Ah, then you should see #6 which says there is no need to do this as RO water has unappreciable (2.5 ppm as CaCO3) alkalinity.
Perhaps you read #24 but don't understand what it is saying.
Base malts don't have the ability to lower mash pH. Acid must be added to do that. Acid must also be added to water to lower its pH. The amount of acid required to lower each grain to a desired pH is called its proton deficit. The proton deficit of a kg of Maris Otter with respect to pH 5.4 is 14 mEq. You must add that much acid to it to get it's pH to 5.4. If you mix that with 3 L of water of alkalinity 100 you have a proton deficit, wrt pH 5.4 of another 5.2 meq and you need that much more acid. If you use RO water, however, its deficit (RE 5.4) is only 0.15 mEq and you only need to add the 14 mEq. None for the water (or so little you can ignore the requirement). The RO water does not contain any base (assumining for purposes of simplified explanation that it is effectively deionized). It will not, when used to sparge a mash, pull the pH of the wort higher because it is not a proton absorber. In fact the mash will pull its pH down (the mash has appreciable buffering, the water 0). Now in the real world RO water isn't deionized. It, in fact, contains a bit of acid (carbonic acid ) and will probably have a pH of 6 or a bit below. There is, therefore, no need to add any acid to it whatsoever. If you do add acid to it then you will have added more acid than you need and the mash pH will go lower than it would otherwise. If you have only added a tiny bit then the pH shift will be small and not something you need worry about. But why bother when it essentially does nothing?
