Mash pH problems using RO water with dissolved CO2 & low pH

[Ceaver]

My RO water has a very low pH due to dissolved CO2 gas in the source water. This results in excess H+ ions, which lower pH, and there is no buffer in the RO to absorb the H+ ions. Below is my attempt at trying to understand the process and what may be causing the trouble of brewing with this kind of RO water. Any input is greatly appreciated!

Please correct me where I'm wrong:

So the pH of water is determined by the presence of H+ and OH- ions, which contrubite to acidic and basic pH, respectively. The alkalinity refers to the buffering capacity, which is the capacity to "absorb" H+ and OH- ions that are introduced, and therefore protect against pH swings. This is related to why Bru'n Water has you add acid to reduce the alkalinity of your sparge water, right, because the acid "uses up" the alkalinity buffer that's in your sparge water and also gets its pH lower?

Grains, especially darker ones, must somehow add H+ ions to your mash liquor, which is why the pH drops during the mash. However, if that drop isn't enough, then we add some acid to the mash water as well, to get the drop into the desired range. RO water doesn't tend to have much alkalinity, so this needs to be taken into account in the calculations. For instance, when I conducted a little water experiment and compared two water profile creations from tap and RO--using Bru'n Water--it had me add acid for the tap batch, but not for the RO batch (typical IPA grain bill and water profile). Would this be because the RO batch had less alkalinity (i.e., buffer) to begin with, so the pH was more easily swayed to the desired level?

So what then is the net effect when there's an additional variable in the mix, such as lots of dissolved CO2 gas in the RO water? It doesn't have much buffering capacity to begin with, and the CO2 reacts with molecules to produce an excess of H+ ions, which results in crazy low pH to begin with (this part, at least, is fact and not conjecture). So then salt additions and possibly acid additions are introduced, as well as the grains, all of which would tend to lower pH, presumably through some kind of contribution of H+ ions through some reactions? If the water profile, resulting from the salt additions, doesn't end up with enough alkalinity (or hardness, as Bru'n Water calculates?), can this result in overshooting the predicted drop in pH?

Btw, I've noticed that Bru'n Water asks for pH and alkalinity of source water on the source worksheet, UNLESS you're using purely RO water, which is then just entered into the water adjustment worksheet with the estimated properties of RO water (i.e., near pure). Therefore, it seems Bru'n Water isn't accounting for differences in RO water, such as high amounts of dissolved CO2, which then produces H+ ions, which will then eventually "use up" some of (a lot of?) the buffering capacity that is added through the salt additions?

I hope that makes sense, and that someone can help me and others understand the process a bit better.

Thanks in advance!

 

[Silver_Is_Money]

Subscribed

 

[jurgenph]

My RO water has a very low pH due to dissolved CO2 gas in the source water.

have you tried boiling the water? that should remove most of the dissolved gasses like CO2


J.

 

[day_trippr]

Kept waiting for it, never saw it: what is the actual pH of this RO output?

 

[Ceaver]

It was in one of the first posts, but the output pH is <4, from two different RO systems in the house. I can't give you an exact figure because at the moment all I have are the pH strips.

I know, I know, they suck, but they are at least sensitive enough for this purpose. In my tap water, they immediately turn the darkest color of their upper range, and in the RO water they don't budge, and the low range is 4.0. When measuring my mashes, they are off by 0.2, which is apparently to be expected, according to the info sheet of Bru'n Water.

I haven't messed with boiling the water yet, as I don't know exactly how long it takes to degas, and for ~8 gallons of water or so that could be a lot of time and resources. I also am going to look into aerating it with a fish pump, as I think I've read that could work and at least wouldn't require a bunch of propane.

Most importantly, I'm already buying a $100+ pH meter and will do an experiment with two batches, one tap and one RO, to see if mash pH is any different and take this from theory to empirical observation. I'll certainly reply with my findings...

 

[mabrungard]

Dissolved gases do preferentially pass through RO membranes into the product water. Having high CO2 content in RO water is typical in some regions. Where this is problematic, the RO water is often run through an air-stripping tower to reduce the CO2 content to normal equilibrium concentrations.

Measuring pH with strips is very problematic. The ionic content of water and wort are low and those strips rely on higher ionic content to reliably produce the colorimetric response. In addition, the wort color can throw off the reading which will alter your perceived pH reading. pH meters are much more reliable, but they require special care and understanding...plus they are more expensive. But, having a meter can help you catch pH excursions in your brewing and help you to correct them.

 

[ajdelange]

My RO water has a very low pH due to dissolved CO2 gas in the source water. This results in excess H+ ions, which lower pH, and there is no buffer in the RO to absorb the H+ ions.

