pH Measurement Theory vol4

In the previous sections the practical aspects of pH measurements were discussed. This chapter will principally deal with the theoretical background to pH measurements and is intended for readers wishing to acquire
a more fundamental understanding of pH theory.

First the basic pH theory is developed, then we will have a look at the sensor theory and at the end some special topics will be dealt with.

4.1. Definitionof the pH value

According to Sørenson the pH is defined as the negative logarithm of the H₃O⁺ ion concentration:

pH = –log [H₃O⁺]

From the equation we can see that if the H₃O⁺ ion concentration changes by a decade, the pH value changes by one unit. This nicely illustrates how important it is to be able to measure even small changes in the pH value of a sample.
Often, the pH theory is described with H⁺ ions in connection with pH values, although the correct ion to refer to is the hydronium (or as it is officially known according to IUPAC: oxonium) ion (H₃O⁺):

H⁺ + H₂O ↔ H₃O⁺

Not only acids and bases show dissociation behavior to form hydronium ions or hydroxide ions, but pure water also dissociates to form hydronium and hydroxide ions:

2 H₂O ↔ H₃O⁺ + OH^–