting it in contact with your plants and add your nutrients after that. As I say, preparing a custom formulation to take into account the 90-150 ppm of Ca might be necessary to achieve good results.

In this case a final solution made from Potassium Nitrate, Potassium hydrogen phosphate (also known as dibasic potassium phosphate), magnesium nitrate, magnesium sulfate, boric acid, iron sulfate, copper sulfate, manganese sulfate, sodium molybdate and zinc sulfate should be adequate. I have a free ebook within my website explaining how you can make your own solutions, you can email me if you want me to adjust the spreadsheet to account for your special needs :o).

I hope this helps :o)

Best Regards,

Daniel

Here is a link that might be useful: Everything Hydroponics

I don't use distilled myself, but I was under the impression that a pH of 5.0 would be considered neutral. That's because it falls directly in the middle of the pH scale of 1 to 10. I was also under the impression that anything above or below a 5.0 would be considered either a acid or base solution.

A pH of 5.5 and 5.3 is not exactly 5.0, but it's quite close. I use RO water myself and I know the pH of our RO water fluctuates, it will be low for weeks (in the 4 range), then suddenly it will become high again (in the 6-7 range), then in a few weeks it is low again. We also change the filters twice a year.

Hello Everyone,

The actual definition of pH sets the neutral value at seven since this is the value where H3O(+) and OH(-) ions have the same concentration 10^-7. Since the equilibrium constant for the self-dissociation of water always has to be satisfied and its value at 25C is 10^-14 then it follows that the product of the concentration of both ions always has to be 10^-14. Since water is neutral when the concentration of both ions is equal then they both have to be 10^-7. Since pH is -Log(concentration of H3O(+)) then a pH value of 7 is neutral and the pH scale in water actually goes from 0-14 and not from 1-10.

Sorry if I got to technical :o), but this is the real explanation of why neutral pH at 25ºC is 7 and why the scale goes from 0 to 14. This value varies very very slightly as temperature changes but for practical purposes 7 can always be assumed to be the neutral pH.

An acidic pH of 5 in distilled water is due to the contact with carbon dioxide in the atmosphere which forms carbonic acid when it reacts with water, therefore dropping the pH from 7 to 5-6. However this is due to the fact that distilled water has absolutely no buffering capacity and therefore a very very slight amount of acid can drastically change its pH. On the other hand, RO water from home purifying systems has low conductivity but still contains much more ions than distilled water. Depending on the nature of the RO system/purification the output can have different pH levels.

I hope this helps everyone understand pH and the difference between RO and distilled water a little bit better :o)

Best Regards,

Daniel

To put it in simple words, the pH scale goes from 1-14 and 7 is considered neutral. This is in fact the most basic knowledge that everyone doing hydroponics should have 'intus'.

I actually appreciate some technical input of yours Daniel, what I am unfortunately missing now and then, are conclusions that are understandable for laymen.
So let me ask by paraphrasing: you say that the only difference between Distilled ad RO-water is no ions at all compared to some ions, right?. That in fact is more like basic - but is there a difference between RO- and distilled water in terms of susceptibility for drastic PH changes with the add of (as you call it) "a very very slight amount of acid"?
And, what about other actual and pragmatical differences between RO-water and distilled water and their affinity for carbon dioxide out of the atmosphere? Is the difference notable, drastic or does it depend on how much and what kind of ions some specific RO-water still has? Let's say RO-water has between 15 and 50 ppm, just to have some workable figures handy. Is there a real difference with a highly purified (10-15 ppm) RO-water and distilled water in this context, and will a content of let's say 40-60 ppm of "impurities" change anything?

What is the "final" conclusion, do you recommend RO-water for the use with nutrient solutions, either for commercial nutrients or for adequate "self-made" nutrients?

And finally, if using your recommended "Ion-exchange resin pH regulation" what would be the outcome if using them with RO-versus distilled water (without the add of any nutrients) - from a purely theoretical point of view of course, because it's not an actual practice nor procedure? Would there be ions to exchange at all with distilled water, when there are no ions...? I am not rhetorical with anything I am asking, - just interested in conclusion I can put into or transfer to actual practice. ;-)

Hello Lucas,

Thank you for your input and questions :o) I will now try to answer some of them here :

So let me ask by paraphrasing: you say that the only difference between Distilled ad RO-water is no ions at all compared to some ions, right?

- Yes effectively this is the most important difference, since the reverse osmosis procedure is not 100% efficient, a lof more ions are left inside the water when compared to distillation.

That in fact is more like basic - but is there a difference between RO- and distilled water in terms of susceptibility for drastic PH changes with the add of (as you call it) "a very very slight amount of acid"? And, what about other actual and pragmatical differences between RO-water and distilled water and their affinity for carbon dioxide out of the atmosphere? Is the difference notable, drastic or does it depend on how much and what kind of ions some specific RO-water still has?

- Of course, as RO water has some ions it can have some added buffer ability. Depending on the nature of these ions the interaction with carbon dioxide might be very different. For example, if some carbonates are left within the water this will tend to creat a carbonate-bicarbonate buffer with time as carbon dioxide from the atmosphere is introduced. However some RO systems include ion-exchange columns that may in fact make the output water acidic. The final ionic composition of your RO water varies depending on the initial water source and the actual purification system.

