See Also pH Values of Water Explained How Can You Tell if a Salt Dissolved in Water Will Increase or Lower pH I'm More Alkaline With Water... What Does This Mean? Basic Types of Water Defined The Importance of Water and Human Health

Is the pH of Pure Water Really Lower than Tap Water?

This question is answered when you understand the equilibrium relationship between CO₂, HCO₃, and CO₃. In a closed system, the relative amount of each of dune compounds vary with pH. At lower pH levels, CO₂ is the predominant species. HCO₃ is the predominant species at mid pH levels, and CO₃ is present at higher pH levels.

Since RO membranes will reject dissolved ions but not dissolved gases, the RO permeate and RO feed will contain roughly the same amount of CO₂. The HCO₃ and CO₃, however, are often reduced by 1-2 orders of magnitude. This upsets the CO₂, HCO₃, CO₃ equilibrium that was established in the feed. In a series of equilibrium reactions, CO₂ will combine with H₂O driving reactions similar to that shown below, until a new equilibrium Is established.

CO₂ + H₂O --> HCO₃ + H⁺

To summarize:

The new equilibrium will always result in a lowering of permeate pH if there is CO2 gas present in the feed water.

The pH drop is usually largest for waters with high amounts of alkalinity or HCO3.

When there is very little CO2, HCO3, or CO3, there is a very small pH drop observed in the permeate.

Therefore it is not true that reverse osmosis filters will always reduce the pH level of water to a noticeable amount. The pH difference after the RO depends on the composition of your input water source; depending on whether you have large amounts of gases such as CO2 in your local water supply. If you are concerned with the pH of your drinking water, you should avoid guess work and get your water tested with an accurate pH tester meter (avoid those $10 paper testing strips, which are very inaccurate with pH readings). If you see that your pH is below 7 with a good amount, you can consider putting a pH increaser filter as a final stage of your reverse osmosis system and correct your acid water problem easily.

Some countries have regulations governing the pH of drinking water. Typically, the pH is recommended to be in the 6.5-9.0 range. It is our understanding that the purpose of these pH regulations are to minimize corrosion of piping materials so that metals like lead and copper do not dissolve into the drinking water supply.

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