Calcium phosphate precipitation in brewing water

I’ve been pre-treating my water with crushed campden tablet, phosphoric acid, and calcium salt (chloride, sulfate, or a mixture).  I’ve seen very small amounts of while solid in the hot liquor tank (HLT) and didn’t really think much of it, sort of assuming it was undissolved material from the tablets.  But after seeing this comment by AJ deLange I wonder if it might be precipitated hydroxyapatite or another calcium phosphate.

If this really is happening in the HLT before I mash in, my impression is that it’s not a big problem.  I don’t believe the precipitation will do anything drastic to the pH, and in any event I can adjust the amount of acid added to correct for this.  I don’t care about the phosphate going missing as the mash will have plenty anyway.  I’m a little more concerned about losing calcium.

Here’s some crude chemistry to figure calcium loss in precipitation with phosphate. For guesstimation purposes, the molecular weights of phosphoric acid, CaCl2, and CaSO4 are the same, and with one of the relevant ion in each compound we can compare the ions directly by weight of their parent compounds.  Conservatively I guess I am using 10 mL of 10% phosphoric acid to treat my water, and probably less in reality.  With a density of about 1.9 g/mL, this means roughly two grams of phosphoric acid.  There are several calcium phosphates but the worst (in terms of taking out calcium) would be the apatite minerals Ca10(PO4)6(OH,F,Cl)2, requiring around two calcium ions per phosphate.  So one would need to add about four grams of the calcium salt to match the two grams of phosphoric acid.  Although I haven’t kept the best records, for the most part I have been adding more calcium than this: depending on the salt, one teaspoon of gypsum or calcium chloride weighs around three and a half grams or more.

This also doesn’t account for the moderate calcium in my municipal water (average 38 ppm according to the utility company).  So using the worst-case assumptions, I believe the calcium will be in excess for me, so the calcium salt additions will still have some effect even if a bunch drops out of solution (and in the case of gypsum, I want the sulfate as well, which stays).

This chemistry was touched on somewhat in Ye Olde HBD (such as here) and might be mentioned in the new brewing water book (which I need to finish).

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