While working in the pharmacy, Scheele sometimes prepared a medicine called
Crocus Martis Stahlii aperitivi by melting a mixture of iron flings, antimony(III)
sulphide and potassium nitrate (saltpetre). He also prepared another medicine called
Antimonium diaphoreticum, which was prepared by igniting a mixture of one part
antimony(III) sulphide and three parts potassium nitrate, after which the residue was
washed with water [7]. Per standard methods, the water used for the washing the
Crocus Martis or Antimonium diaphoreticum (in both cases containing, e.g.
potassium nitrite, KNO 2 ) was of course thrown away, but Scheele, always curious,
decided to analyse it. He found that it gave off red fumes (NO 2 ) when treated with
acids (Fig. 8.4); such a weak acid as acetic acid was sufficient.
1 This is due to a
rapid disproportion of the nitrous acid formed:
KNO 2 þ CH 3 COOH HNO 2 þ CH 3 COOK:
2HNO 2 H 2 O þ NO þ NO 2 :
The fumes could be dissolved in water to a previously unknown acid, similar to
nitric acid but weaker and more volatile.
Scheele found that the new acid, now known as nitrous acid (HNO 2 ), could be
prepared more easily by heating potassium nitrate alone, and adding acetic acid to
the residue after cooling. Evolution of oxygen upon heating potassium nitrate was
not mentioned by Scheele at this point:
2KNO 3 ! 2KNO 2 þ O 2 :
Scheele found that the red fumes (NO 2 ) obtained by dissolving iron or copper in
nitric acid could be dissolved in potassium hydroxide solution to give the potassium
salt of the new acid. He also obtained a white crystalline substance (now known as
nitrosyl sulphuric acid, HOSO 3 NO) by treating nitrous acid with sulphuric acid.
Nitrous acid had actually been observed by Willis,
2 did not realise that it was an
acid, but merely concluded that the aqueous solution was yellow and had a
repulsive taste. The fact that potassium nitrate gave red fumes with acids after
heating had been observed by Boerhaave, but this was unknown to Scheele and
Retzius.
Scheele had to wait a long time for a reply from Retzius. The reason for the
delay is not known, but it may perhaps have taken Retzius quite some time to digest
Scheele’s discoveries, which were quite exceptional. In February 1768, Scheele
wrote back to Retzius, asking whether he had received the report. Retzius reply is
not preserved, but by end of March, he had replied, although Scheele seems not to
have been too impressed by Retzius’ explanations. Apparently, Retzius had suggested that potassium nitrate had reacted with a part of the phlogiston on heating
since Scheele stresses in his answer to Retzius that phlogiston is a principle without
1
Nitrous acid is a stronger acid than acetic acid (pK a 3.39 and 4.75, respectively), but the
decomposition of nitrous acid to gaseous nitrogen oxides drives the reaction.
2
Thomas Willis (1621–1675), an English scientist and co-founder of the Royal Society.
8.2 Scheele’s First Discoveries
103
Crocus Martis Stahlii aperitivi by melting a mixture of iron flings, antimony(III)
sulphide and potassium nitrate (saltpetre). He also prepared another medicine called
Antimonium diaphoreticum, which was prepared by igniting a mixture of one part
antimony(III) sulphide and three parts potassium nitrate, after which the residue was
washed with water [7]. Per standard methods, the water used for the washing the
Crocus Martis or Antimonium diaphoreticum (in both cases containing, e.g.
potassium nitrite, KNO 2 ) was of course thrown away, but Scheele, always curious,
decided to analyse it. He found that it gave off red fumes (NO 2 ) when treated with
acids (Fig. 8.4); such a weak acid as acetic acid was sufficient.
1 This is due to a
rapid disproportion of the nitrous acid formed:
KNO 2 þ CH 3 COOH HNO 2 þ CH 3 COOK:
2HNO 2 H 2 O þ NO þ NO 2 :
The fumes could be dissolved in water to a previously unknown acid, similar to
nitric acid but weaker and more volatile.
Scheele found that the new acid, now known as nitrous acid (HNO 2 ), could be
prepared more easily by heating potassium nitrate alone, and adding acetic acid to
the residue after cooling. Evolution of oxygen upon heating potassium nitrate was
not mentioned by Scheele at this point:
2KNO 3 ! 2KNO 2 þ O 2 :
Scheele found that the red fumes (NO 2 ) obtained by dissolving iron or copper in
nitric acid could be dissolved in potassium hydroxide solution to give the potassium
salt of the new acid. He also obtained a white crystalline substance (now known as
nitrosyl sulphuric acid, HOSO 3 NO) by treating nitrous acid with sulphuric acid.
Nitrous acid had actually been observed by Willis,
2 did not realise that it was an
acid, but merely concluded that the aqueous solution was yellow and had a
repulsive taste. The fact that potassium nitrate gave red fumes with acids after
heating had been observed by Boerhaave, but this was unknown to Scheele and
Retzius.
Scheele had to wait a long time for a reply from Retzius. The reason for the
delay is not known, but it may perhaps have taken Retzius quite some time to digest
Scheele’s discoveries, which were quite exceptional. In February 1768, Scheele
wrote back to Retzius, asking whether he had received the report. Retzius reply is
not preserved, but by end of March, he had replied, although Scheele seems not to
have been too impressed by Retzius’ explanations. Apparently, Retzius had suggested that potassium nitrate had reacted with a part of the phlogiston on heating
since Scheele stresses in his answer to Retzius that phlogiston is a principle without
1
Nitrous acid is a stronger acid than acetic acid (pK a 3.39 and 4.75, respectively), but the
decomposition of nitrous acid to gaseous nitrogen oxides drives the reaction.
2
Thomas Willis (1621–1675), an English scientist and co-founder of the Royal Society.
8.2 Scheele’s First Discoveries
103