14
New Mineral Acids
Three mineral acids were known to the chemists in the seventeenth century: sulphuric
acid, nitric acid and hydrochloric acid. In addition, phosphoric acid was known, but as
the only known source of this acid at the time was urine, it was not recognised as a
mineral acid. Scheele made important and controversial discoveries that forced
chemists to give up the idea of three mineral acids.
14.1 Hydrofluoric Acid
Scheele’s first paper [1] was an investigation of the mineral fluorite (fluorspar;
CaF 2 ; Fig. 14.1) [2]. It may have been this mineral’s property to glow in the dark
that first attracted Scheele’s attention, but Bergman has written in a letter to
Macquer that the investigation was initiated on his suggestion [3]. The important
discovery in this study occurred when he distilled a mixture of fluorite and sulphuric
acid in a retort, with the receiver filled with water (Fig. 14.2). Scheele observed that
the water turned into a fuming acid and that a white crust formed at the water
surface. The acid showed similarities with hydrochloric acid, but had some distinct
properties showing that this was a hitherto unknown acid. On heating fluorite and
sulphuric acid, hydrogen fluoride (HF) is formed. Hydrogen fluoride is one of only
a few substances that rapidly attack glass at room temperature, giving silicon tetrafluoride (SF 4 ) and hexafluorosilicic acid (H 2 SiF 6 ). Upon contact with the water in
the receiver, silicon tetrafluoride is hydrolysed to silisic acid, Si(OH) 4 , which
undergoes condensation to hydrated silicon dioxide species which formed the white
crust that Scheele observed. Scheele’s new acid was thus a solution of hydrofluoric
acid (HF) and hexafluorosilicic acid. Unknown to Scheele, an artist from Nürnberg
named Schwanhardt had found a mixture of fluorite and sulphuric acid useful for
etching glass, and a similar recipe replacing sulphuric acid with nitric acid was
introduced by Matthäus Pauli in 1725 [4]. Apparently unknown to Scheele,
Marggraf had studied fluorite in the late 1760s, and also found that distillation of
© Springer Nature Switzerland AG 2020
A. Lennartson, Carl Wilhelm Scheele and Torbern Bergman, Perspectives on the
History of Chemistry, https://doi.org/10.1007/978-3-030-49194-9_14
203
New Mineral Acids
Three mineral acids were known to the chemists in the seventeenth century: sulphuric
acid, nitric acid and hydrochloric acid. In addition, phosphoric acid was known, but as
the only known source of this acid at the time was urine, it was not recognised as a
mineral acid. Scheele made important and controversial discoveries that forced
chemists to give up the idea of three mineral acids.
14.1 Hydrofluoric Acid
Scheele’s first paper [1] was an investigation of the mineral fluorite (fluorspar;
CaF 2 ; Fig. 14.1) [2]. It may have been this mineral’s property to glow in the dark
that first attracted Scheele’s attention, but Bergman has written in a letter to
Macquer that the investigation was initiated on his suggestion [3]. The important
discovery in this study occurred when he distilled a mixture of fluorite and sulphuric
acid in a retort, with the receiver filled with water (Fig. 14.2). Scheele observed that
the water turned into a fuming acid and that a white crust formed at the water
surface. The acid showed similarities with hydrochloric acid, but had some distinct
properties showing that this was a hitherto unknown acid. On heating fluorite and
sulphuric acid, hydrogen fluoride (HF) is formed. Hydrogen fluoride is one of only
a few substances that rapidly attack glass at room temperature, giving silicon tetrafluoride (SF 4 ) and hexafluorosilicic acid (H 2 SiF 6 ). Upon contact with the water in
the receiver, silicon tetrafluoride is hydrolysed to silisic acid, Si(OH) 4 , which
undergoes condensation to hydrated silicon dioxide species which formed the white
crust that Scheele observed. Scheele’s new acid was thus a solution of hydrofluoric
acid (HF) and hexafluorosilicic acid. Unknown to Scheele, an artist from Nürnberg
named Schwanhardt had found a mixture of fluorite and sulphuric acid useful for
etching glass, and a similar recipe replacing sulphuric acid with nitric acid was
introduced by Matthäus Pauli in 1725 [4]. Apparently unknown to Scheele,
Marggraf had studied fluorite in the late 1760s, and also found that distillation of
© Springer Nature Switzerland AG 2020
A. Lennartson, Carl Wilhelm Scheele and Torbern Bergman, Perspectives on the
History of Chemistry, https://doi.org/10.1007/978-3-030-49194-9_14
203