The pH scale quantifies the acidity or alkalinity of an aqueous solution. This concept is used universally across science, industry, and daily life to monitor everything from water quality to the human body’s blood chemistry. Danish chemist Søren Peder Lauritz Sørensen developed this system, providing a simple, standardized way to express a complex chemical property.
The Inventor and the Context of Discovery
Sørensen was the head of the chemistry department at the Carlsberg Laboratory in Copenhagen, Denmark. Working for the brewery, he was tasked with solving problems related to industrial processes, specifically the quality and consistency of beer production. The primary motivation for his work was the need for precise control over the acidity of fermentation batches.
This control was necessary because the enzymes responsible for the brewing process are extremely sensitive to their environment. Even slight fluctuations in acidity could destabilize these proteins, leading to inconsistent or spoiled products. Sørensen introduced the concept of pH in 1909 as a convenient way to express hydrogen ion concentration. This standardized method was a vast improvement over the less precise color-change tests previously used.
The Conceptual Basis of the Scale
The pH scale measures the concentration of hydrogen ions (H+) in a solution, with the letters standing for “power of hydrogen.” pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration. This logarithmic relationship allows the representation of a vast range of concentrations on a manageable scale, typically 0 to 14.
Because the scale is logarithmic, a change of one pH unit represents a tenfold change in the concentration of hydrogen ions. For example, a solution with a pH of 5 is ten times more acidic than a solution with a pH of 6. The neutral point on the scale is pH 7, which is the value for pure water at 25°C. Solutions falling below 7 are considered acidic, while solutions above 7 are alkaline or basic.
Evolution and Modern Measurement
Sørensen’s initial method for determining pH involved colorimetric techniques, which used chemical indicators that changed color depending on the solution’s acidity. While this was an improvement, it was often subjective and limited in accuracy. A more precise, electrical method involved measuring the voltage generated by a hydrogen electrode, a technique Sørensen himself described.
The practical measurement of pH was revolutionized by the development of the glass electrode. This device uses a thin glass membrane that selectively responds to hydrogen ions, generating a measurable electrical potential proportional to the pH difference between the internal and external solutions. American chemist Arnold Beckman developed the first commercially viable pH meter in 1936. This invention transformed pH measurement from a complex laboratory procedure into a routine, instantaneous test used across diverse fields such as environmental monitoring, medical diagnostics, and food processing.