What pH Levels Are Considered Strong & Weak?

Scientists use pH, a measure of hydrogen ion concentration in a solution, as an indicator the acidic or basic nature of a solution. The pH scale typically ranges from 1 to 14, with lower numbers representing acids, higher numbers, bases. Neutral liquids like water have a pH of 7.

Generally, a strong acid has a pH of about zero to 3. The stronger the acid, the better it dissociates in an aqueous solution, releasing more cationic hydrogen (H⁺) ions. Examples of strong acids include hydrochloric acid (HCl), hydrobromic acid (HBr), perchloric acid (HClO₄), and sulfuric acid (H₂SO₄). However, because pH measures the amount of hydrogen ions released in a solution, even a very strong acid can have a high pH reading if its concentration is very dilute. For instance, a 0.0000001 molar HCl solution has a pH of 6.79. As a strong acid, HCl exhibits 100 percent dissociation, but the extremely low concentration of hydrogen ions it releases in this case gives it a nearly neutral pH.

A weak acid, on the other hand, fails to ionize completely. It releases fairly low concentrations of hydrogen ions in an aqueous solution, resulting in a pH range of about 5 to just below 7. Examples include acetic acid (CH₃COOH), the main component of vinegar, and formic acid (HCOOH), the acid responsible for the sting of ant bites. Again, there are exceptions to this general pH range. A sufficiently concentrated weak acid can still produce a low pH readout. A 1.0 molar CH₃COOH solution, for example, has a pH of 2.37.

Like strong acids, a strong base dissociates nearly completely in water; however, it releases hydroxide (OH^-) ions rather than H⁺. Strong bases have very high pH values, usually about 12 to 14. Well-known examples of strong bases include caustic soda or sodium hydroxide (NaOH), as well as lye or potassium hydroxide (KOH). Hydroxides of alkali or Group 1 metals are generally strong bases.

The pH of a weak base falls somewhere between 7 and 10. Like weak acids, weak bases do not undergo complete dissociation; instead, their ionization is a two-way reaction with a definite equilibrium point. While strong bases release hydroxide ions via dissociation, weak bases generate hydroxide ions by reacting with water. Ammonia (NH₃) and methylamine (CH₃NH₂) are examples of weak bases.