# Hydronium and the Determination of pH

The hydronium cation, also known as hydroxonium is the positively charged polyatomic ion with the chemical formula H3O+. Hydronium, a type of oxonium ion, is formed by the protonation of water (H2O). This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated (bound to a solvent). The reality is far more complicated, and a proton is bound to several molecules of water, such that other descriptions such as H5O2+, H7O3+ and H9O4+ are increasingly accurate descriptions of the environment of a proton in water. The ion H3O+ has been detected in the gas phase.

Determination of pH

It is the presence of hydronium ions relative to hydroxide that determines a solution’s pH. Water molecules auto-dissociate into hydronium and hydroxide ions in the following equilibrium:

2 H2O = OH− + H3O+

In pure water, there is an equal number of hydroxide and hydronium ions. At 25 °C and atmospheric pressure their concentrations are approximately equal to 1.0×10−7 mol∙dm−3 (or mol/L). For these conditions, [H3O+] = 10−pH so pH = 7 is defined as neutral. A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution.

The concept of p[H] was first introduced by Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909 and revised to the modern pH in 1924 to accommodate definitions and measurements in terms of electrochemical cells. In the first papers, the notation had the “H” as a subscript to the lowercase “p”, as so: pH.

The exact meaning of the “p” in “pH” is disputed, but according to the Carlsberg Foundation pH stands for “power of hydrogen”. It has also been suggested that the “p” stands for the German Potenz (meaning “power”), others refer to French puissance (also meaning “power”, based on the fact that the Carlsberg Laboratory was French-speaking). Another suggestion is that the “p” stands for th eLatin terms pondus hydrogenii, potentia hydrogenii, or potential hydrogen. It is also suggested that Sørensen used the letters “p” and “q” (commonly paired letters in mathematics) simply to label the test solution (p) and the reference solution (q). Current usage in chemistry is that p stands for “decimal cologarithm of”, as also in the term pKa, used for acid dissociation constants.

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