Crash Course On Ph And How An Acid Works

Crash course on pH and how an acid works

To fully understand ocean acidification, it is important to have a solid grasp on what exactly “acidification” means. If the sentence “the pH goes down as the concentration of free H⁺ ions in solution goes up” does not feel familiar to you, then it might be worthwhile to read this article.

A little fuss about H+

A typical hydrogen atom consists of just a proton and an electron (no neutron). And since an electron and a proton have exactly the same (but opposite) charge, if we remove an electron then we are left with a single proton, which we can say has a +1 charge. So basically an H⁺ ion is just a proton, and it is perfectly correct to use the two names interchangeably.

If this H⁺ ion* exists in a solution of water, then it will affect pH. Why? Because pH is precisely a measurement of how much free H⁺ is in the water. It literally means the negative logarithm of the concentration of H⁺ in the water. For example, if we had an H⁺ concentration of 10^{^-3} mol, then we would take the negative log of this number:

- log ( 10^-3)

= - (-3)

= a pH of 3.0

So for a solution with an H⁺ concentration of 10^-3, the pH is 3—which is of course quite acidic, since as we know a pH of 7.0 is neutral.

Some people are surprised that the definition for pH is so simple. By remembering this simple definition, we do not have to consider the concentration of the acid itself, the temperature of the water, the alkalinity, or any other molecules or ions in the water. While some of these factors may affect how much H⁺ is able to be dissolved into the water, the only factor that can affect pH is the amount of H⁺ that is dissolved.

The weak have trouble letting go

What then makes some acids stronger than others? Many people are familiar with “sulfuric acid” or “hydrochloric acid” and would consider them dangerous substances. And indeed, they can be—but it is not because of the “sulfuric” or “chloric” part. The strength of an acid has nothing to do with these other atoms, because as we’ve said pH is 100% determined by the amount of free floating H⁺, nothing else.

The sulfur, or chlorine, or oxygen, or whatever other atom an acid molecule might contain are basically just acting as “magnets” for the H atoms that are bound to it. And for some molecules, these “magnets” are relatively weak. It should be intuitive that acids in which the “magnet molecules” have a weak attachment to their H⁺ are strong acids. This is because if the molecule does not hold onto its H⁺ atom very strongly, then it will tend to fall off of the molecule more easily, and thus become free H⁺ in the water—and cause the pH to go down. Conversely, weak acids are those which have a very strong attachment to their H+. 

What determines whether a certain acid will be strong or weak can be complicated, and it often has to do with the polarity, electronegativity, and molecular structure of that specific acid. But each acid has its own specific strength. This is not the whole story however, in that in addition to the strength of the molecule's attachment to its H+, the question of whether or not it gives up its H+ is also determined by how much H+ is in the water to begin with.

Pure and neutral, please

Water wants to be neutral. It is not physically favorable to have too many or too few** H⁺ ions floating around, so as a solution gets farther and farther from 7.0, it will tend to have a “magnetic” force of its own. It’s similar to pulling or pushing a spring: it will always want to go back to its neutral, relaxed position. A very basic solution, then, will essentially crave H⁺ in order to bring it back down to a pH of 7.0.

Because of this, an acid that may be considered weak and which would not normally lose its H+ in a solution of neutral water may very well lose its H⁺ in a highly basic solution. The more basic it is, the more likely this is to happen.

The opposite is the case for a very acidic solution, i.e. if it already contains high concentrations of H+, then the acid will be less inclined to lose its H+.

The tricky nature of the logarithm

One take-away point that I’d like to emphasize here, and that is that pH is a logarithmic value. This means that the concentration of H⁺ in a solution may be changing quite a bit more than one might assume from the change in pH value alone.

For example, a pH of 8.2 corresponds to an H+ concentration of 10^-8.2, or 6.3096 x 10 ^-9 mol. A pH of 8.1 corresponds to an H⁺ concentration of 10^-8.1, or 7.943 x 10^-9 mol.

Dividing 7.943 by 6.3096, and we get 1.259—in other words, a mere 0.1 drop in pH means that the H⁺ concentration has increased by about 26%. Similarly, a 0.2 drop corresponds to a 58% increase, a 0.3 drop corresponds to a 99.5% increase, etc.

*Normally in an aquatic solution, the H+ will bind with a water molecule to form H3O+. Different texts may prefer one definition over the other, so whether you call it H3O+ or H+ is a question of personal preference.

**Technically a very basic solution will not only have very few H⁺ ions, but it will have an abundance of OH- ions. In fact, the amount of OH- and H⁺ always vary inversely proportional to one another; that is, as H⁺ levels rise, OH- levels will fall by exactly the same amount. The opposite is the case for H⁺ levels falling, i.e. OH- levels will rise.