Is Reverse Osmosis (RO) Water Acidic?

Yes, reverse osmosis (RO) water is acidic. But a deeper understanding of basic chemistry and water chemistry is necessary if you want to be able to draw conclusions about the health benefits or risks of RO water. Before we dive into the explanation of why RO water will measure as acidic, we need to define some commonly encountered chemistry terms and set the record straight on alkalinity vs pH.

Alkalinity Is Not pH

This is an absolutely critical thing to understand if you wish to understand pH, water chemistry and the potential role they have to play in health. The pH scale is a 14-point scale where numbers below 7 are considered acidic and numbers above 7 are considered basic. Yes, basic - not alkaline.

There is no such thing as an alkaline pH. It would like be like a purple smell.  It just doesn't make sense. If you wish to confirm this, do a google search for a phrase like "what is an alkaline pH" or "the health impacts of alkaline ph". You won't find any scientific publications that use these phrases, because these phrases are scientifically inaccurate. You will find lots of online materials that are science-y that will substitute basicity for alkalinity, but that's because they're not going to dive any deeper into pH and its relationship to alkalinity. As soon as this is a requirement, it become obvious why this swapping of terms is so confusing and counter-productive to a true understanding of what's going on.

What Does pH Mean?

pH is the potency of Hydrogen ions in a solution. The pH of any solution is a result of the balance between Hyrdrogen ions (H⁺) and Hydroxyl ions (OH⁻). Acids are substances that cause an excess of H⁺ to exist in solution while bases are those substances that cause an excess of (OH⁻). A pH 7 solution (neutral) has equal numbers of both in solution in a state called equilibrium. It's very important to note that the pH scale is logarithmic. Any two adjacent numbers on the pH scale differ by a factor of 10. So the difference between a pH of 4 and a pH of 6 is a 100X difference in the concentration of Hydrogen ions!

What Is Alkalinity?

Alkalinity is the ability to absorb hydrogen ions (acids) without a resulting change in pH. Alkalinity is often referred to as the "buffering capacity" of a solution. Highly alkaline water will be able to absorb large volumes of acids while keeping the pH stable. Eventually the buffering capacity of a solution can be exceeded and the addition of more acid will cause a decrease in pH. Almost every living organism relies on the pH buffering phenomenon to ensure that tissues maintain the proper pH which is absolutely critical to proper cellular function and overall organism health. Different chemical compounds will buffer the solution to a different pH. Alkalinity as a group of compounds will buffer the pH to a value above 7 (basic). This is why alkalinity and basicity have become synonymous is popular discussion. Alkalinity can create a basic pH, but they aren't the same thing.

Alkalinity, pH, and Why Reverse Osmosis Water Tests Acidic

Reverse osmosis systems are used to remove dissolved substances from water, and they do it very efficiently. Water treated through a reverse osmosis process will typically have over 95% of the dissolved substances removed with levels sometimes reaching 97 or even 98 percent. But for the sake of The reverse osmosis process is non-selective - it attempts understanding why RO water will measure as acidic, let's assume for a moment that the RO process removes all dissolved substances and results in absolutely pure water. Pure water has a perfectly neutral pH of 7. Pure water also has no pH buffering capacity at all. This makes sense since we've shown that pH buffering capacity comes from compounds (such as alkalinity) that are dissolved in the water. When water is pure it has no inherent ability to control changes in pH. Extremely small amounts of any dissolved material will cause the pH of the water to fluctuate, sometimes wildly. But any change in pH that caused by a small amount of a chemical compound being dissolved in pure water, can be very easily undone by exposure to another compound that will take the pH in the opposite direction. It's a tug of war that is happening with very little effort on either side.

Water is sometimes called the "universal solvent" because of its highly unique and important ability to dissolve even small amounts of pretty much every substance it encounters. When a glass of RO water is dispensed, it's the universal solvent property of water combined with the lack of buffering capacity that explains why RO water is acidic. Immediately upon dispensing the water into the glass, the water starts dissolving everything it contacts. Part of what it contacts is the air where it dissolves carbon dioxide (CO₂):

Carbon Dioxide + Water = Carbonic Acid

So, pure water measures acidic because it contains a small amount of carbonic acid. But because the buffering capacity of the water has been removed it only takes a tiny amount of it to make the pH measure below 7. What this also means is that as soon as this "acidic" water contacts anything else (the coffee machine, your boiling potatoes, or your mouth) it will be immediately influenced to take on the pH of those compounds without any capability of resisting that change. That makes the acidic pH of RO treated water more or less irrelevant. Another way of thinking about this is that the pH effect that any solution can have on another substance is a combination of the pH of that solution and its ability to maintain that pH.