pH

pH measures a solution's acidity or basicity based on hydronium ion concentration. The pH value is calculated as the negative logarithm of hydronium ion concentration. At neutral pH 7, pure water contains equal concentrations of hydronium and hydroxide ions produced through water autoionization via proton transfer between water molecules.

The pH scale uses logarithmic units to simplify measuring hydronium ion concentrations across a wide range. A solution at pH 4 contains ten times more hydronium ions than pH 5, which has ten times more than pH 6. This logarithmic relationship allows the extensive range of hydronium ion concentrations in water-based solutions to be expressed as simple whole numbers from 0 to 14.

Solutions with pH below 7 are acidic due to high hydronium ion concentrations, like coffee at pH 4.5 and lemon juice at pH 2.5. Solutions above pH 7 are basic or alkaline with lower hydronium ion concentrations, such as saltwater at pH 8 and soapy water at pH 12. The higher the hydronium concentration, the lower the pH value.

Buffers prevent significant pH changes in solutions and are critical because most biological functions occur near neutral pH, approximately 7.35 to 7.45. Blood maintains a pH of about 7.4 through a buffer system involving carbonic acid, bicarbonate ions, and carbon dioxide. Bicarbonate also buffers chyme moving from the stomach to the small intestine, protecting the intestinal lining from acid damage.

Hydronium ions form when water molecules gain an extra proton through chemical reactions and balanced chemical equations describing water autoionization. Pure water at 25°C contains 1×10−7 moles of hydronium ions per liter, corresponding to neutral pH 7. As hydronium concentration increases, pH decreases proportionally on the logarithmic scale.

Most biological functions require near-neutral pH conditions between 7.35 and 7.45. Blood maintains an average pH of 7.4 through buffer systems. Gastric acid in the stomach is highly acidic at pH 1.5, while the small intestine requires buffering to protect its lining from acidic chyme entering from the stomach.

While most solutions have pH between 0 and 14, some extreme solutions exceed this range. Carborane, an extremely acidic compound, has a pH of −18 with 1×10^18 moles of hydrogen ions per liter. These exceptional cases demonstrate that the standard pH scale represents typical aqueous solutions but does not limit all possible pH values.