Reverse Osmosis Drinking Water Systems
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Reverse Osmosis and PH

[H3O+] pH [OH-] Example
1  X 100 0 1  X 10-14 HCl (4%)
1  X 10-1 1 1  X 10-13 Stomach acid
1  X 10-2 2 1  X 10-12 Lemon juice
1  X 10-3 3 1  X 10-11 Vinegar
1  X 10-4 4 1  X 10-10 Soda
1  X 10-5 5 1  X 10-9 Rainwater (unpolluted)
1  X 10-6 6 1  X 10-8 Milk
1  X 10-7 7 1  X 10-7 Pure water
1  X 10-8 8 1  X 10-6 Egg whites
1  X 10-9 9 1  X 10-5 Baking Soda
1  X 10-10 10 1  X 10-4 Ammonia
1  X 10-11 11 1  X 10-3
1  X 10-12 12 1  X 10-2 Drano®
1  X 10-13 13 1  X 10-1 NaOH (4%)
1  X 10-14 14 1  X 100

...Customers often ask questions about water issues because they’ve heard a story on the TV or radio, or, perhaps, heard some bit of information from a friend. Often, what they’ve heard is incomplete information—or even wrong altogether.

.....Water that has been produced by a properly functioning reverse osmosis system is some of the purest water available, yet often RO users are surprised and a little alarmed to find that their very clean water is also “acidic”, that is, that it has a low pH. Neutral pH is defined as 7.00, but RO water typically shows a pH of between 5.00 and 6.00.
.....Given that the pH scale, like the Richter scale for earthquakes, is logarithmic, that means that pH 5.00 water is actually 100 times as acidic as pH 7.00 water. Certainly, that sounds very drastic, but due to the somewhat unique properties of RO water, there really is no cause for alarm.
.....First, it may be helpful to look at what pH measures. pH is a measurement of the relative acidity or alkalinity of a solution. Some substances, such as hydrogen ions, lower the pH of a solution. They are considered acids. Other substances, such as hydroxide ions, raise the pH of a solution. Those are called bases.
.....There are many different acids and bases that affect pH, but for clarity sake it will be easiest to think only in terms of hydrogen ions (H+) and hydroxide ions (OH-). When water has more free hydrogen ions floating around than hydroxide ions, it is acidic (pH less than 7). When there are more hydroxide ions than hydrogen ions, it is basic (pH greater than 7).
.....Water, as we have all learned, is described by the chemical formula H2O. If you could shrink down small enough to be able to see molecules, when you looked at a glass of water, you would see that the water molecules did not simply stay put as H2O, but were always separating and recombining. An H2O would break in to two pieces: an H+ and an OH-, and then get back together. In that glass of water there would be gazillions of water molecules, all splitting and rejoining constantly. If that water were very pure RO water and didn’t have any other substances dissolved in it, the pH would be neutral.
.....Because there are only H2O molecules there, the number of H’s and OH’s would be even. There wouldn’t be more H’s than OH’s, or the other way around, and so the acidity of an H+ would be “cancelled out” by the alkalinity of an OH-. This balance between hydrogen ions and hydroxide ions is what makes a solution neutral. Therefore, extremely pure water is always neutral because it doesn’t contain any other substances to throw off that H/OH balance.
.....Why then does RO water test to have a low pH? The very purity that means it is neutral also makes it very sensitive to the addition of other substances. Technically speaking, RO water has little or no buffering capacity. That means that the addition of even a small amount of acid will have a large effect on pH.
.....Consider this example. A man is blindfolded and put in a room. He is told that he must raise his hand when he hears a baby start to cry. If the room is quiet, free of other’“contaminating” noises, and a baby starts to cry, he will raise his hand immediately. If, however, the room contains a large flock of seagulls, several construction workers with jackhammers, a roaring jet engine, an 80’s heavy metal band, and other noisy things, and then a baby starts to cry, he will not be able to notice it.
.....The empty, quiet room is like the glass of RO water. If only a tiny amount of acid (a single baby crying) is added, the change will be easily noticed. The noisy room is like a glass of tap water, full of amounts of salt and minerals and other things commonly found in water. The addition of one more thing will go basically unnoticed.
.....Regular, untreated tap water typically contains chemicals that act as “buffers”. When some H+ is added to typical tap water, some of those buffers “catch” the H+ and combine with it. Then, even though H+ has been added, the pH doesn’t change because the amount of free H+ floating around is still the same relative to the amount of free OH-. Because a reverse osmosis removes the vast majority of those buffer chemicals, when a little bit of H+ is added, it upsets the balance between free H+ and free OH-.
.....Reverse osmosis membranes do not remove gasses, such as carbon dioxide in water. Also, when RO water is exposed to the air, a small amount of carbon dioxide will begin to dissolve in the water. So RO product water has the buffering alkalinity chemicals removed and the acid causing gasses remaining.
.....Again, if you were small enough to see molecules, what you would see happening is this: You would see the carbon dioxide molecules combine with some of the OH-‘s in the water. That means that there would be relatively more free H+’s floating around, because some OH-‘s had been “caught” by the carbon dioxide in the water. Having more free H+’s than OH-‘s floating about is how we earlier defined an acidic solution, so that is why the pH of RO water is typically lower than neutral.
.....Having low pH RO water should not be of concern because the “acidity” in RO water is very weak. In order to bring RO water to a neutral pH, it takes only a tiny amount of base. For example, if you had a glass of RO water with a pH of 5.0 and you added 2-3 specks of baking soda (a base) that would likely neutralize the tiny amount of acidity in the water.
.....For someone who is concerned about the effect of low pH RO water on the body, I would not recommend going through the trouble to add specks of baking soda to every glass of drinking water. The moment a person drinks some low pH RO water, that water will combine with the saliva, and, moments later, the stomach contents. It will cease to be extremely pure (because it will have saliva and chewed up food in it), and so it will no longer have the unique pH characteristics described above. Additionally, the pH in the stomach of a healthy person is typically less than 2. That is very, very acidic. As soon as the water gets to the stomach, it will become very acidic, also.
.....That acidity is essential to human health. Without the acid in our stomach, we would not be able to digest our food, and we would get sick much, much more often because stomach acid kills many bacteria and other things that we ingest. The only way that drinking low pH RO water could upset the pH balance of the body might be if someone drank ridiculously large amounts, and didn’t eat anything, and continued that for some time. Barring that situation, drinking low pH RO water will have basically no effect on the pH of the body.


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