pH Theory The term pH is derived from a combination of “p” for the word power and “H” for the symbol of the element Hydrogen. Together the meaning is the power of hydrogen. pH serves as a convenient way to compare the relative acidity or alkalinity of a solution at a given temperature. A pH of 7 describes a neutral solution because the activities of hydrogen and hydroxide ions are equal. When the pH is below 7, the solution is described as acidic because the activity of hydrogen ion is greater than that of hydroxide ion. A solution is more acidic as the hydrogen ion activity increases and the pH value decreases. Conversely, when the pH is above 7, the solution is described as basic (or alkaline) because the activity of hydroxide ion is greater than that of hydrogen ion.
Thermo Scientific (2013) pH Electrode Selection Handbook