The Basics of pH

pH is a mysterious term that many people use without understanding what it actually means. Ask Science helps unmask this confusing term.

Lee Falin, PhD
4-minute read
Episode #5

In my last episode on vinegar I mentioned that the low pH of vinegar makes it a good cleaner. “pH” is a handy term because you can use it to describe something without actually knowing what it means. For example, you might know that pure water has a pH of 7 and is considered “neutral.” You may also know that anything with a pH lower than 7 is considered acidic, and anything with a pH higher than 7 is considered basic. But what do acidic, basic, and pH really mean?.

What Does pH Stand For?

The Basics of pH

The exact meaning of pH is actually fairly complicated, which is probably why most people never bother learning what it exactly means. A simple way to think of it is as the inverse logarithm of the concentration of hydrogen ions in a solution. I know what you’re thinking: If that’s supposed to be the simple way, you’d hate to see the complicated ones. But just stick with me on this one.

Even though we might think of pure water as a collection of H2O molecules (two hydrogen atoms bonded to one oxygen atom), that isn’t completely true. Oxygen has high electronegativity compared to hydrogen, which means it really likes to take electrons from things.

Imagine oxygen as a big bully, roaming the schoolyard trying to beat up other molecules and take their hydrogen atoms from them. Sometimes this happens in pure water, so when two H2O molecules bump into each other, sometimes the oxygen atom in the first molecule will steal a hydrogen atom from the second molecule. When they walk away from the fight, they have become H3O+ (a hydronium ion) and OH- (a hydroxide ion).

Pure water has a concentration of 10-7 moles of H+ (or H30+) per liter. That’s a bit of a mouthful, so back in 1909, a Danish biochemist named Søren Peter Lauritz Sørensen decided it made more sense to use the inverse logarithm of the concentration. You can ask Math Dude about the details of logarithms, but taking the inverse logarithm of 10-7 gives you simply 7.

By this point you might have deduced that the “H” in pH stands for Hydrogen, but what about the “p”? Well the sinister truth is that nobody actually knows what Sørensen meant the “p” in pH to stand for. Depending on whom you ask, pH might stand for “potential Hydrogen,” “partial Hydrogen,” or “power of Hydrogen.” In fact, some scientists think the “p” doesn’t really stand for anything at all.  

Since everyone seems to be allowed to make up their own definition, I’ll confess that I like to think of it as “pieces of hydrogen,” as in the pieces that broke off of the other molecule. I should warn you that this isn’t a very scientific definition and if you were to use this definition around other scientists, they might laugh at you.


Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Lee Falin, PhD

Dr. Lee Falin earned a B.S. in Computer Science from the University of Illinois, then went on to obtain a Ph.D. in Genetics, Bioinformatics, and Computational Biology from Virginia Tech.