Is The pH of Pure Water Really Lower than Tap Water?

This question is answered when you understand the equilibrium relationship between CO2, HCO3, and CO3. In a closed system, the relative amount of each dune compound varies with pH value. At lower pH levels, CO2 is the predominant species. HCO3 is the predominant species at mid pH levels, and CO3 is present at higher pH levels.

Pure water contains a low and unstable level of ions, making electrode reading unreliable. In addition, RO membranes will reject dissolved ions but not dissolved gases, the RO permeate and RO feed will contain roughly the same amount of CO2. The HCO3 and CO3, however, are often reduced by 1-2 orders of magnitude. This upsets the CO2, HCO3, CO3 equilibrium that was established in the feed. In a series of equilibrium reactions, CO2 will combine with H2O driving reactions similar to that shown below, until a new equilibrium is established.

CO2 + H2O --> HCO3 + H+

water faucet

To summarize:

The new equilibrium will always result in a lowering of permeate pH if there is CO2 gas present in the feed water.

The pH drop is usually largest for waters with high amounts of alkalinity or HCO3.

When there is very little CO2, HCO3, or CO3, there is a very small pH drop observed in the permeate water.

Therefore it is not true that reverse osmosis filters will always reduce the pH level of water to a noticeable amount. The pH difference after the RO depends on the composition of your input water source; depending on whether you have large amounts of gases such as CO2 in your local water supply.

If you are concerned with the pH of your drinking water, you should avoid guess work and get your water tested with an accurate pH tester meter (avoid those $10 paper testing strips, which are very inaccurate with pH readings). Try to use an alternative measure like resistivity: ultrapure water with a resistivity of 18 MΩ cm provides a trustworthy way of ensuring the water is very close to pH 7.0. If you see that your pH is below 7 with a good amount, you can consider putting a pH increaser filter as a final stage of your reverse osmosis system and correct your acid water problem easily.

Some countries have regulations governing the pH of drinking water. Typically, the pH is recommended to be in the 6.5-9.0 range. It is our understanding that the purpose of these pH regulations is to minimize corrosion of piping materials so that metals like lead and copper do not dissolve into the drinking water supply.

It is important to know that our body naturally regulates our pH levels to find balance and equilibrium. Thus, under normal conditions, it will always maintain a neutral 7.4 pH value. Water pH will automatically change when it is ingested and comes in contact with food in your stomach. Some examples of water pH and beverage pH levels.

Water pH Levels Beverage pH Levels
Sea Water = 8.6 pH Soda = 2.5 pH
Mineral Water = 7.4 pH Sports Drink = 2.9 pH
Tap Water = 6 -8 pH Coffee = 4 pH
RO Water = 5-7 pH Beer = 4.5 pH

The truth is, people, drink acidic beverages all the time, but they are not aware of it. Since almost all fruit juices have lower pH, it is safe to say that it is not only safe to drink some acidic beverages but it is also beneficial. However acidic drinks such as soda should be kept to a minimum due to their high sugar content. Drinking slightly acidic water is also equally safe as long as the water is clean and contaminant-free. Eating plenty of fruits and vegetables every day will also help balance your body pH and keep you strong and healthy. Our body provides us with life every day; the least we can do is to treat it right. Exercise, eat healthily, stay hydrated, and laugh often!

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