14.2 Arsenic Acid
In part as a consequence of the discovery of chlorine (Sect. 15.2) during his studies
of manganese, Scheele discovered another mineral acid, arsenic acid (H 3 AsO 4 ).
Salts of arsenic acid, arsenates, had been studied by Macquer, and arsenic acid had
been isolated by Cavendish in 1767, but his unpublished results were unknown
until 1840. It is also possible that Georg Brandt prepared the acid in 1728, but
without any clear idea of its nature.
Scheele described two methods [17] to prepare the new acid. The first method
involved dissolving arsenic (III) oxide (arsenic) in hot hydrochloric acid (Spiritus
salis) and the addition of nitric acid (Spiritus nitri) to the hot solution. In the second
method, chlorine was bubbled through a suspension of arsenic(III) oxide in water.
Upon heating, hydrochloric acid and arsenic(III) chloride (Butyrum arsenici) distilled over, leaving arsenic acid in the retort. Before proceeding with his experiments, Scheele performed a crude toxicological study:
Before I subjected this acid to my experiments, I was curious to know whether it is as fatal
as arsenic itself. I therefore mixed a little of it with honey, and exposed it to the flies, when I
found that it killed them in an hour. I gave 8 grs. [0.5 g] of the powdered acid, enclosed in a
piece of meat, to a cat, which, two hours afterwards, seemed to be at the point of death; I
then gave it some milk, upon which it vomited violently, and afterwards ran away. [18]
Scheele’s results were very important for several reasons. He had isolated
another new mineral acid, and as indicated earlier, there were only three mineral
acids generally recognised. The discovery of arsenic acid was easier for the scientific community to accept than the discovery of hydrofluoric acid. Arsenic acid
could be isolated in pure form, unlike hydrofluoric acid with its suspicious silica
content, and the solid arsenic acid could hardly be confused with any other known
acid. More importantly, Scheele now realised that, in modern terminology, arsenic
(III) oxide (arsenic) could not only be reduced to elemental arsenic (arsenic regulus)
but also oxidised. In Scheele’s phlogistic terminology, he concluded that arsenic
(i.e. arsenic(III) oxide) was not a true element, but it was actually composed of
arsenic acid and phlogiston.
Scheele also found that he could obtain two different sodium salts of arsenic acid
with different alkali content, i.e. NaH 2 AsO 4 and Na 2 HAsO 4 , and had thus discovered that arsenic acid is a polybasic acid. The same year Bergman noted in his
Attractionibus electivis [19] that tartaric and sulphuric acid behaved similarly. The
existence of a normal and acidic sulphate of potassium had been indicated in 1754
by Rouelle, [20] but the idea of polybasic acids is usually attributed to Scottish
chemist Thomas Graham (1805–1869) [21] and German chemist Justus Liebig
(1803–1873) [22].
In a letter to Hjelm, dated March 6, 1775, while he still lived in Uppsala, Scheele
wrote that he had given the arsenic manuscript to Bergman, who would read it and
forward it to the Academy in Stockholm. The printed version appeared in the
October to December issue of the Transactions for of 1775 [23].
206
14 New Mineral Acids
In part as a consequence of the discovery of chlorine (Sect. 15.2) during his studies
of manganese, Scheele discovered another mineral acid, arsenic acid (H 3 AsO 4 ).
Salts of arsenic acid, arsenates, had been studied by Macquer, and arsenic acid had
been isolated by Cavendish in 1767, but his unpublished results were unknown
until 1840. It is also possible that Georg Brandt prepared the acid in 1728, but
without any clear idea of its nature.
Scheele described two methods [17] to prepare the new acid. The first method
involved dissolving arsenic (III) oxide (arsenic) in hot hydrochloric acid (Spiritus
salis) and the addition of nitric acid (Spiritus nitri) to the hot solution. In the second
method, chlorine was bubbled through a suspension of arsenic(III) oxide in water.
Upon heating, hydrochloric acid and arsenic(III) chloride (Butyrum arsenici) distilled over, leaving arsenic acid in the retort. Before proceeding with his experiments, Scheele performed a crude toxicological study:
Before I subjected this acid to my experiments, I was curious to know whether it is as fatal
as arsenic itself. I therefore mixed a little of it with honey, and exposed it to the flies, when I
found that it killed them in an hour. I gave 8 grs. [0.5 g] of the powdered acid, enclosed in a
piece of meat, to a cat, which, two hours afterwards, seemed to be at the point of death; I
then gave it some milk, upon which it vomited violently, and afterwards ran away. [18]
Scheele’s results were very important for several reasons. He had isolated
another new mineral acid, and as indicated earlier, there were only three mineral
acids generally recognised. The discovery of arsenic acid was easier for the scientific community to accept than the discovery of hydrofluoric acid. Arsenic acid
could be isolated in pure form, unlike hydrofluoric acid with its suspicious silica
content, and the solid arsenic acid could hardly be confused with any other known
acid. More importantly, Scheele now realised that, in modern terminology, arsenic
(III) oxide (arsenic) could not only be reduced to elemental arsenic (arsenic regulus)
but also oxidised. In Scheele’s phlogistic terminology, he concluded that arsenic
(i.e. arsenic(III) oxide) was not a true element, but it was actually composed of
arsenic acid and phlogiston.
Scheele also found that he could obtain two different sodium salts of arsenic acid
with different alkali content, i.e. NaH 2 AsO 4 and Na 2 HAsO 4 , and had thus discovered that arsenic acid is a polybasic acid. The same year Bergman noted in his
Attractionibus electivis [19] that tartaric and sulphuric acid behaved similarly. The
existence of a normal and acidic sulphate of potassium had been indicated in 1754
by Rouelle, [20] but the idea of polybasic acids is usually attributed to Scottish
chemist Thomas Graham (1805–1869) [21] and German chemist Justus Liebig
(1803–1873) [22].
In a letter to Hjelm, dated March 6, 1775, while he still lived in Uppsala, Scheele
wrote that he had given the arsenic manuscript to Bergman, who would read it and
forward it to the Academy in Stockholm. The printed version appeared in the
October to December issue of the Transactions for of 1775 [23].
206
14 New Mineral Acids