pH rise upon filtration - (Apr/20/2012 )

"The pH may rise 0.1 to 0.3 units upon filtration" (This is found in many Tissue Culture publications.)

Example:
pH is adjusted to 7.00. After filter sterilization, the pH becomes 7.10-7.30.

My questions:
Why does the pH rise upon filtration? What's the mechanism behind it?
Does the pH rise only in medium containing sodium bicarbonate? Does it also rise in PBS (Na-Bicarb free)?

Any thoughts are welcome.

Some of it probably depends on the filter type, but I suspect that the majority will be because of absorption of CO2 from the air during the collection part.

absorbing co2 would lower the pH.

sterile filtration will also somewhat degas the solution. the removal of co2 may, therefore, be the cause of rising pH.