It is nearly impossible to measure the pH of RO/DI water. It takes both a special probe
and a specially designed cell to even have a chance of getting an accurate result,and just
because your pH meter gives you a reading doesn't mean it is accurate.

The other problem in measurements with RO/DI, is that you can't take a sample and expose
it to air and get an accurate reading. The resisitivity of 17 Meg-Ohm water and the pH
will drop within just seconds after exposure to air due to CO2 dissolution and absolutely
no buffering capacity in the water. To be accurate, any measurements with water 10
Meh-Ohm almost have to be done on-line.

In short there is not enough conductivity for the pH sensor to work.. The reason for
this is that pH is the measure of a specific ion- H+ (and indirectly OH-). The presence of
H+ will reduce resistivity in the water, but so will any other ionic species.

Here is some theory- H2O molecules have infinite resistance. It is ions that cause
solutions to be conductive. Even perfectly pure water has some ions because of the
equillibrium between H2O, H+ (ions), and OH- (ions). The concentration of the ions H+ and
OH- in perfect water is 0.0000001 molar at 25º C. The pH of this water is exactly 7.
Because of the difficulty (impossibility) in the accurate measurement of pH in ultrapure
water, ASTM has at least removed the requirement from the specification for Type I and
Type II deionized water.

There are some max-min pH values for puure waters, that give you ranges of where you might
be.

http://www.cal-water.com/pdf/ConversionChart.pdf

http://www.purite.co.uk/technical/CH...ISED_WATER.pdf

http://www.coleparmer.com/techinfo/t...+of+Pure+Water


I am working on my post doc... so feel free to use the
big words.

OK, you have them

Some more reading make sure you follow all the links

http://www.advancedaquarist.com/issues/feb2004/chem.htm

Feel free to come to our Chem forum if you have any more questions. The link is below


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