Let's say RO-water has between 15 and 50 ppm, just to have some workable figures handy. Is there a real difference with a highly purified (10-15 ppm) RO-water and distilled water in this context, and will a content of let's say 40-60 ppm of "impurities" change anything?

- Yes, of course. distilled water has a conductivity that is for most practical purposes very close to 0 ppm. Just having and additional 1 ppm changes the buffer and pH responses of the water significantly. The more impurities you have, the lower the response will be to atmospheric carbon dioxide. However, this difference is not extremely important when using the water for hydroponics since the amount of salts is in either case small and the actual pH will be set by the hydroponic nutrients.

What is the "final" conclusion, do you recommend RO-water for the use with nutrient solutions, either for commercial nutrients or for adequate "self-made" nutrients?

- I would recommend RO water because distillation is a much more energy intensive process that yields water quality which is simply "unecessarily pure" for hydroponics. The additional 40-60 ppm of salts dissolved in RO water will most probably not make any noticeable difference while the process to obtain RO water is much less energy intensive.

And finally, if using your recommended "Ion-exchange resin pH regulation" what would be the outcome if using them with RO-versus distilled water (without the add of any nutrients) - from a purely theoretical point of view of course, because it's not an actual practice nor procedure? Would there be ions to exchange at all with distilled water, when there are no ions...? I am not rhetorical with anything I am asking, - just interested in conclusion I can put into or transfer to actual practice. ;-)

- The outcome with both wouldn't be very different after the solutions are prepared. Certainly if you use an ion exchange resin with RO water before preparing the solutions the water will be further purified to eliminate ions in exchange for acidic protons of the resin but in the end, after the addition of nutrients both RO and distilled water sources will have the same response.

I hope this answers your questions :o) Thank you very much for your comment,

Best Regards,

Daniel

Thanks Daniel for the answers that are indeed clarifying all things.
You have probably noticed that these questions weren't exactly random;-)

I guess it's important and useful to have such kind of conclusive aftermath and "question & answer games" with any rather technical content. Some of it isn't exactly obvious for everyone and may even lead to some misconceptions if not discussed further or put in a modified context.

The essence here is that the purer the water the better - and that the add of "your" or a distinct nutrient formula gives such water a different and very specific quality that is best suited for the purpose.

While it is still possible to adopt a nutrient formula to some hard, mineral rich or "special" water, as long as the content is known (by analysis), - this isn't exactly in everyone's reach. Simply because such water would in fact need a specially designed nutrient formula that has to be figured out and "custom made" somehow. Thus it again draws down to the conclusion that the purer the water, the better.

As far as the pH of any water quality is concerned, all "base water" has its own chemical signature, pH properties, affinities to carbon dioxide and other typical qualities, greater or lesser susceptibility to acids or bases. But as soon as one adds a specific (any) nutrient composition in fair amounts, all of it gets actually "reset" chemically, - anecdotically speaking of course... ;-)

Hi Daniel,
Thank you very much for the thoughtful comment on the quality of my tap water. I think I will go ahead and experiment with the DWC system and use it as a stepping stone for a more efficient one. I visited your blog, very helpful and encouraging, but there is so much to learn and my chemistry is more than 25 years old. So I am taking baby steps. Hopefully I will someday be able to use your calculator to make my own nutrients. And thank you everyone for graciously and selflessly sharing your ideas and experiences. You give me hope. (I just notice chlorine is 0.10 - 0.40 mg/L, not 0.10 = 0.40)

Lolak

Wow fantastic info!

So I just want to make sure that I've got this right...if I water my houseplants with distilled water, will they not care about the water PH because the water will just adopt the PH of the soil? Or will it throw PH off?

Background to this question is that I have had PH issues with my plants from really alkaline tap water, so I switched to distilled and I have a few plants (all African Violets) on distilled water, a few plants on distilled water + 3 drops of PH Decrease, and then another set of plants with distilled water, 3 drops of PH decrease + Dyna-Grow Foliage Pro and I'm trying to figure out how to get the PH down without over kill. I'm also growing them in a mix of Pumice, Lava Rock, and Turface which I recently repotted from a heavy peat mix.

The low pH means almost nothing since the moment you dissolve fertilizers in it, since these pure waters (not including mineral water which is not pure in a chemical sense) have no buffering ability, the pH snaps immediately close to the pH of your fertilizers which each have some capacity to buffer pH.

Ignoring the background since that might get complicated, if you put distilled water in soil you are correct in that it will assume the pH of the soil provided we are talking about a system that generally has more dirt than water. Hydroponic systems do not have dirt, that is the defining characteristic. Instead they have supposedly an inert medium. If you are using an inert medium instead of dirt the pH theoretically could stay the same as the the pure water that has experienced carbonation. But even there the moment the nutrients are dissolved in, it becomes simply an academic observation since the nutrients, pH adjusters, and whatever was in the roots will determine the pH and the low measurement initially will prove just to be a ghost, like an engine that turns over because of a tiny amount of fuel vapor left in the fuel line which is immediately consumed as if it were never there.