The potential for a solution to donate or accept hydrogen ions determines whether it is an acid or a base. Acidic solutions donate protons, whereas bases or alkaline solutions can accept protons. Pure water has equal numbers of hydrogen ions to give protons and hydroxide ions to receive them, making it a neutral solution.

pH is a measure of the acidity or basicity of a water-based solution, determined by the concentration of hydrogen ions. In one liter of pure water, there are 1x10^-7 moles of hydrogen ions. However, the extensive range of hydrogen ion concentrations present in water-based solutions makes measuring pH in moles cumbersome. Thus, a pH scale was developed in which moles of hydrogen ions are converted using the negative of the base 10 logarithm. The pH of pure water, then, is 7, representing a neutral solution. Most solutions have a pH between 0 and 14, but some solutions, like carborane (with a pH of -18), exceed this. One liter of carborane has 1x10¹⁸ moles of hydrogen ions.

When free, unbound hydrogen ions accumulate, as with carborane, the solution is acidic, and the pH value falls below 7. Coffee, lemon juice, and gastric acid (digestive juices) are acidic solutions, with pHs around 4.5, 2.5, and 1.5, respectively.

Solutions with pH values above 7 have lower hydrogen ion concentrations and are alkaline. In these solutions, the hydrogen ions become bound to other components, like hydroxyl groups. This decreases the total number of free hydrogen ions and increases the pH value. Saltwater and soapy water are examples of alkaline solutions, with pHs of approximately 8 and 12, respectively.

Buffers prevent big changes in pH by absorbing or releasing hydrogen ions. Because most biological functions occur in near neutral pH conditions, approximately 7.35 to 7.45, buffers are critical. For example, blood becomes acidic with too much carbon dioxide. Bicarbonate acts as a buffer and brings the blood back to a homeostatic pH. Bicarbonate is also used to buffer chyme as it moves from the stomach to the small intestine. Without it, the acidity of the digested food would damage the lining of the intestine.