On Tue, 17 Nov 1998, Linda Jenkins wrote:

>  .. Checked the pH of  our building's distilled
> water as it came out of the tank - it was 5.1
> ... Any thoughts on what could be happening to
> cause such a low pH?

  Carbon dioxide from the air dissolves in the
  water making it somewhat acid. Tap water
  contains enough dissolved stuff to neutralize
  (buffer) this acidifying effect of CO2.
    You can get rid of the CO2 by boiling and
  then immediately securely close the bottle, with
  hardly any air space.
                        John Kiernan
                        London, Canada.