remedy for too much phosphoric acid

[Andrew Walsh]

Silly me accidentally spilled about 10ml phosphoric acid (96%) into boiling wort when doing pH adjustment for a dunkle bock. pH dropped to 4.7 and stayed there but after adding NaOH I eventually got the pH back up to 5.1. Water treatment has the water salt profile prior to the accident at about 50 ppm Calcium, 34 ppm sulphate and 100 ppm chloride.

So now I figure there will be a ton of phosphate buffering the beer. Although the yeast should metabolize some of the phosphate there may well be too much and the phosphate will keep the beer pH too high, somewhere around 4.7 as it was in the boil.

What is the best course of action to fix this? I figure CaCl2 and/or gypsum additions to precipitate out the phosphate. But which one and when to add?

Beer is not bubbling yet. My thinking is to wait until pH gets stuck and do something then. If I add too much of either I risk an acidic beer. There is already a fair amount of chloride in the water so I probably have more headroom with gypsum. But what happens to the sulphate if adding at the end of fermentation?I want to avoid rotten egg beer if possible. Yeast is S-189.

Thoughts appreciated.

 

[ajdelange]

Silly me accidentally spilled about 10ml phosphoric acid (96%) into boiling wort when doing pH adjustment for a dunkle bock. pH dropped to 4.7 and stayed there but after adding NaOH I eventually got the pH back up to 5.1. Water treatment has the water salt profile prior to the accident at about 50 ppm Calcium, 34 ppm sulphate and 100 ppm chloride.

Are you sure it was 96% ? It is usually sold as 85% and I think that's pretty close to saturated. It's pretty syrupy even at 85%.

So now I figure there will be a ton of phosphate buffering the beer. Although the yeast should metabolize some of the phosphate there may well be too much and the phosphate will keep the beer pH too high, somewhere around 4.7 as it was in the boil.

The normality of 85% phosphoric acid to pH 4.7 is 14.63 mEq/mL (i.e. it donates 14.63 mEq of protons for each mL added in bringing a solution to pH 4.7) so we can tell that your wort required 146.3 mEq to shift it from pH 5.1 to pH 4.7 or that its buffering is 146.3/(5.1 - 4.7) = 365.75 mEq/pH. But the normality of 85% phosphoric acid is 14.72 at pH 5.1. This means that if taken to pH 5.1 rather than 4.7 those same 10 mL of phosphoric acid would only deliver 147.2 - 146.3 =0.9 mEq more protons or, conversely, that if we had a solution (your wort) at 5.1 and your yeast need to take it down to pH 4.7 they will have to supply 146.3 mEq to move the wort and 0.9 mEq to move the phosphoric acid you added. Thus your phosphoric acid addition of 10 mL in your kettle amounts to a drop in the bucket in more technical terms.

What is the best course of action to fix this? I figure CaCl2 and/or gypsum additions to precipitate out the phosphate. But which one and when to add?

You have nothing that needs fixing.

Thoughts appreciated.

The reason this works out the way it does is that phosphate does not buffer much at all in the region of beer and mash pH. It's first two pK's are 2.14 and 7.20. Phosphate, as is the case with all other acids, buffers most strongly at those pK's and least halfway in between, i.e. pH 4.57, which is close to the pH's of interest here. The phosphate titration curve (it's in Palmer's Water book) is pretty flat between 4.7 and 5.1.

 

[Andrew Walsh]

Thanks for your reply.
Yes, according to the label it is 96% phosphoric acid. I double checked before posting. I guess we buy from different suppliers but a few % won't make any difference in the scheme of things.
The other area of concern for me was loss of calcium through precipitation of apatite - I should have mentioned that.
I plugged the numbers into Bruwater after posting and it suggested the amount would be closer to 4ml than 10ml for that pH change.
I have no way of knowing how much went in as it spilled from the bottle - I could believe 4ml - but I will never do that again ...
I should have checked the pKs but thanks again for the help.

 

[ajdelange]

Yes, according to the label it is 96% phosphoric acid. I double checked before posting. I guess we buy from different suppliers but a few % won't make any difference in the scheme of things.

No, it wouldn't. Only change the buffering of the acid from 0.9 mEq to 1.1 mEq.

The other area of concern for me was loss of calcium through precipitation of apatite - I should have mentioned that.

I don't think that's something you need to worry about either. At pH 5.1 the PO4-- is only 4E-10*the total acid you added. It's going to take massive calcium concentration to precipitate apatite at that level.

I plugged the numbers into Bruwater after posting and it suggested the amount would be closer to 4ml than 10ml for that pH change. I have no way of knowing how much went in as it spilled from the bottle - I could believe 4ml - but I will never do that again ...

Most of the popular programs are not very good at this sort of thing for reasons I don't understand. It's quite a simple calculation, actually.

 

[Andrew Walsh]

Just a follow up.
My bock finished at pH = 4.74 cf typically 4.4 so way above normal. I added (dissolved) 4g CaCl2 and pH dropped to 4.63. Then I added 2mL 88% lactic and pH went down to 4.57. Still high but acceptable for a malty beer so I stopped mucking around with it. The taste is good end of primary.

I cannot find the reference now but I found a paper that said average beer phosphate levels are around 400mg/L. If I added 10ml 96% phosphoric acid (900 mg/ml) I would have trebled the amount of phosphate in the beer, which is why I added the CaCl2 which helped a little. I added the maximum amount of CaCl2 I was comfortable with. I figure I would have needed at least 10 times that amount to precipitate out all the extra phosphate.

In hindsight:
- don't spill phosphoric acid into wort!
- if you do, don't try and correct it as I did with base (I ended up over-compensating)
- calcium salts can be used end of fermentation to help remove some of the phosphate (I have not seen this mentioned anywhere else)
- taste is logarithmic with respect to salt concentrations in water so I possibly could have tried more salts but since taste was fine I decided to leave it alone
- gypsum may have been better than CaCl2 added to boil (rather than post fermentation) since some of the sulphate is reduced to H2S by active yeast much of which blows off. You are stuck with chloride ions no matter when the salt is added.