Phosphate buffer to achieve a mash pH of 5.4?...???? (with warning and disclaimer)

[Silver_Is_Money]

Per this website: http://clymer.altervista.org/buffers/phos2.html

A phosphate based pH 5.4 buffer with a "buffer strength" of 10 mmoles per liter can be generated via the addition to your mash water of:

1) Monosodium Phosphate, Monoyhdrate at 0.1327% by weight of mash water
2) Disodium Phosphate, Heptahydrate at 0.0104% by weight of mash water

Example for a mash in 9 gallons of water:

9 gallons x 3.7854 L/Gal. = 34.07 Liters
Weight of 34.07 Liters of water in grams ~= 1,000 x 34.07 ~= 34,070 grams
34,070 grams x 0.1327/100 = 45.21 grams of Monosodium Phosphate, Monoyhdrate
34,070 grams x 0.0104/100 = 3.54 grams of Disodium Phosphate, Heptahydrate

Disclaimer 1: Please don't shoot the messenger (I.E., me) as I'm only following along with my understanding of how this website would suggest going about creating a 5.4 buffer.

Warning and Disclaimer 2: I have not researched the potential health hazards of either of the compounds being discussed here. Please do so diligently if you are actually considering attempting this. I'm not at all suggesting that you try this. I'm merely presenting it for scientific discussion.

Conclusion (using the websites guidance and my understanding of it, which may be completely wrong):

If 45.21 grams of Monosodium Phosphate and 3.54 grams of Disodium Phosphate are added to 9 gallons of water, it should nominally buffer a mash at a pH of 5.4

I'm guessing that the manufacturer of 5.2 may have used a calculator such as this web based calculator to formulate their 5.2 buffer.... (and I'm well aware that most have concluded that it doesn't work), but again, I'm only tossing this out as the messenger, and for open discussion.

 

[Silver_Is_Money]

To make it really simple you should** be able to buffer 10 gallons of distilled water at right close to 5.50 pH via the addition of:

50 grams of Monosodium Phosphate and 5 grams of Disodium Phosphate

**if the linked website is both accurate and correct**

 

[Silver_Is_Money]

The buffering inherent within malts is defined as a predominantly phosphate based buffering system, so perhaps the flavor impact of introducing additional phosphate based buffering would not be all that noticeable. ???

One option to avoid over-mineralization via attempting to travel down this road might be to forego the addition of traditionally added minerals. Another beneficial option would be to apply AJ's Zero Effective Alkalinity method to your mash water if you are not starting with distilled or quality RO.

 

[ajdelange]

This one has been pretty much beaten to death here. Yes, you can make a phosphate buffer for any pH between 1 and 12 (and outside that range too) but one is ill advised to do so outside the ranges 2.15 ±1, 7.2 ± 1 and 12.32 ± 1 i.e. within 1 pH of phosphoric acid's pK's (this rule of thumb applies to other buffers as well as phosphate buffers). Thus the 7.00 phosphate buffer we use to calibrate out pH meters is suited to the purpose but the 5.2 product which is a phosphate buffer supposed to "lock in" your mash pH at 5.2 is not. Look at the phosphate system Q vs pH curve to see why this is so.

 

[Silver_Is_Money]

Should it be presumed then that the inherent phosphate based buffering found within barley malts is similarly non-efficacious?

 

[ajdelange]

That really depends on what you mean by "buffering" and we use it in two related ways. Buffering simply means resistance to change in pH, in particular, the amount of acid or base we would need to add to a given quantity of something to change its pH from one value to another. Thus buffering is simply the first derivative (slope) of the Q vs pH curve. That's why I suggested you look at the phosphoric acid curve. It vaguely resembles a stair case. There are flatish portions and places (near the pK's) where the curve plummets. Thus there is buffering everywhere. Buffering in the sense of the ability to set or hold pH occurs where it takes a lot of acid/base to effect a small pH change - the steep parts of the curve.

Now look at the Q vs pH curve for a malt. It is a gentle, almost linear, monotonic curve. There are no steep parts. Thus it does not exhibit buffering in the meaning of pulling something added to a particular pH but it does exhibit buffering in the sense that we need to add acid to get it from its intrinsic pH to a pH of interest.

 

[Silver_Is_Money]

I can see from the image I attached below that there is effectively very little to no phosphate salts induced buffering capacity that exists between the pH's of about 4.1 and 5.8, which is smack within the range of nearly all kilned barley malts DI_pH values, ranging from deep roasted to caramel/crysral and right on through nearly all base malts. And that phosphate buffers are only effective within the vicinity of perhaps +/-1 pH points to either side of the 3 phosphoric acid pKa's, just as you have stated. So any attempt at contemplating an efficacious phosphate salts based buffer that corresponds to the pH's of concern for mashing is for naught.

