Why is RO/Distilled water so Acid on PH scale? - The Planted Tank Forum
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post #1 of 16 (permalink) Old 09-26-2015, 09:37 AM Thread Starter
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Why is RO/Distilled water so Acid on PH scale?

Why is RO/Distilled water so Acid on PH scale?

I would think that purifying water would be the absence of anything including anything acid or base.

Thanks.
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post #2 of 16 (permalink) Old 09-26-2015, 09:54 AM
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RO water has 0 buffering capacity, so it tends to read acid. Since there is nothing there, even small amounts of anything you add can cause a major swing in pH. I found that a 50 gal container of RO water could be shifted to a pH of about 8 with about 1/4 teaspoon of baking soda.
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post #3 of 16 (permalink) Old 09-26-2015, 10:41 AM Thread Starter
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Originally Posted by DaveK View Post
RO water has 0 buffering capacity, so it tends to read acid. Since there is nothing there, even small amounts of anything you add can cause a major swing in pH. I found that a 50 gal container of RO water could be shifted to a pH of about 8 with about 1/4 teaspoon of baking soda.
I took some distilled water and ran it into a zero water RO filter so I would have the most pure water and took it to Petco for testing since I was returning a dead fish. They acted like I handed them a jar of acid they said it was so far off the PH scales.

My real tank water is actually far on the base side.

Thanks for that great information. That actually makes a lot of sense. So the PH test may be a false reading or tainted by something to be acid due to the lack of any buffering?

Thanks
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post #4 of 16 (permalink) Old 09-26-2015, 02:23 PM
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RO water picks up CO2 from the air pretty easily. This lowers the pH.
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post #5 of 16 (permalink) Old 09-26-2015, 02:37 PM
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distilled water with no contact to air is a nice neutral 7.0pH.
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post #6 of 16 (permalink) Old 09-26-2015, 04:40 PM
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Originally Posted by mistergreen View Post
distilled water with no contact to air is a nice neutral 7.0pH.
Which of course no one can do in any practical circumstance.

RODI or distilled water exposed to air usually falls into the low 5's. Almost always well under 6.0. BUT....Distilled or RODI water is also not healthy for fish or plants without remineralization, so frankly even bothering to test RODI water is rather pointless. Mix up your minerals, always include at least a tiny bit of alkaline buffer (baking soda), then measure. Aerate thoroughly (hours). Adjust as needed; less baking soda = lower PH. Absent injected CO2 it takes very, very little baking soda (aka Alkaline Buffer) to drive the result above 7.0, so if you are going for an acidic mix it's a pinch in many gallons.

The myth that distilled water is always a ph of 7.0 has screwed up many people. Expecting a pet store employee to understand water chemistry is even worse than expecting them to, well, understand aquariums.

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post #7 of 16 (permalink) Old 09-26-2015, 04:56 PM
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Quote:
Originally Posted by Linwood View Post
Which of course no one can do in any practical circumstance.
boil to de-gass. Immediately pour into air tight container.

Agreed, distilled water is unhealthy if not kill fish.
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post #8 of 16 (permalink) Old 09-26-2015, 05:14 PM
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Originally Posted by mistergreen View Post
boil to de-gass. Immediately pour into air tight container.

Agreed, distilled water is unhealthy if not kill fish.


The boiling phase is especially useful if your fish have fattened nicely and your plants are collard greens, and you are hungry, but otherwise ....
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post #9 of 16 (permalink) Old 09-26-2015, 06:20 PM
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I remember the first time I used 100% RO/DI: I knew that you couldn't read pH because there are no ions in the water to react with the hydrogen ions in the probe, but I didn't realize that there would be effects of CO2 absorption. So I mixed in some bicarbonate and started with 7.0 pH (yeah), but the next day it was 7.5 (boo). I had to toss it and start over.

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post #10 of 16 (permalink) Old 09-26-2015, 06:49 PM
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A better test for RO:

TDS.
If your membrane needs replacing the TDS starts to creep up. Depending on the quality of the system, RO water ought to test in the single digits.

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post #11 of 16 (permalink) Old 09-28-2015, 02:46 PM
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My RO water comes out at 7.4. I buffer it to get to about 7.0. What is your PH before you run it thru the RO filter? I don't know if the RO filter has an effect on PH but just makes the water very soft.

Last edited by Clear Water; 09-29-2015 at 01:35 PM. Reason: 1
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post #12 of 16 (permalink) Old 09-28-2015, 03:07 PM
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My RO water comes out at 7.4. I buffer it to get to about 7.0. What is your PH before you run it thru the RO filter? I don't know if the RO filter has an effect on PH but just hakes the water very soft.
There's a big difference in RO (only) water and RODI water, are you just saying RO? And in retrospect not sure what the OP was asking about.

RO takes a lot of the dissolved solids out, but leaves behind a good fraction. Depending on how aggressively the RO filters are cleaned, it might take out 80-98% of what was in the water. If you have pretty high TDS going in, depending on filter maintenance, you might easily have 20-60 TDS coming out.

RODI water goes through an additional process (the DI part) that attracts the actual ions (separate but mixed media for positive and negative), so you are getting much closer to zero TDS.

Many people use RO water directly, both for drinking and for fish that like very low TDS, neutral water. Unlike RODI water it may have enough mixed in still to be healthy (depending on your kit). RODI water is definitely not healthy to use by itself (talking fish, not drinking here, it's kind of a myth albeit a complicated subject that RODI is bad for you to drink).

When you say you "buffer it" if you are lowering the PH, that must mean you are adding some kind of acid? That's the reverse problem most people have with RODI water, where they need to raise the PH (of the carbon dioxide mixture with the water) with an alkaline buffer.