And, therefore, it isn't a problem for you. Had the water some buffering capacity there would be a problem as the buffer would release the protons to your mash but as there is no buffering there are only the 1000*10^-4 = 0.1 mmol/L protons to pull your mash pH around. The buffering of water itself is unappreciable.

Please correct me where I'm wrong:

So the pH of water is determined by the presence of H+ and OH- ions, which contribute to acidic and basic pH, respectively.

Don't know how far you want me to go but the pH is related to both the H+ and (OH)- concentrations which are fixed at [H+] = 1000*10^-PH and [OH-] = 1000*10(pKw - pH). The faactor of 1000 is there so the units of concentration are mmol/L. pKw ~ 14 @ 25 °C is minus the log of the dissociation constant of water.

The alkalinity refers to the buffering capacity, which is the capacity to "absorb" H+ and OH- ions that are introduced, and therefore protect against pH swings.

Alkalitity is the buffering between two particular pH's. In a water report those are the source water pH (intrinsic pH) and 4.5 (ISO standard). For a grain the intrinsic pH and the mash pH are the pH values of significance. Alkalinity is the number of mEq of protons which must be added to or subtracted from a unit mass of the substance of interest to bring its pH from its intrinsic value to the value of interest.

As your water has no buffering to speak of it no protons (to speak of) need be absorbed from it in order to change its pH from its intrinsic value of 5 to the higher mash pH of 5.4 or so. In fact, of course, a very small amount of carbon dioxide from the air has dissolved and that carbonic acid must be neutralized but the proton absorbing capacity of the grist is greater than this by orders of magnitude.

This is related to why Bru'n Water has you add acid to reduce the alkalinity of your sparge water, right, because the acid "uses up" the alkalinity buffer that's in your sparge water and also gets its pH lower?

When you mix acids and bases protons flow to the bases from the acids. As the buffering depends on the pH and the material the actual flows may seem tricky to calculate but they aren't. Protons flow until the net absorbed equal the net given up. All one has to do is calculate the flow as a function of pH till everything balances and that is the answer. The various programs attempt to do this empirically and there is no point in trying to do it precisely as the data isn't available.

Grains, especially darker ones, must somehow add H+ ions to your mash liquor, which is why the pH drops during the mash.

Remember that alkalinity is relative to mash pH and intrinsic pH. Light grains have higher intrinsic pH than mash pH but lower than water pH (usually). Buffering capacities vary but typically run about -45 mEq/kg&#8226;pH Dark grains, conversely, typically have intrinsic pH less than mash pH (and water pH) with the same range of buffering capacity (alkalinity). Thus some grains are acids and some are base depending on the goal and the grain. The water is usually a base and, if it is alkaline enough, a big factor. In this case it is an acid but because it has so few protons not one that need be worried about.

So what then is the net effect when there's an additional variable in the mix, such as lots of dissolved CO2 gas in the RO water?

If there were lots of dissolved CO2 in the RO water it would fizz off as soon as it hit the grain and you'd be back to the situation you have: a very weak solution of carbonic acid.

It doesn't have much buffering capacity to begin with

Which is why you don't have to worry about it

...and the CO2 reacts with molecules to produce an excess of H+ ions, which results in crazy low pH to begin with (this part, at least, is fact and not conjecture).

and why there is no CO2 to react. pH 4 is hardly 'crazy low' for relatively pure water exposed to an RO system. If you think there is too much CO2 in this water simply agitate or aerate it and the pH will come up into the high 5's very quickly. Keep in mind that pH strips are pretty worthless for this application and may be coloring your thinking.

 

[Ceaver]

Thank you both for your replies, especially AJ, for providing such detailed information. I have read about and seen schematics of the air-stripping towers and frankly don't want to have to go there. I will try aerating and see how that works. And certainly will hold off on any further evaluation until my pH meter arrives. I do plan to conduct a few side-by-side experiments to try to better confirm my initial observations, as well as to evaluate the effects of some of the more user-friendly treatments, such as aerating the product water overnight before brew day.

Thank you again for your input. And I absolutely do appreciate all the work that has gone into Bru'n Water and was by no means being critical of it. I'll have to read AJ's response a couple dozen more times for it to fully register...

I'll reply soon with data!
Cheers!

 

[ajdelange]

No need to get into stripping towers. Simply letting it stand (in relatively small containers) will do or if that isn't fast enough simply pour back and forth a few times. In no case will the CO2 content be large enough to be a problem for your beer. If the permeate goes straight into a pressure tank, though, there may be enough to promote corrosion depending on what the tank/plumbing is composed of and that's why breweries will strip permeate in some cases. Of course RO water is aggressive enough even with 0 CO2 content that it should not be run in metal pipe. Even in my home brewery it only runs in plastic.