Re: pH test

The short-form answer is that it is rather difficult to measure the pH of a
weakly buffered (or unbuffered) solution, and you can't get much weaker
than pure water.  This is true both for dye indicators and electronic

One of the complicating factors, and the one probably responsible for the
shift from "neutral" to full-blue, is that the indicator dyes are
themselves weak acids (or, if you prefer, weak bases).  The indicator color
change occurs because of this weak acid chemistry.  Thus you probably saw
bromothymol blue doing its weak-base thing in fairly pure water (which can
take a while).  The first color represents (as close as you're going to
get) the "correct" pH; the second represents the equilibrium bromothymol
blue eventually reached with your water.

One more reason to have some buffering capacity in your water chemistry:
with buffering, your pH readings will be more stable and repeatable (and,
to the extent this has meaning with pH, accurate).

Today's trivia:  weak organic acids that show this kind of color change are
pretty common.  Red cabbage is full of one, for instance.

Hope this helps.

  --Martin Harriman
    martin-h at mail_utexas.edu
    (512) 471-5742