 

[ajdelange]

Yes, note that mash pH is just about half way between two pK's where the buffering is minimum. Which does not mean that you cannot formulate, market and successfully sell a phosphate buffer.

 

[Silver_Is_Money]

Based upon pKa's, potentially the strongest 5.40 mash pH buffering candidate appears to be a blend of citric acid and sodium citrate, as the pKa(2) of citric acid is 4.76.

25.45 grams of citric acid and 75.71 grams of sodium citrate will buffer 10 gallons of water at 5.40 pH. But that's a whopper load of both citric acid and sodium citrate. And who knows what bodily harm this combination might bring along with it. So this entire exercise seems futile.

https://www.cusabio.com/m-296.html#a07

 

[Silver_Is_Money]

Per the chart seen at the below listed citrate buffer web-link it appears that the proper quantities to add are a bit different from those seen in the post above this one for a 5.4 pH citric acid and trisodium citrate buffer, and as such this sites recommendation (provided that I'm understanding the chart correctly) is that 20.44 grams of citric acid and 82.90 grams of trisodium citrate will buffer 10 gallons of water at 5.40 pH. And per this chart the weights as seen in my post above would appear to buffer at closer to 5.1 pH. I have no idea as to which data source might be more accurate with regard to generating a 5.4 pH buffering solution.

http://www.mystrica.com/Pages/Applications/citratebuffer.pdf

 

[Silver_Is_Money]

And lastly, this site is the easiest of all to use, and its output differs from both of the above:

https://www.aatbio.com/resources/buffer-preparations-and-recipes/citrate-buffer-ph-3-to-6-2

This site tells me that for a 0.01M buffer at 5.40 pH in 37.9L of water (~10 gallons) I need to add 23.742 grams of citric acid and 75.09 grams of sodium citrate.

Obviously the hard science of making a spot on 5.40 pH buffer out of citric acid and sodium citrate is rather unsettled.

 

[ajdelange]

It isn't hard to formulate buffers. In this example we want pH 5.4. The charge on a mole of citrate at pH 5.4 is Qcitrate(5.4) = -1.903. This means that double charged Cit-2 in Na2HCit would need to pick up 0.0963 mEq protons per mmol whereas the singly charged Cit- in NaH2Cit would have to dump 0.9030 mEq/mmol. Since the number taken up must equal the number released it is clear that we need 0.903/0.0963 = 9.637 times as many mmol of NaH2Cit as NaHCit. Pick the molarity you want and it's trivial from there.

Now while it is true that 5.4 is within 1 pH of a pK of citric acid it is, nevertheless, close to half way between 2 pK's so the buffering isn't as great as it would be at one of the pK's but even so being close to a pK helps a lot. The buffering of 1 mmol of citro is -0.56 mEq/pH whereas for a phosphate buffer it is only 0.04.

 

[Silver_Is_Money]

The buffering of 1 mmol of citro is -0.56 mEq/pH whereas for a phosphate buffer it is only 0.04.

Thanks AJ! Would a buffer providing -0.56 mEq/pH be sufficient to move an average grist close to 5.4 pH (+/- 0.1) during the mash without the assistance of any additions of the likes of lactic acid or baking soda? Lets presume 5 Kg. of grist which mashes unadjusted in 33L of buffered DI water at either of the extremes of 5.0 and 5.8 pH, or 0.4 pH points on either side of a desired target mash pH of 5.4.

 

[ajdelange]

Here again you need to change the way you think about what a buffer is. This example buffer is about 90% NaH2Cit and 10% (molar basis) Na2HCit. For each mmole of Cit in passing to pH 5.4 0.9*0.9030 mEq of protons will be released (by H2Cit-) and 0.1*0963 mEq absorbed by the Na2HCit for a total of 0.9*0.9030-0.1*0.0963 = 0.803 given off. Along with the protons you get 0.9*0.9030 + 2*.1*0.0963 = 0.832 mmol of sodium. If, OTOH, you just add a mmol of citric acid you'd get 1.903 mEq protons and no sodium. So which is the better choice? The buffer or straight citric acid? The 5star "buffer" or phosphoric acid?

 

[ajdelange]

A simpler way of saying what I said in #14 is that citric acid, H3Cit has three protons to give up (and in fact gives up 1.9 of them) whereas NaH2Cit has only two (and, in fact, gives up 0.9) and Na2HCit has but one (and, in fact, absorbs 0.1). Thus, where protons are needed, the acid is the most economical source.