Every source simply states that pure water has a neutral pH of 7. But pH is defined as the negative log of the concentration of hydronium ions in mol/liter and that a pH of 7 is a happy coincidence rather than any natural law. If we had a different definition of a gram or a centimeter (or for that matter, Celsius since water’s pH is only 7 at 25C), then the neutral pH would be a different number.
Given that, water’s true pH can’t possibly be exactly 7. Given enough significant digits, pH has to vary. So what is the “true” neutral pH? I’m having an incredibly hard time finding it.
If the solution is pure water then the only ions present are H+, OH- plus the unionised H[sub]2[/sub]0.
The only way to increase the concentration of H+ is by the dissociation of H[sub]2[/sub]0 which produces an exactly balancing increase on the concentration of OH-.
I would not call it a happy coincidence, but rather the consequence of the pH scale being defined based on the behaviour of weak aqueous solutions of acids and bases. Such a solution by definition is neutral if the concentration of hydronium ions exactly balances the concentration of hydroxyl ions, and this is exactly halfways between the pKa values for the protonation of H2O and the pKa value for the deprotonation of H2O.
Of course, these two values have been determined with ever greater precision, and depend on temperature and pressure, hence are defined for standard conditions (STP).
In the real world, it is really hard to get absolutely pure water, as water readily takes up CO2 from the air, and as a consequence gets acidified due to:
H2O + CO2 <=> H2CO3 <=> H+ + HCO3 -
It is not a coincidence that water has a neutral pH, since water is what we’re using to define the standard of neutrality. It is, however, a coincidence that the neutral pH is very nearly an integer.
This is not my field even a little bit. I read the various wikis on chemistry and I’m going “Yeah, I get it, OK, I suppose, if you say so, I … 'm lost. Damn. Again.”
So why do they sum to 14? And is it exactly 14 or approximately 14? Is that a property of the world or a property of our units? Or both?
Somehow this stuff is slipperier for me than it ought to be. Especially when the pH > 7.
As mentioned, it’s just a freak coincidence that the auto-ionization of water is such that the log of the concentration of either ion is very close to an integer. I asked about this before on this board, and that was the consensus. There’s nothing about how anything is defined that gets the constant so close to an integer, it just so happens to be. Another reason to use the metric system!
It is a property of the world and of the substance; we define pKa, the dissociation constant of an acid, as “the constant you get when you add the -logs of the concentrations of its dissociation products and substract the -log of the non-dissociated form” (aka, the -log of [the multiplication of the concentrations of the dissociated stuff, divided by the concentration of the non-disociated thing]). It is different for different substances, and different for different temperatures.
“Standard temperature” has ranged between 15C and 25C depending on who did the experiment and where (we had to start putting it in the symbols because of the problems that caused), and that’s without getting into other scales. We could always redefine it for purposes of concentration calculations as “the one where the water constant happens to be exactly equal to 14 (which btw is real, real close to 25C)”.
Don’t be so sure; that’s traditional physical chemistry and those are the guys who think that giving 15 decimals with a 10% error is an improvement over giving 7 decimals with a 10% error… nowadays their machines have more lights but the actual methods haven’t gotten that much better.
Thank you Nava. The dawn slowly gathers for me on this stuff.
My tentative gloss on this: At a high conceptual level it’s a coincidence (or design feature :)) of the universe that liquid water has an inherent constant value near 14. Which then drives the “neutral or balance point” loosely defined, to be the middle value or near 7.
Then by choosing our standard temp, which is a quasi-unit since it depends on what arbitrary scale we choose to measure temperatures with, we can “adjust” the constant to be exactly 14. Conveniently for us, the constant is not exquisitely sensitive to temperature, so we can use a wide range of engineering-convenient temps as our “standard” for different purposes without totally trashing our calculations unless we really care about the deep decimal places.
