Magnesium doesn't drop mash pH, but rather it hinders the impact of calcium during the mash - Beer, Wine, Mead, & Cider Brewing Discussion Community.

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Larry Sayre, Developer of 'Mash Made Easy'
Dec 31, 2016
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N/E Ohio
Sometimes I find amazing the things we can learn from old dead people who actually performed real science. The below link to a 1939 published study indicates that magnesium ions do not measurably drop pH during the mash, and when magnesium ions are present they actually hinder calcium ions from fully doing their expected "thing" and dropping pH during the mash to their full extent, but then later on, in the boil, the magnesium hindrance is removed and reversed and at this juncture both magnesium and calcium are free to contribute to pH drop. This renders Kolbach's work which measured and attempted to quantify the pH drop magnitude of calcium and magnesium at "Knockout" (or post boil) quite effectively useless for determining these two minerals pH drop impact upstream during the mash stage of the brewing process, where we primarily concentrate our pH control efforts (for reasons not 'fully' understood) these days. Although the linked document doesn't dwell upon it, the document also seems to imply in passing that sodium ions when present can also have a downward impact upon pH, and no mash pH assistant software to my knowledge attempts to integrate this little tidbit. Here is the study link: