The term pH was originally defined in 1909 by Søren Peter Lauritz Sørensen, a Danish biochemist. Literally meaning “potential for hydrogen,” pH is used to indicate the concentration of hydrogen ions in a fluid. A hydrogen ion is a simple positively charged particle, a proton symbolized as H+.
The number of free hydrogen ions in a solution determines the degree of its acidity and the more hydrogen ions there are present, the more acidic the solution will be.
By showing the concentration of hydrogen ions, pH indicates whether a fluid or compound is acidic, alkaline, or neutral.
All acids contain hydrogen ions in varying concentrations, with stronger acids having a greater concentration of hydrogen ions than weaker acids.
The hydrogen ion is unstable and very reactive. Specifically, it seeks to react with a negatively charged molecule. So hydrogen ions can be thought of as “hiding” in the body ready to react with other molecules. These hydrogen ions, once they become attached to various proteins, alter a protein’s structure, or “denature” it. This is precisely how hydrochloric acid in the stomach aids in protein digestion.
Because an acid gives off hydrogen ions while a base accepts hydrogen ions, acids and bases can react with one another, altering the acid-base balance of a substance. In the body, the bicarbonate ion is a major base that transforms and neutralizes acids by taking on their hydrogen ions. Largely, it is the balance between bicarbonate and hydrogen ions that determines acid-base balance.
The pH scale
It is worth noting that pH values are determined by the concentration of hydrogen ions represented as moles per liter. (The term mole is short for molecular weight.)
Pure water, which is a neutral element, has a concentration of hydrogen ions that equals 0.0000001, or 10–7 moles per liter. By comparison, extremely acidic substances can have hydrogen ion concentrations as high as 0.01, or 10–2 moles per liter.
As these examples make clear, the concentration of hydrogen ions in a substance is
written as a power of 10. To indicate the substance’s pH value, we remove the base
number 10 and the minus sign.
Thus, a pH of 7 is the value of pure (neutral) water while a pH of 2 indicates very high
acidity, and a pH of 12 indicates very high alkalinity.
An increase of one point of pH is equal to a ten-fold decrease in hydrogen ion
concentration. Equally a decrease of one point of pH equals a ten-fold increase in
hydrogen ion concentration. If we understand this we realize that small changes on the pH scale represent a big change in acid concentration.