A brief history of acidity - reviewed by Brian Og Mc Alary Y13 Class of 2003/4

A review of
‘A brief history of acidity’
from ‘Education in Chemistry’, volume 41, number 1 January 2004
by Brian Og McAlary (Y13 Class of 2003/4)

Many scientists have tried to understand ‘acidity’ from Glauber in 1648 to Lowry in 1927, with many showing the wrong understanding, though their work helped other scientists to eventually come to terms with it. (Link)

Tasting of chemicals was in the early days the only way to recognise a chemical, and the name acid came from the Latin word acetum, meaning sour after it had a sour taste when tested. These were found to turn plant dye pigments red form blue and be able to precipitate sulphur from alkaline solutions.

In 1648 Rudolph Glauber thought that the reacting of an acid and an alkali produced a salt, and that a salt could then be broke up to an acid and an alkali. Robert Boyle disagreed saying that acids were sour due to sharp angular particles within the substance, and when they formed salts; the particles became embedded in soft particles of alkali.

In 1977, Lavoisier said that an acid should be defined as a compound of oxygen with a non-metal, as oxygen was the component that turned the non-metal into an acid. Humphrey Davy disagreed with this proving that muriatic acid contained no oxygen, and that acidity wasn’t down to the elements, but how they were arranged. He said, “organic acids are compounds containing hydrogen in which the hydrogen can be replaced by a metal.”

In 1884, Arrhenius believed that some substances became ionised in solution but this was dismissed. He believed that substances dissociated in water into hydrogen ions and cations- this bringing up the theory of bases-which was a substance which dissociated in water into hydroxyl ions and cations. The hydroxyl ions, and hydrogen ions joined to form water and the remaining ions formed the salt. In 1903, chemists started to understand Arrhenius and he was given a Nobel prize for the work, though he couldn’t recognise the basicity of insoluble substances, or of oxides of silicates-the Bronted-Lowry theory was later formed form this work.

In 1923 Bronsted believed that acid-base reactions involved the giving of protons from the acid to the base, which could take into account insoluble bases. Lowry didn’t recognise NH₄⁺as an acid or CH₃CO₂^-as a base (or such like these) existing. He was able to notice that it wasn’t just H⁺ present in solution but that H₃O⁺ existed.

Since acidity was understood, a scale known as the pH scale was formed for measuring it. This was discovered by a chemist called Sorensen in 1909, when he hydrogen ion concentration. He formed the scale to see to what extent does an acid become dissociated and he used this idea to 0.1 normal HCl, which didn’t dissociate fully. He noted that in a solution whose normality based on hydrogen ions equals 0.01, ‘pH’ can be written briefly as 10^-2. He was able to recognise that the pH of water was approximately 7.

This used the hydronium ion that Lowry thought off than the hydrogen ion. The activity of the hydronium ion remains useful as the bases for a modern understanding of the concept of pH.

Peoples understanding of acidity now, is much greater than that back in the 17^th Century were all they could see is acidity being applied to aqueous solutions.