Do you know why the pH of distiled water is more than 8? - (Apr/27/2012 )

Do you know why the pH of distilled water is more than 8?

I have checked it in IRAN in National Genome Center and in Iranian Institute of Pasteur and
in Canada, in McGill University and in Quebec Genome center.

The pH of distilled water is near 8 and the pH of 18mega ohm water in 10.

is the CO2 of air is the cause?

Absorption of CO2 would result in an acidic pH. The pH of deionised water is difficult to measure as any tiny contaminant will alter the pH by many units. Having said that, the majority of times I have seen this question, people wonder why the MilliQ is pH 6 or thereabouts, which is due to absorption of CO2.

It may be that the pH you are measuring is actually the result of contamination of the pH meter (perhaps the calibration fluid)

As said by bob1, pH for MilliQ should be around 6, usually a range of ±0.5. A given distillator/purification system uses to be quite constant in this value while there is no contamination. pH 8 is too high for MilliQ

If there is no problem with the pHmeter...

In these systems or in the reservoirs is quite easy that some bugs can proliferate. My former lab had problems with this.

To detect a potential contamination I suggest (using a MilliQ purification system as model) to measure the pH directly from the purification system (without passing through the filter), after the filtration and in the water leaving the tank/deposit. This is roughly what I did to detect the contamination source... It was the deposit. It had some inhabitants inside... and a good wash was an effective solution

I usually get 5.5 with MilliQ. Therefore, I am usually careful with the pH of buffers I make and I check them in the end. I never had issue with higher pH of water. As bob1 said, it could be problem with the pH meter. Or, it could that in Iran the soil is basic, and it might affect the tap water. But I hear Tehran has the best tap water in the world.