Asked in Units of MeasureChemistryAcids and BasespH Levels
What is pH and what does it measure?
February 10, 2013 10:28PM
The pH is a measure of how acidic or basic (alkaline) an aqueous solution is.
Pure water is neutral, with a pH = 7. Things with a pH <7 are acidic, and things with a pH >7 are basic.
The theoretical pH of an aqueous solution is defined this way:
pH = -log10 [H3O]
The pH approximates the negative logarithm, base 10, of the Hydronium ion (H3O+) molar concentration of an aqueous solution. It's an indicator of the relative acidity or basicity (alkalinity) of the solution.
What makes a solution acidic is the amount of H3O+ in it. A highly acidic solution will have a high concentration of H3O+, and so it will have a low pH. A basic solution has a very low concentration of H3O+, and so it has a high pH. Note that H3O+ and H+ really refer to the same thing, but H+ doesn't actually exist by itself in water: it immediately combines with an H2O molecule to form H3O+ instead.
Substances with very high, or very low, pH are dangerous to human hair, eyes, and skin.
Examples: pH of common substances are:
Stomach acid -- 1
Lemon juice -- 2.2
Pure water -- 7
Milk of magnesia -- 10.5