pH Definition and Equation in Chemistry

pH levels below 7 are acidic, while levels above 7 are alkaline

litmus paper next to pH measurements
pH is a measure of hydrogen ion concentration in a water-based solution. A pH of 3.5 for an orange indicates the fruit is acidic. marcu alina/Getty Images

pH is a measure of hydrogen ion concentration, a measure of the acidity or alkalinity of a solution. The pH scale usually ranges from 0 to 14. Aqueous solutions at 25°C with a pH less than 7 are acidic, while those with a pH greater than 7 are basic or alkaline. A pH level of 7.0 at 25°C is defined as "neutral" because the concentration of H3O+ equals the concentration of OH in pure water. Very strong acids might have a negative pH, while very strong bases might have a pH greater than 14.

pH Equation

The equation for calculating pH was proposed in 1909 by Danish biochemist Søren Peter Lauritz Sørensen:

pH = -log[H+]

where log is the base-10 logarithm and [H+] stands for the hydrogen ion concentration in units of moles per liter solution. The term "pH" comes from the German word "potenz," which means "power," combined with H, the element symbol for hydrogen, so pH is an abbreviation for "power of hydrogen."

Examples of pH Values of Common Chemicals

We work with many acids (low pH) and bases (high pH) every day. Examples of pH values of lab chemicals and household products include:

0: hydrochloric acid
2.0: lemon juice
2.2: vinegar
4.0: wine
7.0: pure water (neutral)
7.4: human blood
13.0: lye
14.0: sodium hydroxide

Not All Liquids Have a pH Value

pH only has meaning in an aqueous solution (in water). Many chemicals, including liquids, do not have pH values. If there's no water, there's no pH.

 For example, there is no pH value for vegetable oil, gasoline, or pure alcohol.

IUPAC Definition of pH

The International Union of Pure and Applied Chemistry (IUPAC) has a slightly different pH scale that is based on electrochemical measurements of a standard buffer solution. Essentially, the definition uses the equation:

pH = -log aH+

where aH+ stands for hydrogen activity, which is the effective concentration of hydrogen ions in a solution. This might be slightly different from the true concentration. The IUPAC pH scale also includes thermodynamic factors, which may influence pH.

For most situations, the standard pH definition is sufficient.

How pH Is Measured

Rough pH measurements can be made using litmus paper or another type of pH paper known to change colors around a certain pH value. Most indicators and pH papers are useful only to tell whether a substance is an acid or a base or to identify pH within a narrow range. A universal indicator is a mixture of indicator solutions intended to provide a color change over a pH range of 2 to 10.

More accurate measurements are made using primary standards to calibrate a glass electrode and pH meter. The electrode works by measuring the potential difference between a hydrogen electrode and a standard electrode. An example of a standard electrode is silver chloride.

Uses of pH

pH is used in everyday life as well as science and industry. It's used in cooking (e.g., reacting baking powder and an acid to make baked goods rise), to design cocktails, in cleaners, and in food preservation.

It's important in pool maintenance and water purification, agriculture, medicine, chemistry, engineering, oceanography, biology, and other sciences.