You’re looking to achieve a mash pH of around 5.4 because this is the sweet spot for the enzymes to do their magic and produce a good quality fermentable wort. Mash pH is not the same as your water pH which has little impact on the mash pH.
If you use distilled water (no alkalinity) you are likely to get a mash pH around 5.6-5.8 for light coloured grains and maybe below 5.0 for some of the darker grains. Adding calcium salts will lower this mash pH even further through its interaction with phosphates in the malt but you do need calcium, somewhere between 50 and 150ppm is common.
So without alkalinity the mash pH might be okay for very pale ales but the mash pH of dark ales is likely to be well below where you want it. Alkalinity moves the pH up and resists the acidification of the mash and needs to be higher in darker brews (assuming the dark grain is added to the mash).
There are rough values for alkalinity that work in practice and that most people use if they’re not relying on brewing recipe apps to do all the working out for them. For black beers you might want 150ppm alkalinity, for very pale beers maybe 0-20ppm and in between pick a number that feels about right for the colour.
Armed with this you then need to know what the alkalinity of your water is and how it can be changed. Reducing alkalinity can be done in many ways including dilution with distilled/RO water, addition of other salts (including more calcium), or addition of acid. Increasing alkalinity can be done by adding high alkalinity water from some other source, or adding carbonate salts (eg sodium bicarbonate or calcium carbonate). For example 0.1g of sodium bicarbonate per litre of water adds 60ppm of alkalinity.
Right, I’ve finished my pain au chocolat so that’s all you’re getting for now
