Calculating Water PH after Gypsum not Mash PH

Just a quick question hopefully. I'm using Bru'n Water and love it.

My water is high in bicarbonates and low in sulfates. For hoppy pale beers, I usually treat my tap water by diluting it 50% with distilled and then adding back gypsum. I usually treat the brewing water all at once except for sparge acidification. For this particular beer, I'm adding 1.3g of gypsum per gallon.

Using the sparge acidification tab of Bru'n water, I've also been using 10% phosphoric acid to lower the sparge water PH. The sparge acidification tab lists the starting water PH...per my water report, it's 6.9. However, didn't I already lower the water PH with the gypsum additions? If so, how do I calculate the the new liquor PH so I can recalculate the sparge acid addition?

Thanks!

Gypsum (calcium sulfate) is the salt of a strong acid (sulfuric acid) and a strong base (calcium hydroxide). Therefore, it does not have an appreciable effect on the pH of solutions to which it is added. There are, of course, exceptions. And one of those is when it is added to a solution which contains a sufficient amount of phosphate at high enough pH that sufficient phosphate is in the tribasic form in which case calcium will coalesce with it precipitating as apatite. The loss of tribasic phosphate causes dibasic to convert to tribasic to try to re-establish equilibrium as well as the conversion of monobasic to dibasic to make up for the loss of dibasic and of the acid to monobasic to make up for the loss of monobasic. Each of these results in the release of hydrogen ions which lower the mash pH. This is why calcium is often supplemented in brewing. It lowers mash pH by reacting with the phosphate in the malt.

Gypsum doesn't lower water pH by itself. Its only when combined with malt in the mash that there is an acid-producing reaction that creates a pH reduction. Sparge water pH (alkalinity really) must be reduced via an acid addition.

Thanks guys. You two are the best. I'll continue using 6.9 as the water input when acidifying the sparge water with phosphoric acid.

If I may, another question.

If I use all RO for my sparge, will the pH of the sparge rise? Assuming my mash pH is 5.4,will that go up?

If I may, another question.

If I use all RO for my sparge, will the pH of the sparge rise? Assuming my mash pH is 5.4,will that go up?

In my experience, RO water for sparging does not require any pH (or alkalinity) adjustment since its alkalinity is already very low. It is not likely to exhaust the acidity of the mash which would lead to a rise in pH rise and all the bad things that may result from that rise.