« Back to Glossary Index

pH is a measure of acidity (proton (H⁺) concentration). The lower the pH, the higher the proton concentration, [H⁺], and the more acidic. The higher the pH, the lower the proton concentration, [H⁺], and therefore the less acidic (more basic/more alkaline). This inverse relationship can be confusing but it comes from the fact that pH is the inverse log of proton concentration: pH = log10(1/[H⁺]).

Dividing by something bigger gives you something smaller, and all the log part does is make the numbers smaller and easier to deal with, so the more protons there are (the more acidic the solution) the lower the pH.  We call pH 7 “neutral” – above it (fewer protons) & we call a solution basic or alkaline – lower than 7 (so more protons) and we call it acidic 

I like to think of a sort of “proton cookie jar” where the jar fullness corresponds to pH (fuller jar -> lower pH -> more acidic) & how full the jar is at any time depends on how many proton-givers (acids) & proton-takers (bases) there are and how “stingy” they are – how willing are to give or take? Acid/base strength is reported as pKa, the pH at which 1/2 of the molecules of an acid are likely to be deprotonated: the higher the pKa, the greedier the molecule and the weaker the acid (stronger the base); the lower the pKa, the stronger the acid (weaker the base).

A buffering agent (“buffer”) is a weak acid that you use near their pKa, where you have almost even numbers of the acid & base forms at the point where they’re “on the fence” about giving or taking protons so they’re easy to “convince” with small changes in proton concentration. This allows them to keep the pH at a relatively constant level (if the jar gets too low (high pH) protonated copies donate protons, and if it gets too full (low pH), the deprotonated copies take some).