Is Reverse Osmosis Water Acidic or Alkaline? Learn Here!

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A common question that pops up regarding reverse osmosis water is whether it’s acidic or alkaline.

The reverse osmosis filtration process is widely considered the most thorough home water filtration method – as it essentially strips everything but H20 from your feed water. This process changes the pH level of your water. But how?

Let’s take a closer look at this somewhat confusing topic – and also consider the consequences of consuming RO water.

Key Takeaways

  • As a consequence of the ultra-fine reverse osmosis filtration process, beneficial minerals like magnesium, calcium, and potassium are removed from water. This has the effect of lowering the pH level of RO water – leaving it slightly acidic.
  • Typical RO water will have a pH level between 6 and 6.5 – meaning it’s slightly more acidic than most tap water which ranges between 6.5 and 8.5.
  • RO water contains very little in it besides pure H2 This means it’s low-mineral and low-TDS water. Because of this, it loses its buffering ability and will rapidly change to the pH level of anything it comes in contact with. Typically, RO water will absorb carbon dioxide from the air, which forms carbonic acid and lowers the pH level rapidly.

Is Reverse Osmosis Water Acidic or Alkaline?

So, is reverse osmosis water acidic or alkaline? It’s slightly acidic!

Reverse osmosis water is some of the purest water available, but as a result of the membrane filtration process, minerals are stripped away along with harmful contaminants.

Dissolved minerals – particularly magnesium and calcium – increase the pH level of water. When these minerals are removed through the filtration process, the pH level of the water decreases, resulting in a more acidic final product.

pH scale

To be more precise, typical tap water in the US will have a pH level between 6.5 and 8.5. This is a fairly wide range, and the EPA states that water in this range is safe to drink.

Reverse osmosis water will have a pH of approximately 6 to 6.5. This makes it slightly more acidic than tap water, but the difference is fairly minor.

Consider the pH of commonly consumed beverages like black coffee (4), soda (2.5), and sports drinks (3), which our bodies have no issue dealing with. If we can drink these beverages regularly, then the small pH decrease in RO water is not a major cause of concern.

Why Is Reverse Osmosis Water Acidic?

As mentioned previously, the stripping away of certain minerals during the filtration process affects the water’s pH level. But the whole story is actually a bit more complex than that.

To understand what’s going on, we’ll need to take a brief look at some of the chemistry going on behind the scenes.

No Mineral Content/TDS

Reverse osmosis filtration involves forcing feed water through an ultra-fine membrane filter. This traps any impurities on one side of the membrane – and leaves ultra-pure filtered water on the other side.

This water has little to no mineral content or total dissolved solids (TDS), which essentially makes it highly susceptible to changes from the surrounding environment. Put another way, the buffering capability of RO water is greatly decreased – so it will quickly take on the pH of whatever it comes into contact with first.

CO2 in the Air

What happens next is the RO water is stored in a tank or poured into a glass or pitcher, where it comes into contact with carbon dioxide (CO2) in the air. The water reacts with the CO2and forms carbonic acid – lowering its pH level.

If RO water is poured and left out for an hour or so, its pH level can actually drop down to just 5 to 5.5 from the carbon dioxide effect.

What’s important to keep in mind though – is that this pH effect is a weak effect. This means that while the pH may drop to 5, it will just as quickly adapt to whatever environment it’s exposed to. So, when you consume a glass of water, it will adapt to the pH level in your mouth without any ability to resist the change.

Acidity vs Alkalinity

While the term alkaline is thrown around a lot these days – especially by proponents of alkaline water – the term is generally misused when referencing drinking water. The opposite of acidic on the pH scale is not alkaline, but rather basic.

The actual definition of alkaline is the ability to absorb hydrogen ions without a resulting change in pH. Another way of putting this is the buffering capacity of a solution.

Nevertheless, the two terms often get used interchangeably when referring to drinking water.

Alkaline (or basic) water has not been proven to have any particular health benefits, despite many claims to the contrary.

Does That Mean RO Water Is Bad for You?

Critics of reverse osmosis water often claim that low-mineral water is harmful due to a 2004 World Health Organization report which suggested that drinking said water could increase the risks of heart disease, low-birth rate, and the risk of fracture in children.

However, according to a 2015 Water Quality Association paper “there are no known scientific data which clearly demonstrate that the consumption of low TDS (total dissolved solids) water by humans will or will not lead to harmful effects on the human body.”

Additionally, large populations regularly drink low-mineral water for extended periods without any health effects. This includes the US Army, US Navy, and NASA – all of which provide service members with this type of water while on duty.

Critics of RO water highlight RO water’s lack of mineral content as a potential cause of mineral deficiency. This is not a strong claim though, as only 5% of our mineral intake comes from drinking water, with the rest coming from our diet.

woman sitting in front of water glass

How to Test Your RO Water’s pH

While a slight decrease in pH is not a cause for concern, if you’re interested in determining the pH level of your reverse osmosis water, there are several ways to go about it.

pH Test Strips

pH test strips are one of the cheapest and easiest ways to test your RO water’s pH. These are cheap, disposable plastic strips that you soak in your water and then change color to give you a pH reading.

There are more precise versions of pH strips that will give you a clear reading for slightly acidic and slightly basic – which is what you want when measuring RO water.

Digital pH Meter

Another option for measuring pH levels is a digital pH meter. It will give you the most precise pH reading out of any method but may be overkill if you only want to test the water once.

Litmus Paper

Litmus paper is sort of the dumb method of testing pH but it does work. It will not give you any indication as to the degree of acid or base your water is, only which one it is. When you soak litmus paper in water, it will change into one of two colors to tell you if you’ve got an acid or base.

Using Red Cabbage

This is a bit of an unconventional testing method – but it does work believe it or not. Red cabbage contains a dye that changes color when it comes into contact with an acidic solution.

To perform this method:

  1. First, blend the piece of red cabbage with water (not the water you’re going to test).
  2. Then, strain the mixture, and cabbage pieces leaving only liquid.
  3. Then pour some of your RO water into the mix and stir well.
  4. If the solution changes color and turns red then you’ve got acidic water.

The effect with RO water won’t be as profound as using a strong acid like lemon juice or soda, but you may still notice some color change.

Remineralization: How to Make Reverse Osmosis Water Alkaline

If you’re still not convinced about the safety of reverse osmosis water, then there is a solution to increase its pH level and mineral content. This involves adding healthy minerals like magnesium, potassium, and calcium back into the water as a final stage in the RO system.

These minerals are hard minerals and will increase the pH level back into neutral to slightly basic range of 7 to 7.5.

Many RO system manufacturers offer optional remineralization filter stages that can be added to an existing system.

Another option is adding an electrolyte blend or mineral drops to the filtered water. This is certainly a cheaper option, but it does add extra work to the filtration process.

If you have any thoughts about the question, is RO water acidic or alkaline, please don’t hesitate to leave a comment below!

About the Author Alexandra Uta

Alex is a content writer with an affinity for research and a methodical attention to detail. Since 2020, she has fully immersed herself into the home water treatment industry only to become an expert herself. Alex has been using water filters and similar products for years which has gained her lots of hands-on experience.
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