Читать книгу pH of the Skin: Issues and Challenges - Группа авторов - Страница 13

Today, the terms acids/acid and bases/alkaline are ordinary components of our colloquial language and are often used in technical, biochemical, and medical contexts. To most of us, the terms are more common in connection with nutrition. Everyone has a concept of acid (vinegar, lactic acid in sour-milk, citrus fruits, etc.) and alkaline (alkaline foods [“deacidification for health”], spring water, etc.) and the wellness and spa industry propagate and offer both acid and alkaline foods and balneotherapies.

Originally, the concepts of acidity came from the ancient Greeks who defined “sour-tasting” substances as oxein, which metamorphosed into the Latin word for vinegar acetum, which became anglicized to acid. Acid substances were eventually found not only to taste sour but also to change the color of litmus paper and corrode metals. In contrast, bases were typically defined and studied by their ability to counteract acids – and thus followed behind the acids in their chemical characterization. Their more rigid terminology (alkaline) is derived from an Arabic root word associated with “roasting” because of the fact that the first bases were characterized from the soap-making substances obtained from roasting ashes and treating them with water and slaked lime. In the early 14th century, the Spanish scholar, Arnaldus de Villa Nova (1235–1311), began to use litmus for studying acids and bases. This compound, extracted from a lichen, had been used as a dye since at least the time of the Vikings, but he was the first person known to use it as a test of acidity. This idea was expanded by Robert Boyle (1627–1691), who found that certain plant-derived substances changed color in the presence of acid or basic substances. One example was a liquid fraction of violets, which is blue in a pH neutral environment but turns green when exposed to bases and red when mixed with acid [2, 3].

The first scientific concept of acids and bases – originally based on mass analysis or titrimetrics – was proposed by Antoine Lavoisier (1743–1794) in the 1770s. His knowledge of strong acids was mainly restricted to oxoacids such as nitric acid and sulfuric acid. He defined acids in terms of their containing oxygen, which in fact he named from Greek words meaning “acid-former” (from the Greek οξυς (oxys) meaning “acid” or “sharp” and γεινομαι (geinomai) meaning “engender”). Other theories followed, for example, Liebig’s hydrogen theory of acids. This definition was based on the chemical composition of organic acids, finishing the doctrinal shift from oxygen-based acids to hydrogen-based acids. Justus von Liebig’s (1803–1873) definition, while completely empirical, remained in use for almost 50 years until the adoption of the Arrhenius definition. The first modern definition of acids and bases in molecular terms was devised by Svante Arrhenius (1859–1927). The hydrogen theory of acids led to Arrhenius receiving the Nobel Prize in Chemistry in 1903. An Arrhenius acid is a substance that dissociates in water to form hydrogen ions (H⁺); that is, an acid increases the concentration of H⁺ ions in an aqueous solution. This causes the protonation of water or the creation of the hydronium (H₃O⁺) ion. Thus, in modern times, the symbol H⁺ is interpreted as a shorthand for H₃O⁺ because it is now known that a bare proton does not exist as a free species in aqueous solution. An Arrhenius base is a substance that dissociates in water to form hydroxide (OH⁻) ions; that is, a base increases the concentration of OH⁻ ions in an aqueous solution.

Later, the Danish chemist Johannes Brønsted (1879–1947) and the English chemist Thomas Lowry (1875–1936) independently proposed a modification of the definitions, retaining the connection of the proton release to acids but defining bases more broadly as any substance capable of binding protons. Thus, measurements of the hydrogen ion became key to defining the level of acidity, and bases truly were relegated to the mirror-companion of acids as simply receptors. The above definitions, relying on protons and hydroxyl ions as it did, generally related to reactions in water solution. Gilbert Lewis (1875–1946) refined the acid and base concept to include dissolution events in nonaqueous solvents, where free protons are not involved. Developing along with these theoretical considerations, the practical measurement of acidity became the foundation of analytical chemistry and ultimately of the analytical instrument business. The concept of pH was perhaps the most important in facilitating this evolution. It was the work of Hermann Walther Nernst (1864–1941) that, in 1889, gave the theoretical foundation for the use of electrode potential to measure the concentration of an ion in solution. With Arrhenius’s definition of acidity as the result of hydrogen ion concentration, a scale of acidity based on the results of electrode potential was created. In 1909, Søren Peter Lauritz Sørensen (1868–1939) developed a new colorimetric assay for acidity [4]. But more importantly, he defined the concept of expressing acidity as the negative logarithm of the hydrogen ion concentration, which he termed pH. And he was one of the first to attempt the use of electrostatic methods to discern pH. The usefulness of his pH concept was first recognized by Leonor Michaelis (1875–1949). He published a monograph – “Die Wasserstoffionenkonzetration” – that helped to convince biochemists and later chemists and others of the critical importance of pH to analytical research [5]. From then on, measurement of pH, especially in the life sciences, was considered of increasingly greater importance. The modern measurement of pH was revolutionized by 2 major innovations. The first was the development of a superior glass electrode capable of responding to hydrogen ions. The second occurred when Arnold Beckman (1900–2004) invented a sensitive and sturdy acidometer that used a pair of vacuum-tube signal amplifiers and glass electrodes to monitor fruit acidity during the production of pectin and citric acid. The original Beckman acidometer became the inspiration for many improvements and adaptations in what would universally be known as the pH meter, giving profound impetus to the rise of the international equipment business. For example, the Swiss engineer Bertold Suhner (1910–1988) developed one of the first European pH meters for Metrohm, only a few years after Beckman introduced his device [6–9].