Overview

pH is a numeric scale used to indicate the acidity or basicity (alkalinity) of an aqueous solution. On the conventional scale, values lower than 7 are acidic, values higher than 7 are basic, and a value of 7 is neutral at standard conditions. The term describes both a practical scale and a precise chemical quantity; see the pH scale for common ranges and the word alkaline for synonyms and related concepts.

Definition and primary formulas

In chemistry, pH is most often given by the familiar equation pH = −log10[H+], where [H+] denotes the molar concentration of hydrogen ions (often written as [H3O+] for hydronium). This practical form is widely taught and used; a compact representation of that formula is commonly shown in textbooks and diagrams. Standard formula depictions frequently accompany introductory discussions.

For rigorous work the formal definition uses activity rather than concentration: pH = −log10(a_H+). Activity accounts for non-ideal behavior in real solutions, especially at high ionic strength. The difference between concentration and activity is important in accurate measurements and in some industrial or environmental analyses.

What the components mean

The bracket notation [H+] refers to the amount of hydrogen ions per volume, typically measured in moles per litre (molarity). Hydrogen ions are protons released in acid–base reactions; more formally they exist as hydronium ions in water. These charged species are sometimes simply called protons, and the H stands for the hydrogen ion itself. The etymology traces to S.P.L. Sørensen, who introduced the notation in 1909; see the entry on Sørensen and the year 1909. The p in pH comes from the German word potenz, meaning power or exponent, while the H represents hydrogen ion.

Measurement methods

  • pH meters and electrodes: Electronic meters with glass electrodes give direct readings of potential that are converted to pH; they are standard in laboratories and field work.
  • Indicators: Chemical dyes or paper strips change color over particular pH ranges and are useful for quick or approximate measurements.
  • Titration: Analyzing how much strong acid or base is required to change the pH of a sample provides precise acidity or alkalinity values for solutions and solids.

Examples, ranges and extremes

Most everyday aqueous substances fall between about 0 and 14 on the conventional scale: for example, stomach acid is typically very acidic (close to 1–2), household vinegar around pH 2–3, neutral pure water near 7, blood tightly buffered near pH 7.4, seawater slightly basic around pH 8, and many cleaning agents such as dilute bleach have pH values above 11. Under concentrated or unusual conditions, solutions can have pH values below 0 or above 14; these lie outside the simple 0–14 picture but are consistent with the logarithmic definition.

Uses, importance and buffering

pH influences chemical reaction rates, solubility, biological activity and corrosion. In biology, enzyme activity and cellular processes depend on narrow pH ranges; in environmental science, pH of soil and water affects nutrient availability and ecosystem health. Many systems use buffers—mixtures of weak acids and bases—to resist pH change. The Henderson–Hasselbalch relationship links buffer component concentrations to pH and is a practical tool for designing and understanding buffers used in biochemical and industrial contexts.

Alkaline solutions typically have excess hydroxide ions (OH−) rather than hydrogen ions; see the entry on hydroxide. pH is related to but distinct from acidity measures such as total acid or basicity, and from redox potential which describes electron-transfer tendencies. Many practical fields—medicine, agriculture, water treatment, chemical manufacturing—rely on accurate pH control. For further technical details and reference data, consult specialized resources and standards such as those that discuss measurement technique and calibration.

Terms and further reading: substances with extreme pH, general notes on alkalis, and the original historical references to Sørensen and early pH work are available in chemistry texts and online scientific collections.