Alkalinity of Reverse Osmosis Water | What You Need to Know
Written by: Gene Fitzgerald // Last Updated: Mar 31, 2023
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You would be forgiven if you automatically assumed that healthy, reverse osmosis water also had high alkalinity. After all, it is healthy water, so why wouldn’t it have?
Curious to know the precise alkalinity potential of RO water, you jump on Google, only to find the results are not what you expected.
RO water doesn’t have high alkalinity at all! But why? Let’s delve into a bit of the science.
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Key Takeaways
- Reverse osmosis water doesn’t have any alkalinity potential at all. That’s because it has been stripped of all its mineral content.
What’s the Alkalinity of Reverse Osmosis Water?
Reverse osmosis water does not possess alkalinity at all. Alkalinity in water only occurs when there are dissolved minerals present, capable of buffering any acids that come into contact with the water. These minerals are usually calcium and magnesium carbonates, which are removed by the reverse osmosis process entirely.
Don’t Confuse Water pH With Alkalinity
Acidity is measured on a scale called the pH scale. pH stands for “potential hydrogen”. 7 on the pH scale is neutral, any value under 7 is acidic, and any value over 7 is known as a base or alkaline.
The pH of water does not necessarily indicate alkalinity, as pH and alkalinity are two separate things.
What Is Alkalinity?
Alkalinity is a measure of how well a solution can buffer acids without changing its pH. The higher the level of alkalinity, the more acid you will need to change it, as the solution can buffer a lot of acid while keeping the pH stable. Alkalinity is usually provided by concentrations of dissolved minerals.
What Is Acidity?
Acidity measures the concentration of hydrogen atoms in the water. The more hydrogen atoms, the more acidic something is.
Why Is Reverse Osmosis Water Acidic?
Reverse osmosis water is acidic because all minerals are removed from the water, meaning it has low alkalinity. As a consequence, RO water doesn’t have anything to buffer acids.
As water is a solvent, and it really likes to dissolve things, it’s hungry to find some impurities to snack on. In the case of reverse osmosis water, the first thing it usually gets a chance to interact with is air, and the carbon dioxide in the air mingles with it to create carbonic acid, which is acidic.
That’s why reverse osmosis water has a pH between 5 and 7, usually 6.5 before exposure to air and 5 to 5.5 afterwards.
What Makes Alkaline Water Alkaline?
Alkaline water is simply water with a pH above 7. This can be achieved by adding baking soda to it, for example. The baking soda neutralizes any hydrogen atoms, and once no more hydrogen atoms are left, hydroxide ions accumulate in the water providing for an alkaline pH level.
Note that this is different from neutralizing acidic RO water by remineralizing it. Adding calcium and magnesium carbonates to the water increases its alkalinity which allows it to neutralize acids. Thus, the pH level goes up.
How to Measure Water Alkalinity
Water alkalinity is tested using a process called titration, which measures how much acid is needed to lower the pH of a water sample down to 4.5. To do this, small amounts of strong acid are added to the water until it reaches the desired level. The test is referred to as M-Alkalinity, or sometimes a P-alkalinity test is used for a pH above 8.3.
Measuring your feed water alkalinity can be important to predict how likely carbonate scaling is to occur on your reverse osmosis membrane. A pH above 8.3 will increase the scaling potential of the feed water quite a bit.
Further Reading:
How to Measure Water pH
If you want to check the alkalinity of your water, you can do so using a digital/electronic pH meter, pH strips, or even cabbage!
Digital pH Meter
Digital pH meters are easily purchased online and are probably the most accurate way to measure your water pH.
- Put the pH meter into the container of water.
- Press and hold the button.
- Wait for the screen to hold a stable reading.
If the value on the screen is above 7, this indicates that the solution is a base, and under 7 indicates the solution is an acid.
pH Strips
pH strips change color to show the approximate pH of a solution. The downside is that the colors can sometimes be hard to differentiate, so it is best to get one with a range of 5-8.5 to measure RO water, as the color range isn’t as broad and hard to read.
Add the strip to the purified water, wait for the color change to show, and check on the packaging to see what pH the color indicates.
Litmus Paper
Similar to a pH strip is litmus paper, though it isn’t as accurate in terms of giving you a numeric pH reading, it simply indicates if a solution is acidic or basic.
The “Red Cabbage” Test
Red cabbage contains a pigment that is reactive to acid and, upon exposure, will change color.
First, blend the cabbage with plain water, and strain the liquid to remove the cabbage chunks.
Secondly, pour the reverse osmosis water into the cabbage liquid, and stir. If it turns red, it is acidic, and if the color does not change at all, then it means it is a base.
If you have any questions about RO water alkalinity please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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