Brad,
What do you mean the electrode is regulated by the meter?
The meter doesn't regulate anything with them. All the meter does is sense a potential difference between the two electrodes. The same things is done with a temperature sensor that you place in the water for an electronic thermometer. Nothing is regulated. All that happens is two different meterials react differently in the solution. The reaction of the probe sensetive to Ions in a solution is so different to the reaction that the reference probe has that an electric potential is created. This potential is measured by a meter and then displayed as a pH reading.
DI water is of such a low Ionic strength that you can't get a proper reading with the pinpoint monitor.
A pinpoint meter reads the Ionic strength of a solution. If you remove the Ions(De-Ionisation) how do you read the strength? You can but it has to be done with a probe made to measure in pure(Low Ionic strength) water.
Here is what Randy Holmes-Farley just passed onto me.
<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> Measuring the pH of pure water is problematic for a variety of reasons. These include real pH changes caused by just about any contaminant around (including atmospheric CO2) and phantom changes (e.g., water motion around the electrode). The root of all of these problems is that there is very little buffering capacity in pure water, and the pH is free to easily drift over the range from about 5-9.
So the short answer is that measuring the pH of pure water is difficult. However, if you put a pH electrode in "pure" water and get a pH reading that is stable, it is likely a real reading, and can be used for most purposes.
I've measured my RO/DI water and the pH is on the high side. Something real is in the water that is raising the pH, though I've not tried to determine what it is, or whether it comes through the RO/DI, or is only coming into the water as it sits in my plastic trash can. <HR></BLOCKQUOTE>
[ 15 January 2002: Message edited by: DJ88 ]