Given the large amount of darkly-roasted grains in a Russian imperial stout, the pH is almost guaranteed to be lower than desirable when the strike water has a low level of bicarbonate ions (<50 ppm). (Strike water is the water used to mash your grist.) If you use any of the standard water chemistry calculators, the amount of carbonate ions they suggest for Russian imperial stouts is quite high. And in practice, you usually don’t need that much bicarbonate to approach proper mash pH. This is because darkly-roasted grains are not more acidic than dark crystal malts — the correlation between color and acidity breaks down when you jump to the darkly-roasted grains. Here is one way to deal with mash pH in a Russian imperial stout. There are certainly others, but I like this approach for a couple reasons (that will be clearer when we discuss lautering).