Bicarbonate question

[Rake_Rocko]

So I'm sitting here wondering about one last thing in this whole water chemistry dealio.

I understand (or atleast I think I do) why you would add or omit calcium sulfate, calcium chloride and acids. But my question is with the bicarbonates.

I've read countless times that mash pH is the portion that matters most. The water I use has low bicarbonate (14 ppm). But I usually seem to see in brunwater that my estimated mash ph is 5.7 or so (obviously depending on the recipe and if I have more dark malts in the mash). So if I use acid to bring down the pH, the spreadsheet shows that I will have negative bicarbonate. Does this matter as long as I have the correct mash pH even though the desired water profile has a much higher ppm for the bicarbonate?

Hopefully that makes sense. I'm also asking this question in general, not tied to a specific recipe or situation. Thanks all!!


Sent from my iPhone using Home Brew

 

[ajdelange]

The role of acid is to supply protons to overcome the proton deficits of the bicarbonate in your water and the base malts. If you add too much acid and/or too much dark malt they will release more protons than are needed to cover those deficits and pH will be too low. Counting protons is really the simplest way to keep track of what is going on but in the US we have the unfortunate practice of using calcium carbonate as a proxy for protons expressing alkalinity (a proton deficit) in terms of 'ppm as CaCO3'. There are certain conveniences to the ppm as CaCO3 concept if all you are dealing with is the alkalinity and hardness of natural water: If 100 mg of limestone is dissolved in 1 L of water using CO2 as the acid the alkalinity of the water is approximately 100 ppm as CaCO3 even though the water may contain little or no calcium carbonate. This can be confusing but at least the relationship between ppm as CaCO3 and mEq/L, the units in which protons are measured, is simple. Divide ppm as CaCO3 by 50 and Bob's your uncle.
Now if you want to confuse people even further you can use bicarbonate as a proxy for protons and divide the 'as bicarbonate' number by 61 to get the mEq/L you actually need to do calculations.

Now as I said, this confuses people and I am one of them so I am not sure what exactly negative bicarbonate means in a spreadsheet but I can speculate. I assume that if the water has an alkalinity of 100 the spreadsheet will convert that to 100/50 = 2mEq/L and then multiply that by the equivalent weight of bicarbonate ion which is 61 thus expressing the alkalinity at 122 'as bicarbonate'. If the water's pH is 7 this is pretty close to the actual bicarbonate content which is 121 mg/L. If the water's pH is 9 it is 111 mg/L. Now if you add 2 mEq acid to 1 L of this water (alkalinity 100, pH 7) it's pH is reduced to 4.4, the pH which I assumed was the end point of the alkalinity titration and, and its bicarbonate to 1.6 mg/L. But the spreadsheet will tell you that the bicarbonate is 0 because it considers the acid to be 2*61 = 122 mg/L as bicarbonate and 122 - 122 is 0. This is a small error. Now if you add another mEq/L acid the spreadsheet subtracts another 61 mg/L as bicarbonate and tells you the bicarbonate is -61. What this is saying is that enough acid has been added to carry the pH of the water to the alkalinity titration end point and that 61/61 mEq/L more than that has also been added.
Thus the bicarbonate number in such a spreadsheet expresses the 61 times the water proton deficit with respect to alkalinity end point titration pH. If the number is positive this is close to the actual bicarbonate content provided the original pH was between say 6 and 8.

Clear?

 

[Rake_Rocko]

AJ, Thanks for the very in depth reply, and Ive grown to know that your replies are nothing short of extremely in depth and I respect that.

However, I read your post multiple times and its a little bit over my head. With that said, I asked sort of a generalized question so it seems you answered my question in a general way. That is my fault for doing that, I should have made it more specific in the beginning bad approach.

The water profile I use is as follows:

Bicarbonate: 14 PPM
Calcium: 5.8 PPM
Chloride: 10 PPM
Magnesium: 2 PPM
Sodium: 5.7 PPM
Sulfate: 8 PPM


So I am mashing:

10 lbs pilsner (2 L)
1 lbs special roast (50 L)

When I input all the info into Brunwater and build my desired profile, in order to hit my numbers I was adding calcium sulfate (dont have the exact number in front of me, at work) and also calcium chloride. AFter that, my pH was showing an estimate of 5.7. I am wanting to get the pH down to 5.5-5.4. So I add the correct amount of acid. But by doing that, my bicarbonate goes in the "negative." It shows that I have a "negative" PPM for the bicarbonate.

So, I guess my question remains, should I worry about that bicarbonate level being in the "negative" or does it really matter, as long as my pH level is in the correct range?

AJ, it seems to me from your response that the "negative" ppm isn't the major factor here. Correct me if I am wrong, but the bicarbonate acts as an alkaline buffer, so if that is the case, then really, I shouldn't have to worry about it, and worry more about the mash pH.

I just read this as well from you AJ in another thread. "Since carbonic is dissolved CO2 gas and that is of limited solubility it departs the solution at proper mash pH and you are left with very little bicarbonate when pH is set correctly. Thus if you bring the mash to proper pH you will have disposed of the bicarbonate by converting it to CO2 gas." This actually seems to answer my question. Feel free to reiterate anything that I missed, or sounds confusing.

 

[ajdelange]

All the confusion, IMO, stems from the fact that bicarbonate is being used as a proxy (and a poor one) for proton deficit. If there be a field in the spreadsheet labeled 'bicarbonate' it should contain a number representative of the bicarbonate content. There should be a field labeled 'proton deficit' that contains a number representative of the total proton deficit. One has to be a little careful here is answering the question 'Proton deficit of what?' The proton deficit of water only is the sum of the proton deficits of things in the water which include those of carbonate, bicarbonate, the water itself and any things put into it such as acids or alkalis. The proton deficit of the mash includes the proton deficits of the malts. The total proton deficit for a system is always 0 but the proton deficits of individual components need not. Thus I think the proton deficit in the spreadsheet you are looking at, proxied as bicarbonate, must be the proton deficit of the water. If you add enough acid to the water to overcome the alkalinity and then add more the proton deficit of the mixture, with respect to mash pH will be negative and I'm sure that is what is being done here. So no, the fact that the 'bicarbonate' is negative would not a problem as long as my assumptions about how it is computed are correct.

 

[Rake_Rocko]

AJ, Thanks again for the response! This one I did fully understand and again appreciate the fact that you take so much time into explaining everything fully in your posts. I do feel more comfortable about the whole bicarbonate issue.

I do also apologize also if I use some amateur language or anything like that because I am 100% NOT a chemist and a water chemist at that. Never really took chemistry in school and so I am kind of learning this as I go. I have learned quite a bit from reading Palmer and Strong and what they say on the issue of brewing water chemistry.

Thanks again.

 

[ajdelange]

I do also apologize ...

Don't! Most brewers are not chemists or not chemists beyond high school or freshman college. The whole 'proton deficit' idea is an attempt to recast the chemists 'proton condition' into terms that make this business understandable to people who are not chemists.