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Measurement limitations (pH)

I mentioned some limitaions of the test kits in an earlier posting.
I should mention another.  Chemical indicators are materials that
change colour as the pH changes.  They do this by changing form:

	HA (yellow)  <->  A- (blue) +  H+

This means that the indicator is itself a buffer.  If your solution
to be tested is very dilute, and you add too much indicator, the 
indicator will have a significant, and perhaps even dominant, effect
on the pH.

Use the minimum possible.

Paul Sears        Ottawa, Canada