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pH of Reverse Osmosis Water: Is RO Water Acidic?

Scott Winfield
Last Updated on
by Scott Winfield

RO water treatment removes toxins, microbes, lead, and debris from water. This often leads to a change in pH compared to unfiltered water.

Continue reading to find out about RO water’s pH and acidic properties.

What Is the pH of Reverse Osmosis Water?

Reverse osmosis water will have an average pH of between 5 and 7 after being treated. This pH level is considered healthy for most human consumption, including drinking.

The acidity of water is measured on the potential hydrogen (pH) scale. This scale tells us the concentration of hydrogen ions and how active they are in water.

The scale goes from 0 to 14, and an increase of one whole number on the scale represents a tenfold difference in acidity.

In general, a higher level of potential hydrogen (pH) means the water will be less reactive with other elements and compounds it comes into contact with.

This fact is because more hydroxyl ions means that there is less free hydrogen that isn’t already in an ion to react with new substances.

Conversely, water with a low pH is considered acidic, and can corrode metals and eventually dissolve them over time.

pH of Reverse Osmosis Water

Importance of the pH for Reverse Osmosis Water

The pH of RO water will determine its solubility and the biological availability of chemicals.

The solubility of water determines the amount of substances that can be dissolved into 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 humans and animals need, as well as nutrients that help plants grow when they are watered with liquid containing them.

Does Reverse Osmosis Change the pH of Water?

Reverse osmosis changes the natural pH of water from 6 – 8 pH to 5 – 7 pH.

The reason for a change in pH is due to the RO process removing certain minerals from water.

Without these minerals, the water will react to carbon dioxide when exposed to air which will allow more carbolic acids to form. These carbolic acids lower the pH of the water and turn it more acidic.

This process can vary depending on the water chemistry of the initial sample, so it is hard to predict how much the water’s pH will change when going through the reverse osmosis process.

Is Reverse Osmosis Water Acidic or Alkaline?

Reverse osmosis water falls between 5 and 7 on the pH scale, making it generally acidic.


On the pH scale, anything below a seven is considered acidic.

Acidic water dissolves many substances which leads to an antimicrobial effect because microbes have a more challenging time 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 continually consumed.


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

The reason neutral water is regarded as pure water is that it has an even balance of hydrogen ions and hydroxide ions.


On the pH scale, anything above a seven 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.

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.
Digital pH Meter
Digital pH Meter

If you get a reading that doesn’t fall within the five to seven range of RO water, it could be a sign of something wrong with your RO treatment system or the source of your water.

To be very accurate, also test water from the RO faucet’s and compare readings to water from the RO membrane.

pH Test Strips

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

However, most pH test strips are not accurate enough to effectively test RO water since RO is a lot purer than unfiltered water and can be off by one whole point on the pH scale.

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

pH Litmus Paper

Litmus Paper
Litmus Paper

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

To use a pH litmus paper:

  1. Fill up a clean container with test water. Ensure enough water in the sample to cover the strip completely.
  2. Dip a paper into the test container for 15 seconds.
  3. As you are dipping the paper, you will be able to observe the paper starting to turn 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.

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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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