First off, as has been mentioned, RA really isn't terribly useful as a means of managing mash pH largely because some of the calculator authors are confused as to what it really is. It is really the pH of the mash that you should be concerned with and getting that right requires that a certain portion of the alkalinity of each alkaline mash component needs to be neutralized. "Neutralized", in the brewing context, doesn't mean what you were probably taught it means in school. In brewing it means that all the alkaline components in the mash have been protonated (acid is a source of protons) to the extent that the mash is at the desired pH (pHz) and that, in turn, means that the solvent, water, has been protonated such that -log[H3O+] = pHz. [H3O+] is the concentration of H3O+ which is a protonated water molecule. Often when writing this the water is left out and we just write pH = -log[H+]
The alkaline components of the mash are
1)Carbonate ion: CO3-- (in the water but very little of it)
2)Bicarbonate ion: HCO3- (in the water)
3)Bases in the malts: A-
4)Water itself: H2O
As you add acid the following reactions take place:
1)CO3-- + H+ --> HCO3-
2)HCO3- + H+ --> H2CO3 --> CO2 + H2O
3)A- + H+ --> HA
4)H2O + H+ --> H3O+
Which of the 4 types of alkali gets the added protons depends on how strong a base it is. The bases listed here are listed in order of strength so that CO3-- takes up the added protons (H+) much more readily, forming HCO3- than water takes them up (to form H3O+). Thus, as you add protons, most of them, at first, go to convert CO3-- to HCO3-. But even then some go to all the other reactions too, even the last so that [H3O+] is increased and the pH drops. After most of the carbonate has been "neutralized" (all of never will be - a tiny amount remains even at pHz) most of additional added protons go to "neutralize" bicarbonate and, as more and more are added, more and more bicarbonate gets turned into CO2 and water while at the same time some A- molecules are being protonated too as are some water molecules so the pH continues to drop. If we continue to add protons (acid) until the pH reaches pHz we would find that, in general, we will have neutralized about 90% of the water's alkalinity (the actual fraction depends on the water's starting pH) and this can be a useful fact if we approach pH management by setting the mash water to the desired mash pH and then determine the amount of acid required to get all the A- "neutralized". Thus, at pHz, there is still quite a bit of HCO3- present as well as A- and, of course, H2O, so that if you continue to add more acid still more HCO3, HA and H3O+ will continue to form and the pH will continue to drop.
We mentioned earlier that which of the alkalis absorbs how much of the added protons depends on their strengths. Their strengths are a function of pH so that at high pH CO3-- gets the lion's share of the added protons while at medium pH (in particular at 6.38) the majority go to HCO3-. In the region of desirable mash pH most are taken up by malt alkalis (A-) with the total uptake by all the species except water diminishing as pH is lowered. This means that once you get near desired mash pH a small amount of added acid will decrease the pH more than it did at higher pH as a higher proportion of the added protons will go to making H3O+ than they did at higher pH. So you need to be careful. Note that this phenomenon is, of course, much more noticeable if there is no A- present as would be the case in acidifying sparge water. As you approach mash pH the pH drop will zoom for a given incremental acid addition relative to what it was when the first increment was added.
You don't specify your alkalinity but we can tell from the reported bicarbonate content that it is about 0.2 mEq/L (10 ppm as CaCO3). This is very low. To "neutralize" this alkalinity to mash pH would require about 0.9*0.2 = 0.18 mEq of acid per liter of the water. If you add that much acid to the water then, as far as pHz is concerned the alkalinity of the water is 0. You must then add additional acid - enough to protonate enough of the A- in the grains to get the system pH to pHz. You can estimate what this is by telling the spreadsheet program that you are using water with 0 alkalinity.
You ask about the science behind this. Judging from the number of people who seem to understand it it must be very complex indeed but it is, in fact, just the science of weak acids and bases reacting. There is a rather simple way to handle calculations involving it but I cannot seem to get it across which speaks to my abilities as a teacher. The basis for these calculations is in the stickies in this forum. How to do the calculations is explained there but not why they are done this way. I will be happy to answer questions about the why's but as I noted above most people seem to have trouble grasping the basic concepts.
