pH rise with precipitation of CaCO3

[foragedbrews]

If your water pH is high, it's possibly down to hardness. If you boil water, it precipitates out calcium carbonate, CaCO3, which causes temporary hardness. Logically this means pH should then drop, even if boil off occurs because what should be left is less dissolved calcium carbonate. Not only is this not the case but pH can in fact rise (I have found it does). Supposedly this is down to compounds that cause permanent hardness which doesn't precipitate out. However, calcium and magnesium chloride and sulphate are almost all below pH 7, this should be reducing the post boil pH of water.

So what compound(s) is/are concentrating to result in a higher pH than before?

I've tried searching the forum without any success, but maybe I've been looking for the wrong thing, so apologies if this has already been addressed. If any of the water chemistry books mentioned cover this, please let me know. I'm studying and so don't really have the time to be spent looking through books for information that might not be there.

 

[hottpeper13]

It's my understanding the you need to have more calcium (permanent hardness) then CaCO3 when boiling to reduce the alkalinity. It's that residual alkalinity that will determine the pH of the mash. My RO water has an 8.2 pH but I only need to add 4 oz of acidulated malt to get my light beers to 5.3 pH.

 

[cire]

pH of brewing liquor, water, is not of significant value. It is largely a result of dissolved CO2. Boiling your water drove off the dissolved CO2, and as a result it's pH increased.
What is important is the amount of alkalinity present, which can be measured with a Salifert KH kit. It is a cheap titration kit that enables a swift and accurate measurement.

 

[mabrungard]

The reason that boiling water causes the resulting water pH to rise, is that the boiling causes CO2 to leave the solution. CO2 is largely the compound that creates 'acid rain' by creating carbonic acid in the water. The boiling is essentially removing carbonic acid from the water and the pH rises.

 

[VikeMan]

Supposedly this is down to compounds that cause permanent hardness which doesn't precipitate out. However, calcium and magnesium chloride and sulphate are almost all below pH 7, this should be reducing the post boil pH of water.

Calcium salts that cause permanent hardness (such as CaCl2 and CaSO4) don't affect the pH of water, i.e. they are pH neutral. (They affect mash pH, because the calcium reacts with phosphates from the malt, making protons.) Same with Magnesium Chloride. Also, I can't think of any sulfate containg salts that would affect the pH of water.

 

[foragedbrews]

Thanks everyone! This has cleared up a number of preconceptions!