Is Reverse Osmosis Water Acidic or Alkaline?

If you’ve made it onto this guide, I’m going to assume that you already know the basics about reverse osmosis (RO) water filtration. You understand that a reverse osmosis filter is one of the most capable water purification solutions on the market, being able to remove up to 99.9% of all total dissolved solids from water.

But something that’s not so clear about reverse osmosis systems – and what manufacturers are unlikely to touch on in their marketing materials – is how they make water taste. It makes sense that removing the minerals, ions and antioxidants that are responsible for the alkalinity of drinking water will adjust the water’s pH somewhat.

But does that mean your reverse osmosis water will be acidic? And how will this filtration process affect your water’s taste? I’ve covered everything you need to know in this blog post.

What is Acidity?

The definition of acidity gets a little difficult from a scientific perspective, but if you’re just wondering about how acidity will affect your drinking water, here’s the lowdown: acidic water is anything that has a pH of lower than 7.

Acidic water usually has quite a sour taste, and may even take on a metal flavor depending on how acidic it is and whether it has come into contact with metal pipes (acidic water is known for corroding metal).

ph scale

What is Alkalinity?

Alkalinity is just the opposite of acidity: alkaline or alkalized water is a pH of higher than 7.

Usually, alkaline water has a pH of around 8 or 9. Like acidic water, you may notice that your alkaline water tastes different from regular drinking water; it may have a bitter edge if it’s particularly alkaline.

However, many of us prefer water with a higher-than-average pH, like mineral water.

On that note, alkaline water typically has a higher essential mineral content, making it better for our health all-round.

Do remember, though, that we get plenty of minerals from the food we eat in our diet, and your body should not rely on its mineral intake from your water alone. Food like fruits and veggies are staples in a healthy diet and can also provide the body with the healthy minerals and ions that it needs.

So, Is Reverse Osmosis Water Acidic?

In short, yes, reverse osmosis water is acidic. The pH of reverse osmosis water can actually drop all the way down to 5, which would give it a noticeably different taste to what you’re probably used to.

But with that said, reverse osmosis water systems aren’t capable of making your water dangerously acidic, even after removing all the alkaline minerals.

Why is Reverse Osmosis Water Acidic?

You may be wondering why reverse osmosis water wouldn’t have a neutral pH level of 7, considering it’s clean, pure water with no health-harming contaminants or impurities, such as lead, fluoride, sodium, bacteria, and chemical compounds. But there are several factors that contribute to the acidity of reverse osmosis water.

First off, when mineral ions such as calcium and magnesium are removed by an RO water treatment system, this causes a dip in pH, as these minerals naturally make water more alkaline. But this isn’t the main problem when it comes to the decrease in RO water’s pH level.

The biggest issue occurs when RO water is exposed to air. Industry research indicates that because RO water is pure water, it’s “hungry” for impurities. As a result, when it comes out of the system and into contact with air, it pulls carbon dioxide out of the air.

The longer you leave a glass of pure RO drinking water out, the more acidic it’ll become. Expose your pure water to air for about an hour and it’ll end up with a pH level as low as 5.5. But it’s highly unlikely that your water will become any more acidic than this, no matter how long you leave it out for. It’s certainly nothing to be concerned about from a diet or health perspective.

reverse osmosis water sitting can become acidic

How to Test Acidity of RO Water

pH Test Strips

pH test strip sets are the cheapest and easiest way to see if you have acidic or alkaline water from your RO filter system. Soaking a pH strip in reverse osmosis water will provide you with an almost instant result, telling you how acidic or alkaline water is based on the color of the strip.

Keep in mind that water treated by a reverse osmosis filter system is likely to have a pH range of between 5.5 and 8.5, so look for strips that are particularly designed for slightly acidic to slightly alkaline water. If you purchase a testing strip set that measures all the way from 0 to 14, you’ll probably find it more difficult to determine an accurate measurement based on the color hue of the strip.

pH strip sets are widely available and can be purchased online for $5-$10 or so for a pack.

They’re useful to keep on hand, whether it’s to initially assess water from a soft water reverse osmosis filter system or to check that your process of remineralizing water after reverse osmosis is working effectively.

litmus ph test strips

Lab Testing

If you’re looking for the most accurate measurement of how acid or alkaline water from your RO system is, you’ll benefit from a lab review.

There are thousands of state-certified laboratories across the US that can assess water for a variety of electrolytes and hard water minerals, like calcium and magnesium, among contaminants like lead, chlorine and other chemicals, in the water you drink.

The big advantage of this process is that you’ll receive a result that tells you, to the exact figure, what your water’s pH is.

That’s why it’s a great option if you’re after a more accurate result than what you’d get from pH sets. Lab reports can also determine your overall water quality, which may be important if you’re keen to examine the benefits of RO filtration in your home.

tap score water test

Litmus Paper

Litmus paper is a bit like the pH strip’s less capable younger brother. It works in pretty much the same manner to determine whether you have acid or alkaline water, but it tends to be not so accurate. You might have used litmus paper back in the day in your school science lab, so this may ring a bell.

To determine the pH of the water you drink with litmus paper, simply soak it in water from your reverse osmosis system. The paper will turn one of two colors to indicate whether you have acid or alkaline water. You won’t get the full spectrum of color shades as you do with a pH strip, which makes litmus paper not as appealing for anyone looking for accuracy.

Another issue with litmus paper is that tap water from a multi-stage RO water system is likely to only be weakly acidic. This means that the litmus paper might not even detect acidity in your water, making it a pretty useless assessment method.

Red Cabbage Indicator Test

If you want to have a bit more fun when determining how acidic or alkaline water from your RO system is, try the red cabbage indicator test.

Yes, it’s exactly how it sounds: you use red cabbage to determine your water’s pH. The reason why this works is that red cabbage contains a unique pigment that actually changes color depending on the pH level of the water it’s mixed with, a bit like a pH strip.

👉 To perform the red cabbage test on your reverse osmosis water:

  1. First combine the cabbage with water in a blender. The water used in this case doesn’t necessarily have to be water from your RO system – you could just use normal tap water.
  2. After blending the cabbage, strain the solution to remove any large cabbage pieces.
  3. You can then add water from your RO water filtration system to the cabbage liquid and stir to combine.
  4. The water may take on a shade of red, and in that case, it’s a sign that it’s slightly acidic.

Though the cabbage testing method typically won’t provide you with a clear indication of how alkaline water from your reverse osmosis system is, it will at least give you an idea – and as an added bonus, it’s a pretty fun novelty activity that you can do with your kids.

Try comparing your results with the same amount of vinegar or lemon juice and compare the shades of red (hint: vinegar or lemon juice will produce a bright red color in comparison).

red cabbage ph test

How to Make RO Water Alkaline

The best and easiest way to achieve healthy alkaline drinking reverse osmosis water is to purchase a remineralization or mineral-adding filter for an extra bit of money.

Mineral-adding water filters can be installed onto most under-sink or whole-house reverse osmosis systems after health-harming contaminants have been removed and act as final-stage filters. These filters can improve water quality by introducing a rich dose of healthy alkaline minerals like calcium and magnesium. These minerals also improve water’s flavor, which can be affected by the RO water filtration process.

You could opt to pay a lower upfront cost by purchasing mineral drops if you prefer, which add a measured amount of minerals back into your water to support your health. Mineral drops are cheaper, but they do require more effort than if you were to simply attach an extra filter to your RO machine – but it’s your call.

If you’re interested in the health benefits of remineralizing RO water, my guide on the different reverse osmosis remineralization options should prove useful to you.