Rainwater with a pH of 4 to 5 ...

Rainwater with a pH of 4 to 5 IS dangerous to health if it is in contact with aluminium metal. Although short-term effects are unlikely to cause problems, the long term effect of drinking acidic water that has been in contact with aluminium can be severe. It is one of the environmental factors that are responsible for the development of Alzheimer's Disease, by stimulating the amyloid cascade process that leads to plaque development in the brain. The risk of AD doubles i areas where the water supply contains only 0.1mg/l of Al. I experienced this problem at first hand in Britain thirty years ago, and was involved in collecting brain samples for analysis later by Chris Exley at Keele University, UK. A number of people almost certainly died with early-onset dementia as the consequence of their exposure. In that case the Al concentration was far above anything you are likely to experience in practical terms, but any action that solubilises aluminium, by allowing water with a low pH to come into contact with it, rendering it bioavaiable, should be avoided at all costs. So by all means collect the water - but avoid allowing it to have any contact with aluminium pans etc, unless the pH has first been raised above 6, at which level aluminium is virtually insoluble.