pH and pOH: Crash Course Chemistry #30

This may come as a bit of a shock to you, but I'm not super into personal grooming.

Like, I understand soap and shampoo, but there is all this other stuff now, and I keep seeing references to pH balance everywhere.

pH balanced soaps and shampoos and deodorant and makeup abound in supermarkets and drugstores, and I've even seen pH balanced water.

We've talked a lot about balance over the last couple of weeks, and pH balance is related to the equilibrium state of a reversible reaction.

You're also probably familiar with the pH scale, and you know that it has to do with acids and bases.

But what is pH exactly, and why is it weirdly written with a lowercase p and a capital H? And also, what about pH's alter ego, pOH? The capitalization thing is probably the easiest to answer, because there is no answer.

No one really knows what the p means.

The Danish chemist who came up with the term, a guy with the absolutely amazing name Søren Sørensen, never explained his reasoning.

Some people think that it comes from some form of the word power, whether it's puissance in French or maybe Latin, pondis.

But it probably just came from a common habit chemists have of differentiating a test solution labeled P from a reference solution called Q.

But thinking of the P as standing for power does help us remember the meaning more easily.

The H part is even easier.

It stands for hydrogen, because hydrogen ions or protons are pivotal to the behavior of acids and bases, which is what pH describes.

So you can think of pH as basically the power of hydrogen in a solution, the strength of the acid or base

character of a substance.

And it all revolves around one very important point of focus, our old friend water.

If you've been watching Crash Course Chemistry from the beginning, you've gotten the message that water is special in more ways than I can list, and pH is just one more of those ways.

We normally think of water as a perfectly neutral substance, neither acidic nor basic, and that's true.

But, as I've mentioned before, water can also function as an acid, releasing hydrogen ions, also known as protons, and as a base, consuming them.

How on earth is that possible? In order to explain, we first have to understand what the pH of a substance really tells us.

While chemically we say that pH represents the power of hydrogen in a solution, it's mathematically defined as the negative of the base 10 logarithm of the concentration of hydrogen ions in solution.

Okay, so now that you're terrified, I'm here to help.

So yeah, logarithms can seem a little bit scary at first, but the ones that we're using here are super easy.

And bonus, once you get familiar with them here, it'll be that much easier to understand them in math class.

So now that we got the scary mathematical definition, let's do the simplest mathematical definition.