Ever since I discovered that chalk, when simply added to water or mash, does not raise the pH as much as an equal amount will do when it is dissolved with CO2 (see these old bog posts: How much alkalinity does 1 ppm of CaCO3 (Chalk) really add? and Undissolved vs. dissolved chalk in the brewing water) I was wondering why.
Recently the discussion got some traction again and got more of A.J. deLange’s attention than before. He proposed that chalk dissolves only slowly at mash pH. Slow enough that the time it takes us to mash doesn’t dissolve all the chalk. In hindsight this looks like a logical explanation. It also breaks with the common (home) brewing wisdom that chalk added to water or mash will dissolve since the mash pH is low enough. And then there has been this “bug” in John Palmer’s water calculator which for a while has been seen as the only reference for mash pH calculations. His water spreadsheet assumes that chalk adds only half its alkalinity potential to the mash. While it may not have been intended it ended up being a close enough approximation of chalk’s behavior in the mash.
A few days ago I did a simple experiment. I rinsed another 500 ml of 7 Plato wort from the spent grain of a batch of IPA I was brewing. To 250 ml I added ~110 mg chalk and to the other I added ~130 mg baking soda. Based on mash titration experiments these salt additions should have been capable to raise the pH by more than 1. I was not really looking for quantitative results but for a qualitative measurement of the pH behavior when chalk or baking soda are added to wort.
After the salts were added I kept stirring and recorded the measured pH values over time. The resulting graph is shown below.
It is apparent that the pH change caused by baking soda is sudden and remains fairly constant over time while chalk causes a sudden initial rise and a further gradual rise that continues over 15-20 min. Even though the 110 mg chalk added about 1.57 mEq residual alkalinity and the 140 mg baking soda added about the same amount (1.55 mEq), baking soda caused 2x the pH change. This lines up with previous observations I made regarding undissolved chalk.
If chalk does not completely dissolve we also need to assume that it releases only part of its calcium to the mash. This has been corrected in the current version of my Kaiser_water_calculator.xls which assumes that undissolved chalk contributes only half its calcium and half its alkalinity.