The calibrated pH probe reads that pure water is acidic.
This is normal: By definition, pure water has a pH of 7 because it has equal numbers of H+ and OH- ions; however, any "pure" water that has been exposed to the atmosphere will have a pH of ~ 6 because CO2 will dissolve in the water and create carbonic acid.
Because the pH of pure water is unpredictable and easily changed by the slightest amount of contamination, it should never be used as a calibration point. Use only bufferered solutions for calibration.
Creation Date: 02/13/2002
Last Modified: 06/12/2012