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Decrypting pH

Ready for some chemistry lessons? Here’s a more in-depth understanding of pH and what it means for your aquarium water
CATS Classified In The Straits Times - October 9, 2010
By: Wong Wei Chen
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Decrypting pH

Seasoned aquarists or those who have been following this column should be no strangers to the term “pH”, as I’ve broached the topic before.

But in brief, pH is a measurement of water acidity or alkalinity. A reading of 7 means that the water is neutral, while readings below 7 indicate that the water is acidic. Readings higher than 7, on the other hand, indicate alkalinity.

So is that all to know about pH? If your concern is merely to maintain the right degree of acidity or alkalinity in your aquarium water, we can stop probing further. However, that barely scratches the surface of the topic, and the truth is – most of us don’t really know that much about pH.

Out of curiosity, I’ve rolled up my sleeves and taken a plunge into the unfamiliar world of chemistry and maths to find out more.

What we mean by pH 7
Let’s start with the basics. A water molecule is composed of two hydrogen atoms (H) and one oxygen atom (O). The chemical composition of water is thus H­2O.

For any given volume of water, a certain amount of water molecules gets ionised, meaning that the H2O molecule is broken down into a hydrogen ion (H+) and hydroxyl ion (OH-). Such ionisation is a natural phenomenon, and not the result of artificial intervention. In other words, this is a given.

In pure (or distilled) water at room temperature, approximately one molecule in every 10 million (107) is ionised. If you have a volume of water that contains 10 million molecules, one of these will be ionised; if you have a total of 20 million molecules, two will be ionised, and so on. The formula for computing the number of ionised molecules, N, is:
                              

Notice that the hydrogen ion carries a positive charge, while the hydroxyl ion carries a negative charge. In order for water to be neutral, the number of positive ions and the number of negative ions have to be equal, so that the charges cancel one another out.

In pure water, neutrality is maintained, since for every H+ produced there is a corresponding OH-. We also know from the formula above that the number of ionised molecules in pure water is (T x 10-7). In short, pure water has a pH of 7, and this reading is used as the standard for neutrality.

What happens when pH changes?
If for some reason the proportion of H+ and OH- changes, the water will deviate from neutrality. If the number of H+ ions increases, the water becomes acidic. Conversely, if the number of OH- ions increases, the water becomes alkaline.

Suppose that the amount of H+ increases 10 times. There will then be 10 H+ ions per 10 million water molecules (one H+ ion per one million water molecules). Using the formula, we can compute the number, N, as follows:
                             

As you might have guessed by now, (T x 10-6) is equivalent to a pH reading of 6. A decrease of one point from 7 on the pH scale therefore represents 10 times more acidity. Using the same logic, an increase of one point above 7 likewise represents a tenfold increase in alkalinity.

Now that we’ve reached such an understanding, you can perhaps better appreciate the proverbial saying that fish should not be exposed to sudden swings in pH. To illustrate the severity of drastic pH changes, imagine yourself suddenly breathing air that is 10 times more polluted than what you’re inhaling now!

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