Re: Can a pH meter measure the pH of de-ionized water?

In fact, attempts to measure the pH of highly de-ionized water results in 
erroneous reading.  If you go to you local library and get a US 
Pharmacopeia / National Formulary (this is the book that compiles all of 
the test methods used in the pharmaceutical industry) you will find a 
procedure that outlines how to measure the pH of very pure water.  While 
it has been several years since I've worked in that industry, I recall 
that prior to measurement, the analyst must add a salt, like sodium 
chloride to the water so that a correct pH value can be obtained.  Glass 
electrodes that are used to measure pH also have a small response to 
sodium ions and that is how a "pH style" eletrode has been developed that 
allow the measurement of sodium in solution.  The web site for the US 
Pharmacopeia is at www.usp.org, however, they don't list any of the 
methods online.