pH refers to the logarithmic scale used to indicate the acidity or basicity of an aqueous solution. It is the negative of the base 10 logarithm of the activity of the hydrogen ion. Solutions with a pH less that 7 are considered acidic and the solutions with a pH greater that 7 are considered to be basic. A pH of 7 is neither an acid nor a base and is considered neutral.
An acid has a high concentration of hydrogen ions (H+) and a base has a low concentration of hydrogen ions. Higher the hydrogen ion concentration, pH is acidic. Among the given substances, lemon juice has a pH of 2, urine has a pH of 6, tomato juice has a pH of 4 and black coffee has a pH of 5. Since lemon juice has a pH of 2, it has the lowest –OH ion concentration.
