Aside from the issues discussed above concerning the effluentÃ¢â‚¬â„¢s pH when the DI resin becomes depleted, the final pH coming out of an RO/DI system should not significantly concern reef aquarists. Many aquarists with low pH problems have asked, for example, if their aquariumÃ¢â‚¬â„¢s low pH may be caused by their replacing evaporated water with RO/DI water that they measure to have a pH below 7. In short, the answer is no, this is not a cause of low pH nor is it something to be generally concerned about, for the following reasons:
1. The pH of totally pure water is around 7 (with the exact value depending on temperature). As carbon dioxide from the atmosphere enters the water, the pH drops into the 6Ã¢â‚¬â„¢s and even into the 5Ã¢â‚¬â„¢s, depending on the amount of CO2. At saturation with the level of CO2 in normal (outside) air, the pH would be about 5.66. Indoor air often has even more CO2, and the pH can drop a bit lower, into the 5Ã¢â‚¬â„¢s. Consequently, the pH of highly purified water coming from an RO/DI unit is expected to be in the pH 5-7 range.
2. The pH of highly purified water is not accurately measured by test kits, or by pH meters. There are several different reasons for this, including the fact that highly purified water has very little buffering capacity, so its pH is easily changed. Even the acidity or basicity of a pH test kitÃ¢â‚¬â„¢s indicator dye is enough to alter pure waterÃ¢â‚¬â„¢s measured pH. As for pH meters, the probes themselves do not function well in the very low ionic strength of pure freshwater, and trace impurities on them can swing the pH around quite a bit. "