pH is a measure of the hydrogen ion concentration (acidity and alkalinity) of an aqueous solution expressed by a number on a 0–14 scale, for which the value of 7 represents neutrality (pure water registers as 7); lower numbers indicate increasing acidity and higher numbers increasing alkalinity. That neutral 7 can be described as a solution in which hydrogen ions (H+) and hydroxide ions (OH-) are in equal proportion. As the H+ proportion increases (and pH decreases), the solution becomes more acidic. Humans register that decreasing pH as sourness.
In the pH scale, each unit of change represents a tenfold change in acidity or alkalinity, so that the difference between a pH of 6.9 and 7 represents a one-point difference (not one-tenth of a point), measured in hydrogen ions by moles per liter. Thus a beverage that has a pH of 4 can be seen as crisp or tangy while a beverage with a pH of 3 can be frighteningly tart for many people. Solutions with very low pH can damage human tissue.
Pure ethanol is slightly alkaline, with a pH of 7.33. While commercial spirits vary in their pH level, in general a well-made, neutral spirit will have a pH in the 6 to 7 range, although some vodkas register as alkaline due to the water used to reduce them in proof (indeed, this can be seen as one of the hallmarks of the eastern European style of vodka) and others as acidic due to added citric acid (used to mask flaws in distillation). A barrel aged spirit will generally fall somewhere around 4.5, with anything less than 3.5 indicating serious errors in the distillation.
“pH Scale.” American Chemical Society. https://www.acs.org/content/acs/en/education/resources/undergraduate/chemistryincontext/interactives/water-everywhere/ph-scale.html.Valaer, Peter. “Brandy.” Industrial and Engineering Chemistry 31 (1939): 339–353.
By: Doug Frost