Are we talking 5.3 at mash temps or room temps? Two completely different things. If it is at room temp than that is well within the range and nothing to worry about, in fact it is ideal.
RO water has a very small buffering capacity as all of the alkalinity is generally not removed. But, yes there is less than 10ppm. You are also forgeting that malt has a natural buffering capacity as well, hence Kai's experiment showing that DI water and 2-Row lock in at approx. 5.6. Color/SRM is a poor predictor of mash pH adjustments (there has been a lot of discussion about this lately) unless one is using dark roasted malts like black patent, roasted barley, carafa, etc. in the case of a stout or porter. You can also get around this issue by cold-steeping the roasted/crystal grains or adding them at sparging to avoid conflicts with mash pH. Crystal 60L will have little effect on mash pH regardless of the water and its alkalinity used. Yes, Martin, I agree with your example of a dark mild, but in your case you are using 25% crystal/chocolate vs. 12.5% crystal 60L in the above recipe. And as I have mentioned above more than once, for a stout or porter I WOULD use water with alkalinity. I am not anti-alkalinity
. But for an IPA, it is not needed, look at Firestone Walker's Union Jack. A simple blend of a known percentage of water and its profile is way easier to conrol than adding chalk since it is simple dilution. Especially if he does not have access to any salts at the moment. Plus, chalk must me added in an acidic environment for it to dissolve otherwise it just precipitates out (a mistake a lot of people make by adding it to the brewing liquor). Even with adding a known amount to the acidic mash it will not completeley dissolve. Yes, lime is better choice and probably easier for him to obtain at the moment You can do more harm than good to your beer by playing around with water if you don't know what you are doing. I would just blend 10-20% of his de-chlorinated tap water with his RO water and call it a day. Adjust salts according to desired flavor profile and to provide calcium for the yeast. Besides, Anchor is the bench mark for Cali Commons and I believe San Francisco water is fairly soft, if my memory serves. Hence, the great beers. Now, I don't know if they adjust their alkalinity. But, I bet not.
**EDIT** Yup, just checked. SF water has an alkalinity of 50ppm. Just blend a small percentage of filtered, de-chlorinated water and save yourself the headache of possible messing up your beer.