What does pH measure?
pH is a measure of the activity of hydrogen ions (H+) in a solution and, therefore, its acidity or alkalinity. For dilute solutions, however, it is convenient to substitute the activity of the hydrogen ions with the molarity (mol/L) of the hydrogen ions (however, this is not necessarily accurate at higher concentrations[1][2]). In aqueous systems, the hydrogen ion activity is dictated by the dissociation constant of water (Kw = 1.011 × 10−14 M2 at 25 °C) and interactions with other ions in solution. Due to this dissociation constant a neutral solution (hydrogen ion activity equals hydroxide ion activity) has a pH of approximately 7. Aqueous solutions with pH values lower than 7 are considered acidic, while pH values higher than 7 are considered alkaline. The concept was introduced by S.P.L. Sørensen in 1909, and is purported to mean “pondus hydrogenii” in Dutch.[3] However, most other sources attribute the name to the French term pouvoir hydrogène.
the numerical number of pH is the negative log of the hydrogen ion concentration. It is an easier number to compare sample to sample than the actual hydrogen ion concentration. (Hey! That’s the idea behind all logarithmic functions… to make numbers more manageable.) The ability to give up a proton (hydrogen ion, H+) is how a Bronsted acid is defined. So pH is typically considered to measure acidity. On the pH scale, most common substances have pH values between 0 and 14. Neutral water, where the hydrogen ion concentration is balanced with hydroxide ion concentration, has a pH value of 7… so things with a pH of less than 7 are classified as “acidic” because they have a hydrogen ion concentration greater than neutral water. Things with a pH greater than 7 are considered basic, or alkaline, since they have a smaller hydrogen ion concentration that neutral water.
