Originally Posted by Malstorme
Distilled water IS right at 7pH but that is neutral, not acidic. As for "keeping" it, at 89 cents a gallon what's the point? +1 on mixing up some Starsan...
If the distilled water is exposed to the atmosphere, then the CO2 present in the atmosphere will turn into carbonic acid in the water as it's absorbed. The pH of the solution will eventually end up around 5.2 if left alone long enough.
Distilled and RO water is corrosive. Generally, drinking water, if treated by municipal plants, has a pH of 7.5-8, in order to reduce corrosion of the city's pipes. That's why I said it's "pretty acidic" compared to normal municipal drinking water.