When pure water comes in contact with the atmosphere the most prevelant gas available is Carbon Dioxide. The CO2 is quickly adsobed into the ion hungry water, forming a weak acidic carbonic acid with a pH as low as 5.5.
A conductivity/resistivity meter is used to measure this gas. A strong base anion exchange resin is the only way to remove CO2. Oxygen sensing electrodes are are used to monitor oxygen, the most common non-ionized gas. Oxygen can cause corrosion of metal surfaces and can be removed by anion exchange resins in the form of sulfite.