pH Meter - History and Operation

How pH meters help us observe the world

The advancement of science is strongly dependent on our ability to observe the world. In the early development of chemistry, there were many observations that were made that we wouldn't consider making in the laboratory today. For example, as scientists were trying to understand the nature of an acid, they found the organic acids they had discovered tasted sour. We now know many more things, not the least of which is that we shouldn't taste chemicals in the lab. Still, the idea that an observation of an acid based on taste would certainly not help measure how much acidity is present. This was also recognized by early chemists, so they found ways to measure acidity, at least for some acids, mostly by carrying out titrations.

The move from titration to instruments that could measure pH based on electrochemistry was motivated by practical needs. Citric acid in fruits like oranges and lemons plays an important role in their flavor, and being able to measure the pH of a citris based food was important. A chemist named Arnold Beckman was given a challenge to measure acidity in citrus, and he built some of the earliest pH meters in the 1930's. A site describing a national chemical landmark about his work has a lot of the details of his invention: click here.

Aside from the ingenuity of making an invention to measure a chemical quantity that was not routinely measured before, the truly amazing thing about pH meters was the way they allowed scientists to interogate nature with much greater precision. pH measures the amount of hydrogen ion (H+) in solution. The amount is often quite small. Even an acidic lemon juice solution only has an H+ concentration of a 0.01 M or less. Neutral water has an H+ concentration of 0.0000001 M and solutions that are basic have even smaller values of hydrogen ion concentration. So, for a chemical that once was measured as present or not-present based on experiencing a sour taste, chemists can now get highly precise numerical values for the amount of a specific chemical (H+) present to indicate how acidic (or basic) a solution is. When this measurement capability is combined with a theoretical knowledge of the chemical equilibrium between acids and bases, the way we are able to look at nature is very different.

The pH meter changed the way we can investigate nature by making it possible to have accuate, quantitative measures of concentrations of important chemicals. It is the instrument that makes it possible to investigate the acid/based behavior of almost any water solution. Living on a water planet, this ability is very important.