pH: definition in chemistry
pH: A scale that measures how acidic or basic a water-based solution is, based on its concentration of hydrogen ions. At 25 °C, pH 7 is neutral, values below 7 are acidic and values above 7 are basic.
pH is defined as the negative logarithm of the hydrogen ion concentration, pH = −log[H⁺], so each step of one unit is a tenfold change. A solution of pH 3 therefore has ten times more H⁺ than one of pH 4 and a thousand times more than one of pH 6. pH is measured with indicators, test strips or a pH meter, and most everyday solutions fall between 0 and 14, although very concentrated acids and bases can go beyond that range.
Examples
- Lemon juice — about pH 2
- Pure water at 25 °C — pH 7
- Human blood — pH 7.35 to 7.45
- Household ammonia cleaner — about pH 11
On the periodic table
Metal oxides from the left of the table, such as Na₂O and CaO, give solutions with a high pH in water, while nonmetal oxides from the right, such as SO₂ and CO₂, give a low pH.