One last comment... I've seen people using RO and RODI filters that were made for home drinking water use. Some (not all) of those actually have an additional cartridge that remineralizes the water. This is to make it taste better for drinking (most people's idea of "pure" water is flavored by a certain amount of dissolved solids, and truly pure water "tastes funny" to many). These can be really bad for fish if people think they are starting with real RODI water, and then remineralize it again. Not suggesting any poster has that situation, but just for people who might read this later.

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post #13 of 16 (permalink) Old 09-28-2015, 03:37 PM
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Originally Posted by Linwood View Post
One last comment... I've seen people using RO and RODI filters that were made for home drinking water use. Some (not all) of those actually have an additional cartridge that remineralizes the water. This is to make it taste better for drinking (most people's idea of "pure" water is flavored by a certain amount of dissolved solids, and truly pure water "tastes funny" to many). These can be really bad for fish if people think they are starting with real RODI water, and then remineralize it again. Not suggesting any poster has that situation, but just for people who might read this later.


Oh thanks for mentioning that! I have a RO unit I use for drinking water and was going to use it for Caridina shrimp, good to know that before I ordered them!

You mention now all have the remineralizing cartridge, how do I check/know if it does or doesn't? I suppose I can use the same test kits that I use for the fish water (API liquid test kits, pH, KH, GH, will get a TDS meter very soon)
Water tastes clean and drinkable to the taste buds, no nasty/funny taste to me. Maybe it does have the remineralizer or it doesn't and I just like the taste?
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post #14 of 16 (permalink) Old 09-28-2015, 04:02 PM
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Oh thanks for mentioning that! I have a RO unit I use for drinking water and was going to use it for Caridina shrimp, good to know that before I ordered them!

You mention [not] all have the remineralizing cartridge, how do I check/know if it does or doesn't? I suppose I can use the same test kits that I use for the fish water (API liquid test kits, pH, KH, GH, will get a TDS meter very soon)
Water tastes clean and drinkable to the taste buds, no nasty/funny taste to me. Maybe it does have the remineralizer or it doesn't and I just like the taste?
You have to ask or check, or if you see the physical unit the cartridges are labeled.

Your basic RO system consists of a pre-filter which removes sediment (it has no active chemical components), a carbon filter for removing various chemicals (including most chlorine), and then the RO membrane for removing most of the dissolved solids that remain.

Some systems add additional carbon filters before for more thorough use with chlorinated water (especially water with chloramines which are more widely used all the time).

This is all the basic "RO" part.

The ones that remineralize will then have a separate cartridge that is plumbed AFTER the RO membrane. So even if not labeled, seeing a cartridge after the membrane (assuming it is not a DI cartridge) is usually a remineralization cartridge.

On a distantly related note, home systems which have pressurized accumulation tanks are generally not as good for aquariums as otherwise, if you are trying for the most pure water. In any RO system, when the flow of water is stopped, there is a certain amount of seepage through the membrane (which depends on pressure differential to work -- when water stops flowing the pressure equalizes). Every time a RO filter is stopped for a bit of time and then restarted, the water for the first few minutes is much higher TDS than in normal run time.

If you are trying to make really pure water, it is useful to discard the first 2-3 minutes of run from the RO membrane when you start, and then don't stop it until you are done. If you have RODI filtration, it is actually helpful to have a divert between the RO and the DI part and let the first couple of minutes run off before switching it into the DI filter -- the DI is likely to remove all the excess stuff if you do not, but it will eat more into the lifetime of the DI media.

I guess I should also mention in all of this that for 99% of the people using any of these for fish or plants it just doesn't matter. Most tap water is just fine for most aquariums (suitably treated for Chlorine of course). It is always worth asking "what specific problem am I trying to solve", unless you (like me) are a bit OCD like me and just insist on trying for zero. Among the conditions that really need an RODI filter are very high PH (and you really need low PH, really need it), and tap water that varies a lot from day to day (and so you would be hitting the fish with lots of changes every water change). Most fish and almost all common plants deal just fine with high and low TDS, reasonably high and low ph, etc. They don't like rapid change, but can acclimate, even if their "native' land is very specific. Remember, most fish are dozens or hundreds of generations from their native lands and have already acclimated to some other environment, as many are raised in outdoor ponds that for darn sure don't use RODI water, and most of the rest in huge indoor facilities that also rarely use RODI water.

But if you decide you want to do RODI, it's nice to study it a bit; if you are going to put all the expense and effort into it, it is a shame to waste all that time and effort and get water that isn't really pure, even if you then just dirty it up again to put it in the tank (i.e. remineralization).

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post #15 of 16 (permalink) Old 09-28-2015, 05:39 PM
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Just for clarification on the original post. In water dissolved CO2 + H2O = H2CO3 or Carbonic acid. This is a weak acid meaning it will sometimes (not always) ionize into H+ & HCO3-. The free proton (H+) is what the pH scale measures in a liquid. pH means "Potential Hydrogen".

Strong acids like HNO3 (nitric acid) amost always ionize into H+ and NO3-(nitrate). This is why old tank syndrome usually has very low pH levels.

RO, RODI, or DI water is used extensively in saltwater setups where the mix of salts and minerals are critical to be correct. This purer form of water is used to avoid changing mineral balance during mixing or top-offs.

Also don't be thinking that the purer the water is the better for you it is. Triple distilled water is some of the purest you can get. It is an extremely strong solvent that has to be stored in high quality glass bottles. I will dissolve just about anything given time.
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