pH What . . . What pH?

by Mike Jacobs

There have been a bunch of rumors running around about pH. Most of them are incorrect but some are really true, but most of all, if you are going to try to breed fish you really ought to pay a little closer attention to the ideas of pH and softness. There are other factors of water that are important as well but primarily, for a start, lets talk pH . . . in particular why a move from pH of 8 to pH of 7 is 1/2 the move of pH of 8 to pH of 6 . . . this is VERY important!

If you remember your amateur chemistry, pH is quite simply the measure of something. It is just that the something has a pretty "sophisticated" name and the unit of measure is not familiar to you. Remember pH is a measure, a quantity of something. Well the something is the hydrogen ion. Ok . . . that's out of the way, it doesn't have any bearing on the discussion anymore . . . that is the something. There are a bunch of these little hydrogen ions in water and we are going to measure how many on average there are in the surrounding area of the tester. pH would not be a problem if we just measured with normal units, ". . . well Louie, there are 4 hydrogen ions in the water . . . therefore the pH would be 4!" It's not that simple . . . but it's not that tough either. First of all remember that the only part of the water you are testing is the part that immediately surrounds the tester. I mean, if you have a bath tub of water and you dump a bunch of acid in the north end and quickly measure the south end you will get a different reading than what it will be very soon . . . meaning that if you are changing the pH of you water you must give the water a chance to be mixed and the sample you are testing is representative of the entire batch. OK, we now have a uniform solution, what is the unit of measure? OK . . . tighten up the belt . . . we're going to get mathematical for a moment but when the smoke clears you should understand the essence of what I'm trying to say.

The concentration of hydrogen ions in water is written in "moles/liter" . . . don't panic! Moles is usually written as a number times a negative power of 10 (VERY SMALL) -- example: 1.0x10e-7. This is just a "simpler", "more compact" way of writing .0000001. Again . . . 1.0x10e-7 = .0000007 . . . you are not use to it but scientists are and they prefer 1.0x10e-7. Just allow that for a moment. Let's use 1.0x10e-7 instead of .0000007.

A "neutral solution" . . . they picked it as neutral not me . . . so the measure of concentration of hydrogen ions ([H+]) in a "neutral solution" of water is:

[H+] = 1.0x10e-7 M/l . . . which means there are .0000001 moles of these silly little hydrogen ions per liter of neutral water . . . everything revolves around the neutral point. ALMOST DONE WITH THE CHEMISTRY AND MATH.

The definition of pH is: pH is the "negative log of the hydrogen-ion concentration." Therefore at neutral: pH = -(log[H+]) . . . we're getting close, hang in there!

Therefore at neutral: pH = -log(1.0x10e-7)

. . .math:

1) log of a product is the sum of the logs . . . believe me here!!!
2) log of a number is the power of 10 that produces the original number (example: log 100 = 2 because 10e2 = 100.

Therefore starting from the above equation:

pH = -log (1.0x10e-7)
pH = -(log 1.0 + log 10e-7)
pH = -(0+e-7) . . . yes, log 1 = 0, 10e0 = 1 . . . trust me here too!
pH = -e-7 = 7 . . . therefore saying that the pH of a neutral solution is 7 . . . we knew that, so what? Well first, here is a nifty chart to remember:

Note the correlation of pH value and the power of 10. Here now is the real reason for the article. If you realize that the pH number is a power of 10 then you can understand why people will tell you it is not just two numbers difference between a pH of 8 and a pH of 6 . . . there is a difference of . . . ta da ta da . . . 2 power of 10 or 10e2 or 100 times the concentration of hydrogen ions. That's significant folks. Going from a pH of 8 to a pH of 6 is not just 2 times the concentration of ions but 100 times the concentration.

We'll talk about the effects of pH next time around . . . test on Thursday . . . ; )