[steve.zoller]

I have high mineral water per a test by Ward.  So I bought an RO filter and feed it with softened water per manufacturer's recommendations.  My test papers tell me the softened water pH is >6.2 (paper limit) and the RO water is <4.6 (paper limit again).  Does this make sense that the pH drops so much?  Is water this acidic OK for mashing?

Thanks in advance to anyone responding.
Steve

[nateo]

I've accidentally over-acidified my water. I think I got down to 5, IIRC. It still hit the proper pH in the mash though. Once you neutralize the alkalinity, it only takes a little bit of acid to drop the pH way down. Also, RO water is known to be corrosive, though I think your strips are probably wrong.

[punatic]

Yes. it makes sense.  Dissolved carbon dioxide in the water passes through the RO membranes while minerals do not.  Dissolved CO₂ forms carbonic acid (H₂CO₃) and acidifies water.  The demineralized water on the permeate side of the RO membranes has little buffering in it to resist the pH change caused by the dissolved CO₂ passing through the membranes.

[BrewArk]

Yes. it makes sense.  Dissolved carbon dioxide in the water passes through the RO membranes while minerals do not.  Dissolved CO₂ forms carbonic acid (H₂CO₃) and acidifies water.  The demineralized water on the permeate side of the RO membranes has little buffering in it to resist the pH change caused by the dissolved CO₂ passing through the membranes.

Being unbuffered, the water should be fine for mashing (the pH will come back up some when you mash).  You do want to back add some minerals (blend with your untreated water?) for your flavor profile.

[mabrungard]

As mentioned above, RO membranes are dissolved gas permeable.  Hydrogen sulfide and CO2 are common issues in RO treatment systems.  We often pass the treated water through an air stripping tower to get those excess gases out of the water. 

Also as noted above, any heating of the RO water will drive off that dissolved CO2 and return the water pH to a more moderate pH closer to 7.

PS:  Don't worry too much about the pH of any water source in brewing.  Its the alkalinity that counts.

[anje]

Has your RO water sat in a container for a while, or is it newly generated? As I understand it, the pH of water in general tends to drop over time after water is distilled, simply due to CO2 from the air going back into solution. RO water probably behaves similarly, though I'm sure the above posters are correct in saying that at least some H2CO3 passes through the filters from the start.

In any case, after RO, the water isn't going to have any buffering capacity to speak of. It'll all be about your grains and what minerals you add to it, not the water itself.

[punatic]

Low TDS (distilled, RO, rain water) will equilibrate with carbon dioxide in the atmosphere to achieve a pH of around 5.6. 

Think about that.  It means rain is naturally acid.  However, rainwater has no buffering, so about all you have to do is whisper "base" over a bucket of rainwater and the pH jumps up. 

[steve.zoller]

Thanks to all for the great feedback.  It was very helpful.

On to brewing...
Cheers!

[yso191]

PS:  Don't worry too much about the pH of any water source in brewing.  Its the alkalinity that counts.

Oh my.  There is so much I don't know.  This statement at my current level of understanding makes no sense.  Alkalinity is just one side of the sliding scale we call pH, isn't it?  Alkalinity is just the opposite of acidic on this scale isnt it?

As indicated above, I am confident that you are right, but this points out a significant misunderstanding of the concept in my head.  I am wondering if I need to start thinking of a triangle rather than a line, with Acid and Base on the bottom and Alkaline on the top... Please enlighten me.

Steve

[nateo]

Think of alkalinity as the water's resistance to acidification. Think of the pH spectrum as acidic to basic. Alkalinity represents the water's ability to neutralize an acid, and has no direct impact on pH.

My water, for instance, has insanely high alkalinity but typically has a pH of 7.4. Since it has a lot of alkalinity, it takes a lot of acid to move the pH downward, as the alkalinity is "consumed" by the acid. Once the acid eats all the alkalinity, there is nothing in the water to stop the acid, and so any more acid drops the pH by a lot.

[mabrungard]

Oh my.  There is so much I don't know.  This statement at my current level of understanding makes no sense.  Alkalinity is just one side of the sliding scale we call pH, isn't it?  Alkalinity is just the opposite of acidic on this scale isnt it?

There is a definition of alkalinity that presents it as the opposite of acidity.  Unfortunately, that is not the definition in use here.  Alkalinity is a measure of the buffering capacity of the liquid or resistance to acidification as Nate mentions.  In typical drinking water, alkalinity is the measure of carbonate and bicarbonate ions in the water.  They are the ions primarily responsible for alkalinity.  Therefore, alkalinity is not equivalent to pH.  They can be quite different. 

[anje]

There is a definition of alkalinity that presents it as the opposite of acidity.  Unfortunately, that is not the definition in use here.  Alkalinity is a measure of the buffering capacity of the liquid or resistance to acidification as Nate mentions. 

To me, it's just "buffering capacity." Quite possibly, this is a case where different disciplines use different terms for the same thing.

[nateo]

To me, it's just "buffering capacity." Quite possibly, this is a case where different disciplines use different terms for the same thing.

Wouldn't buffering capacity be the resistance to change in pH in either direction? Alkalinity only buffers acids, and has no effect regarding bases (AFAIK).

[yso191]

Wow.  Great replies.  That really clears things up.  Thank you

Steve

[punatic]

You forgot the eye-roll emoticon Steve... 

Acid-base chemistry is funny stuff.  It's all about those pesky little protons (aka H⁺ ions)