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pH of Reverse Osmosis Water: Is RO Water Acidic?

Scott Winfield
Written by Scott Winfield
Last Updated on

Reverse osmosis water treatment is effective at removing toxins, microbes, lead, and debris from water, but many minerals are lost in the process because of the 0.0001-micron pores of the RO membrane. This lowers the pH of the water, making it slightly acidic.

Let’s explain how that change occurs, why it’s important, and how you can test the pH levels of your water in more detail.

What is the pH of Reverse Osmosis Water?

After treatment, reverse osmosis water has an average pH of between 5 and 7. This pH level is considered healthy for human consumption.

Water (H2O) consists of two different ions: free hydrogen ions (H+) and hydroxyl ions (OH-) which also contain hydrogen (H). 

Hydroxyl ions (OH- ) can break down into their constituents and produce more H+, affecting water’s potential hydrogen (pH) level. 

While the already-present free hydrogen ions determines the acidity of the water, hydroxyl ions affects its alkalinity. When they’re equal in numbers, the water is neutral with a perfect pH level of 7.

pH Scale
pH Scale

The acidity/alkalinity is measured on the potential hydrogen (pH) scale, which tells us the concentration of H+ and OH- ions and how active they are in the water. The more the H+, the more acidic the water is, and the more the OH-, the more alkaline it is.

Acidity means that the free hydrogen ions inside in water can interact and corrode surfaces it comes in contact with. As a result, acidic water can corrode metals and cause stains on faucets, sinks, and appliances.

On the other hand, a higher level of potential hydrogen (pH) means that the water will be less reactive with other elements and compounds it comes into contact with. This is because more hydroxyl ions mean less free hydrogen to react with new substances.

Importance of the pH for Reverse Osmosis Water

The pH of RO water will determine its solubility. It also affects how certain chemicals and heavy metals the water carries interact with the biological organisms they’re introduced to.

The solubility of water determines the number of substances that can be dissolve in the water. As the pH of the water decreases, more and more elements and chemicals can be dissolved and carried into the water.

Solubility in RO water is necessary to carry healthy nutrients like nitrogen, phosphorus, and carbon. These are essential nutrients that humans and animals need, and that help plants grow. However, the same applies to dangerous heavy metals like lead, cadmium, and copper. As they’re in a more dissolved state in low-pH water, they become more toxic for humans and animals.

Since RO treatment already removes all these dangerous contaminants, it shouldn’t be a cause for concern.

Does Reverse Osmosis Change the pH of Water?

Reverse osmosis changes the natural pH of water from 6 – 8 pH to 5 – 7 pH, since the RO process removes certain minerals from the water.

The alkalinity of water relies upon the presence of calcium bicarbonate and magnesium bicarbonate. Without these minerals, hydroxyl ions will react to carbon dioxide when exposed to air and form more carbonic acids while releasing free hydrogen into the water and lowering its pH.

That said, this process depends on the chemical properties of the initial sample. Therefore, it’s hard to accurately predict how much the water’s pH will decrease after the reverse osmosis process.

Is Reverse Osmosis Water Acidic or Alkaline?

Reverse osmosis water has a pH of between 5 and 7, which makes it acidic.

Here’s a quick breakdown of acidic, neutral and alkaline water:

Acidic Water:

On the pH scale, any substance below 7 is considered acidic.

Acidic water dissolves many substances and has an antimicrobial effect because microbes have more difficulty maintaining their structure inside an acidic substance.

However, because acidic water can dissolve things more efficiently, it often contains more metals like lead, arsenic, nickel, chromium, or zinc, which have severe health effects when consumed.

Neutral Water:

On the pH scale, any substance at exactly 7 is considered neutral. Water that has a pH rating of seven is regarded as pure water.

Neutral water is regarded as pure water because it has an even balance of hydrogen ions and hydroxyl ions.

Alkaline Water:

On the pH scale, any substance above 7 is considered alkaline. Alkaline water has less free hydrogen available, so it will not dissolve or react with different substances.

How to Test Acidity Level of RO Water

There are different ways to test RO water’s acidity level. It’s best to use the digital pH meter method for the most accurate result.

1. Digital pH Meter:

The steps to follow when using a digital pH meter to test the acidity level of RO water are:

  1. Attach the digital meter to a container of water that has left the RO membrane.
  2. Press the calibration button and hold it.
  3. Wait for the digital meter to give you a stable reading.
  4. Release the button and dry the meter off to use again when needed.

If you get a reading that doesn’t fall within the 5 – 7 pH range, it could be sign of a problem with your RO treatment system or water source.

Digital pH Meter
Digital pH Meter

2. pH Test Strips:

Using pH test strips is an alternative method for testing pH.

To use pH test strips:

  1. Take the strips out of the container.
  2. Dip the strip all into the water.
  3. Shake off any excess water after removing the strip.
  4. Place the pH strip on a horizontal surface and wait 15 seconds for the strip’s colors to change.
  5. Compare the changing colors to the chart on your strip instructions to determine the pH reading of your water sample.
pH Test Strips
pH Test Strips

3. pH Litmus Paper

Litmus paper is one of the easiest ways to test RO water.

To use a pH litmus paper:

  1. Fill up a clean container with test water. Ensure enough water is in the sample to cover the strip completely.
  2. Dip a paper into the test container for 15 seconds.
  3. As you dip the paper, you will be able to observe the paper turning red or blue.
  4. If the paper turns blue, it is an alkaline solution. If the paper turns red, it is an acidic solution. If the paper doesn’t change color, then the sample is neutral.
Litmus Paper
Litmus Paper


The RO process removes minerals alongside hazardous contaminants. Removal of minerals means that the free hydrogen ions inside the water are higher than before, which lowers the pH level from 6 – 8 to 5 – 7 and makes the water more acidic.

Acidity in water means that the water is more soluble. Therefore, it can cause corrosion in metal objects and lead to stains on faucets and sinks.

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Scott Winfield
Scott Winfield
My name is Scott Winfield and researching and writing about water filters and other strategies to purify water has become my full time passion in recent years. I'm glad that you found our site and you can look forward to authoritative and well researched content here to help you get the best in water.
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