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Understanding pH

 Age of Aquariums > Aquarium Articles


Water is indeed a very interesting compound. In May '98 I attended a Physics Congress, and one of the most disputed lectures there was by an invited speaker who presented his newest discoveries about...water! Odd as it may seem, despite being one of the most well known substances in the world, even today it's still widely studied by scientists, and there are many new properties still being discovered. Anyway, one of its well known, but very interesting properties, is water's ability to dissolve into itself.

What?!?!? That's right...just as when you add common table salt (NaCl) to pure water, which quickly breaks the Na-Cl bond and dissolves it into Na+ and Cl- (called ions), when you "add pure water" (H2O) to pure water, part of it dissolves into H+ and OH-. The main difference is that, while with salt you can add several spoons into a glass and virtually all of it gets dissolved, only a very small amount of pure water gets dissolved into water.

How much? Well, at room temperature, about 1 molecule in every 10 million (107) is dissolved. This means that, in a typical swimming pool full of pure water, only a few teaspoons of water would be dissolved. Now, that little number 7 up there near the 10 looks familiar, doesn't it? That's because it's exactly the number used to define "neutral pH". Note that, since each dissolved molecule of H2O results in 1 ion H+ and 1 ion OH-, these two ions are in equal amounts in pure water. The term "neutral" here means just that: the same amount of H+ and OH- ions. As mentioned above, at room temperature there's about 1 of each for every 107 molecules of water, and therefore we say that neutral water has pH=7.

And what about non-neutral water? If for any reason, the relative amount of H+ and OH- ions changes, then the water begins to drift from neutrality. If the amount of H+ ions increases, the water becomes acid, if the amount of OH- ions increases, the water becomes alkaline. For instance, suppose that the amount of H+ becomes 10 times greater than in pure water. Then there'll be about 1 H+ ion for every 1 million molecules of water (106) and therefore this water will have pH=6. Note that a change in 1 point in pH represents an increase of 10 times in the amount of H+ ions (in math this is known as a logarithmic scale). Since the amount of H+ never goes below 1 in 107 (at room temperature), the pH value for acid water will always be between 0 and 7. The value pH=0 means that there's 1 H+ ion for every molecule of water (1=100).

The same idea is used to represent increases in OH- ions. There's another scale used for this ion, called pOH, which works exactly the same: if the amount of OH- becomes 10 times greater than in pure water, then the new water will have pOH=6. For the same reasons explained above, the pOH values will always be between 0 and 7.

But using 2 scales complicates things unnecessarily, so it's more common to put both of them together in a single scale - pH. Now, instead of going only from 0 to 7, it goes from 0 to 14. The first half (0 to 7, or more accurately 7 to 0) represents increases in H+ (acid water). The second half (7 to 14) represents the increases in OH- (alkaline water). So, if you take pure water and increase the amount of OH- 10 times, the pH will raise from 7 to 8.

Fine, now that we know how pH works, how can we apply that knowledge in our hobby? Here's a sample application:

DIY Calibrated Acidifier
Suppose you get your hands on a solution of 10% Hydrochloric Acid (HCl). Assuming that all of the acid is dissolved into H+ and Cl-, then there's 1 H+ ion for every 10 molecules of water (101) and therefore this solution has pH=1. If you take 1 ml of this solution (10-3 liters) and dissolve it in 1 liter of pure water, then the relative amount of H+ decreases 1000 times, and the pH value will increase by 3 points, becoming pH=4. If, instead, you dissolve that 1 ml of the 10% solution in 100 liters, the pH will increase by 5 points, becoming pH=6. Aha! Now we're getting into the range of interest to fishkeeping (the great majority of freshwater species live in waters with pH ranging from 6 to 8).

The reasoning above allows us to arrive at the following simple rule: adding 1 ml of a 10% HCl solution to a 100 liter tank will contribute an amount of H+ ions equivalent to 10x that of pure water. So, if the tank's initial pH is at 7, it will lower to 6. If it's at 8, it'll lower to 7.

If you got the hang of this calculation, you can easily adapt it to the size of your tank, and to the desired change in pH. Just keep in mind that logarithmic scales don't behave as intuitively as linear scales, where doubling the amount of one factor simply doubles the amount of the other. For pH, it works like this:

  •  Amount Change
  • 1.3x1.6x2x2.5x3.2x4x5x6.3x8x10x
  •  pH Change

    Here's an example of how you can use the previous calculation and the table: if your tank has 200 liters (instead of 100), then dissolving that same 1 ml of that 10% solution in your tank will result in a relative amount of H+ only 5x greater than in pure water (instead of 10x). According to the table, in your tank this would lower the pH from 7 to 6.3.

    One more example: suppose your tank does have 100 liters and pH=7, but you only want to lower it in 0.3 points (to 6.7). According to the table, you should only add as much of the 10% solution in order to end up with 2x as much H+ as in pure water. So, instead of 1 ml, add only 0.2 ml.

    Cool, huh? However, it's important end this article by saying that, although the ideas above should work and lower the pH as expected, whether it stays at the new level or not will depend on another important property of aquarium water called buffering capacity or alkalinity, which is the ability of water to resist changes in pH. But that'll be the subject of a whole new article.

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