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.
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
7.0: pure water (neutral)
7.4: human blood
14.0: sodium hydroxide
The pH scale ranges from 0 to 14, with 7 being neutral. pHs less than 7 are acidic while pHs greater than 7 are alkaline (basic).
pH and Water
No, you don’t often hear your local news broadcaster say “Folks, today’s pH value of Dryville Creek is 6.3!” But pH is quite an important measurement of water. Maybe for a science project in school you took the pH of water samples in a chemistry class … and here at the U.S. Geological Survey we take a pH measurement whenever water is studied. Not only does the pH of a stream affect organisms living in the water, a changing pH in a stream can be an indicator of increasing pollution or some other environmental factor.
pH in soil
Classification of soil pH ranges
The United States Department of Agriculture Natural Resources Conservation Service, formerly Soil Conservation Service classifies soil pH ranges as follows:
Denomination pH range
Ultra acidic < 3.5
Extremely acidic 3.5–4.4
Very strongly acidic 4.5–5.0
Strongly acidic 5.1–5.5
Moderately acidic 5.6–6.0
Slightly acidic 6.1–6.5
Slightly alkaline 7.4–7.8
Moderately alkaline 7.9–8.4
Strongly alkaline 8.5–9.0
Very strongly alkaline > 9.0
pH in nature
Lemon juice tastes sour because it contains 5% to 6% citric acid and has a pH of 2.2. (high acidity)
pH-dependent plant pigments that can be used as pH indicators occur in many plants, including hibiscus, red cabbage (anthocyanin) and red wine. The juice of citrus fruits is acidic mainly because it contains citric acid. Other carboxylic acids occur in many living systems. For example, lactic acid is produced by muscle activity. The state of protonation of phosphate derivatives, such as ATP, is pH-dependent. The functioning of the oxygen-transport enzyme hemoglobin is affected by pH in a process known as the Root effect.
pH in living systems Compartment pH
Gastric acid 1.5-3.5
Human skin 4.7
Granules of chromaffin cells 5.5
Blood (natural pH) 7.34–7.45
Cerebrospinal fluid (CSF) 7.5
Mitochondrial matrix 7.5
Pancreas secretions 8.1
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.