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
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The Basics of pH
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 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.
The pH Scale
Anything that increases the amount of hydrogen ions when dissolved in water is considered an acid. Anything that decreases the amount of hydrogen ions is considered a base. The pH value of a substance is determined by just how much it changes the amount of hydrogen ions.
Since the pH scale is a logarithmic scale, this means that something with a pH of 4 is 10 times more acidic than something with a pH of 5 and one hundred times more acidic than something with a pH of 6. Likewise, something with a pH of 9 is ten times more basic than something with a pH of 8 and a hundred times more basic than something with a pH of 7.
pH Indicators
A fun experiment that you can do at home is to measure the pH of different substances. While most of us used Litmus paper in school to test pH, many substances that occur in nature can act as pH indicators, usually by changing color.
For example, hydrangea bushes will produce blue flowers if the soil they are grown in has a pH of 5.5 or lower, pink flowers in a soil with pH of 6.5 or higher, and either purple or a mix of pink and blue if grown in soil with a pH between 5.5 and 6.5. Red cabbage is another pH indicator that you can use at home.
If you don’t like the smell of boiling cabbage, you can usually find pH indicator strips at your local aquarium supply store or online. (These are my personal favorites.)
Since the concentration of hydrogen ions also affects how well a solution conducts electricity, you can also find electrical pH testers. However some of the cheaper instruments are notorious for being inaccurate.
Conclusion
That’s all the pH fun for this week. Next week, we’ll talk more about the special qualities of acids and bases.
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Resources
Nørby, “The Origin and the Meaning of the Little p in pH.”
Image courtesy of Shutterstock.
