First introduced by Beckman in 1936, pH meters have become a mainstay of academic, industrial, and manufacturing laboratories. pH meters determine the acidity or alkalinity of a solution by measuring the concentration of hydrogen ions; H+; or, more accurately, the hydronium ion; H3O+. pH is defined as the negative log of the hydronium ion concentration.
A pH meter is actually a voltmeter that uses a hydronium ionselective glass electrode to measure ion concentrations in the vicinity of the probe. Meters tend to be small, about the size of a shoe box. Most are benchtop units, but numerous vendors now sell battery-powered portable meters for field use.
pH meters are used in many industries: chemical, biological, environmental, forensics, consumer products, foods, and many others for which acidity measurements are warranted. A significant application is monitoring titration, a lab method that quantifies the concentration of an analyte in solution. Acid-base titrations measure concentrations of an acidic or alkaline substance. From the quantity of neutralizing species (acid or base) added at the point where pH is 7, or neutral, one can calculate the original concentration of the acid or base in the solution.
Titrations are also employed to adjust the pH of products or, in the case of biology, of buffers or standard solutions.
As George Porter, titration product manager at Metrohm (Riverview, FL) notes, most pH meters may be used as solution voltmeters, which greatly expands their utility beyond acidity and alkalinity. Meters can perform salt concentration measurements by noting the dramatic drop in electrical potential when, say, a halide is precipitated with silver ion. “pH is a derived value calculated from a voltage. With salt concentration measurements you’re titrating to a set millivolt point,” Mr. Porter observes.