Here is an interesting pH behavior that I observed today when I was testing the behavior of phosphoric acid additions:
These were 6 test mashes mashed at about 60C. various amounts of phosphoric acid were added 5 min after dough-in. Mash pH was tested 20 and 60 min after dough-in.
What is interesting is that during the mash the mash build up a stronger buffer. I.e. the amount of added acid had less on an effect on the mash pH after 60 min (~30 mEq were needed for each kg of malt and a drop of 1 pH unit) than it had at 20 min (only 15 mEq/(pH*kg) were needed. After doing the initial pH measurements I thought something was wrong with the amount of acidity I contribute to the phosphoric acid since I was expecting 30 mEq/(pH*kg) but then I had the 2nd pH tests and they made more sense.
I don't know if this is something that is unique to phosphoric acid. Previous experiments, that evaluated the effect acids and bases have on the mash, were not set up like this one. But the ones that tested pH at different times show the same trend. A mash buffer strength of ~30 mEq/(pH * kg) is what I was expecting from previous experiments. I'll have to pay attention to this effect in future mash experiments.
In practical brewing you may have seen that the initial test of the mash pH was much lower than expected while later tests would have shown an increased mash pH. In fact I have seen changed of mash pH during mashing w/o the addition of acids or salts. But the extend of the pH change was not always the same. It is very much possible that it takes some time for the pH active substances to be released during mashing, which is why I think getting your mash chemistry correct at dough-in and thus preventing the need for pH corrections, has its merit.