Where is the pH? Calibrate a potentiometer stick in solution and it will tell you the chemical potential in terms of [H+]. Or for a home aquarist use a test kit.
That's the problem with basic chemistry, or blackboard chemistry. Its simplified for pedagogical purpose and doesn't tell you whats really going on. Saying that there equal parts 1:1 H+ and OH- is pH 7 is only true if there are no buffers in the water that can abstract protons. You're using the pKw of water to calculate pH and are making the assumption that the ion product water is the only dissociation reaction occurring in solution. If there are other buffering species in water you have to account for these as well and you end up with a long series of equations. These other buffering salts will abstract protons or hydroxide so calculating the ratio is meaningless because you can't calculate either concentration [H+] or [OH-] to any degree of accuracy . In any aquarium this is most certainly the case, unless your tank its barebottom and you use 18M ohm cm-1 water (ultra pure). The best way to get pH is best done by direct measurement rather than a blackboard calculation. I keep mentioning pH=-log[H+] because this is the easiest quantity to measure and calculate and is the most fundmanteal.
You use the equation pkw = pH + pOH
This is derived from
kw = [OH-][H+]
taking the logarithm
logkw = log[OH-][H+]
multiply out
logkw = log[OH-] + log[H+]
multiply everything by -1
-log kw = -log[OH-] + (-log[H+])
pkw = pOH + pH
notice how I just used the definition of pH to derive this?
To reiterate, the link you gave is absolute true if we are only considering pure water or water with only strong acids/bases and no buffering species.
Yes, I know how the formula is derived .What's the point to re-type it here? But thank you, it confirms what I've used is correct.
And yes, I've simplified things, so have you to prove me wrong. My point is still that pH measure is meaningless, It points to a ratio between acids and bases. You have no idea what type of acids and basis are affecting your tank, concentrations of hydrogen and hydroxyl ions, total numbers, etc...There are so many factors in play that one can't have a meaningful picture of their water quality by knowing the pH, because.....in your own words: "you can't calculate either concentration [H+] or [OH-] to any degree of accuracy"
But we don't need accuracy. All one needs to know is that the pH changes when the ratio is skewed but it is the cause and the initial "other" water properties that are important, not that the ratio or the pH, etc...have changed, because again....having this bit of info on its own tells you zilch.
For example, there are often people worried about pH being changed by CO2 injection, thinking the swing in pH will affect fish....which it doesn't . It affects fish because CO2 in certain concentration is toxic to certain species of fish, and high CO2 is generally toxic to all living beings in a tank. So if fish are affected, it has nothing to do with the swing in pH...It doesn't even change the water properties much either....It is plain toxic, that's all.
I gave a previous example, which I will repeat, that one can arrive at exactly the same pH for several different reasons, RO water being added, old tank syndrome and CO2 injection, completely different water properties but one can arrive at the same exact pH. ...So what does the pH tell you?
