Originally Posted by flabyboy
Won't your hot alkaline sparge water bring the pH of your mash up to levels where tannins may be leached?
It might or might not. It would depend on the alkalinity and the volume of sparge water. In traditional sparging the problem comes late in the sparge when most of the acids have been rinsed from the grains and there is nothing to hold back the pH rise.
As to the original question: acidification of mash and sparge water accomplishes two distinct things. In both cases you hope to neutralize the alkalinity of the water. Here "neutralize" means eliminate the effect of i.e. it does not mean bring to pH 7 as you might remember from high school or college chemistry. In fact it means to add enough acid to bring the pH to a value lower than 7. In the case of the mash it means adding enough acid to neutralize the alkalinity of the water and
the malt to a pH of 5.4 or so. In the case of sparge water it means adding enough acid to reduce the pH of the water alone to 6 or less as if the water is at pH 6 or less it cannot possibly dilute the remaining mash (at pH < 6) to a pH > 6.
If the alkalinity of the mash water is low enough (and that's one of the main goals of water treatment - to reduce alkalinity) then the water will not dilute the remaining mash down to the point where the pH exceeds 6 before it has diluted the sugars to the point where it is not worth collecting any more runoff.
You should never have to worry about the solubility of the salts in the water you are preparing as you should never add anything to water that doesn't dissolve. In the event you find mash pH too low and choose to raise it with (insoluble) calcium carbonate you would add that to the mash as you would any other alkali.