This is actually not the case. The bicarbonate content of the water has no significant impact on the buffer capacity of the mash since at mash pH the carbonate system is not providing a strong buffer. I do have experiments that show that.
Kai has a good point there, but that is not a significant concern here. The bicarbonate content does however have a strong affect on the pH of the mash. In fact the entire Residual Alkalinity concept is strictly based on that fact. The buffer capacity of the mash is highly complicated by the strong influence of the phosphate buffer system, but for the overall effect on mash pH, the bicarbonate content has a very strong correlation.
Kai's other point regarding the quandry that he uncovered regarding Rahr malt and its apparently elevated acidity is something that may be difficult to resolve. It appears that this maltster has possibly sprayed their malt with an acid solution prior to drying in much the way that Acid Malt is prepared. Another possible explanation is that this maltster uses a steeping water that has very low alkalinity or has been acidified to help enhance this acidic character of the finished malt. There could be other causes, but the fact is that this particular malt produces a roughly 0.2 unit drop in pH compared to other typical 2-row pale malts. Good work by Kai in finding this.
There are plenty of good reasons to do this. Since most brewers suffer with water supplies that have too much alkalinity and the brewer may not know how to adjust for that, the mash pH may not always fall into the most desirable range (5.3 to 5.5 @ room-temp measure). In this case, the extract and fermentability of the grist would suffer. So for Rahr to "help" the typical brewer out and add a little acidity to their malt is sort of a win-win. The brewer sees better performance and extract and this maltster's malt seems to be the reason.
The problem is when a brewer that does know what they're doing with their water or that uses an already low-alkalinity water source (this includes RO and DI), then the mash pH may fall too low. Many of you may have noted my recommendation that mash pH should really stay above about 5.3 to avoid producing a possibly thinner or tarter beer than desired. This malt may create a problem in this case.
As I mention above, this Rahr malt has an Acid Malt character. It appears that it may be neccesary to model it in Bru'n Water as an Acid Malt. Kai, did your analyses indicate how much acidity per kilogram the Rahr malt contributes?
Possibly I need to add a special malt category for this malt? The best solution to all of this would be for all maltsters to test their malt acidity and publish the result as they do for Lovibond, EBC, Lintner, Extract, etc. Then programs like Bru'n Water or similar could dispense with correlating malt color and malt type to acidity and use that value directly. Maybe that day will come. I'll make it a point to start asking maltsters and I'll introduce this concept directly to them so that they are on board too.
Fellow Brewers, we will improve the technology of mashing with a more work and understanding. I'd say that you have just set off a quest that we homebrewers need to press for, malt acidity data for all malting products.