calcium sulphate. One would expect of 70 g NaCl, 29 g of MgCl 2 ∙ 6H 2 O ad 1,7 g
of CaSO 4 ∙ 2H 2 O, but it is difficult to judge Bergman’s accuracy, since the conditions at which his salts were dried are not known. Also, Bergman’s extraction
procedure was not completely selective.
An important step in Bergman’s development of analytical methods took the
form of a dissertation defended by one of his students, Johan Peter Scharenberg, in
June 1778 [28]. For some reason, the printing of the thesis was interrupted after the
first seven sections. As it was custom to print the first word of the following page at
the bottom of each page, it is clear that a §VIII was intended. The complete work
appears in the first volume of Opuscula [29]. This publication is a complete manual
for analysis of mineral waters, but the actual analytical procedures appear in the
latter part that was omitted in the original thesis.
Bergman’s procedure was, in short, first to separate and analyse any volatile
components such as CO 2 . The water was evaporated and consecutively extracted
with ethanol, cold water and boiling water, finally leaving an insoluble residue. The
ethanol extract mainly contained calcium and magnesium chloride and magnesium
nitrate and occasionally barium chloride. This mixture was re-crystallised from
water and the crystals examined with regard to shape, physical properties, taste and
reactions with reagents. The cold water extract was treated the same way, while the
hot water extract mainly contained calcium sulphate. Extraction of the insoluble
fraction with acetic acid dissolved calcium and magnesium carbonates, which could
be separated as sulphates (calcium sulphate is much less soluble than magnesium
sulphate). The residue insoluble in acetic acid typically contained oxides of iron,
silicon and aluminium, occasionally also barium and manganese. Iron and aluminium oxides are soluble in hydrochloric acid, leaving the silica undissolved.
From the acid extract, iron could be precipitated with potassium hexacyanoferrate
(II) and aluminia could be precipitated with alkali.
The second part of the publication makes extensive use of stoichiometry:
The weight of the precipitate may often, indeed, be of considerable use even in that view, as
shall presently be shewn, though it has not yet been employed for that purpose” [30].
For example, mineral waters frequently contain calcium and magnesium carbonates which are soluble in the presence of carbon dioxide. Bergman dissolved
them in sulphuric acid and separated the soluble magnesium sulphate from the
sparingly soluble calcium sulphate. To determine the amount of the corresponding
carbonates that had originally been present in the water, the sulphates could be
dissolved in water and precipitated with alkali carbonate, but “this tedious process
may be avoided, if we recollect that 100 parts of gypsum [calcium sulphate dihydrate] contain about 34 [should be 33] of pure lime [calcium oxide], which are
equivalent to nearly 62 [should be 58] of aerated lime [calcium carbonate]. [31]
This was the birth of gravimetric analysis and given the pioneering nature of this
work, the accuracy is quite impressive.
To help the analyst, Bergman gave a description of all salts that frequently occur in
natural waters, including their quantitative composition. It is noteworthy that all of
Bergman’s salts contain water, even salts such as sodium chloride and potassium
23.3 Water Analysis
317
of CaSO 4 ∙ 2H 2 O, but it is difficult to judge Bergman’s accuracy, since the conditions at which his salts were dried are not known. Also, Bergman’s extraction
procedure was not completely selective.
An important step in Bergman’s development of analytical methods took the
form of a dissertation defended by one of his students, Johan Peter Scharenberg, in
June 1778 [28]. For some reason, the printing of the thesis was interrupted after the
first seven sections. As it was custom to print the first word of the following page at
the bottom of each page, it is clear that a §VIII was intended. The complete work
appears in the first volume of Opuscula [29]. This publication is a complete manual
for analysis of mineral waters, but the actual analytical procedures appear in the
latter part that was omitted in the original thesis.
Bergman’s procedure was, in short, first to separate and analyse any volatile
components such as CO 2 . The water was evaporated and consecutively extracted
with ethanol, cold water and boiling water, finally leaving an insoluble residue. The
ethanol extract mainly contained calcium and magnesium chloride and magnesium
nitrate and occasionally barium chloride. This mixture was re-crystallised from
water and the crystals examined with regard to shape, physical properties, taste and
reactions with reagents. The cold water extract was treated the same way, while the
hot water extract mainly contained calcium sulphate. Extraction of the insoluble
fraction with acetic acid dissolved calcium and magnesium carbonates, which could
be separated as sulphates (calcium sulphate is much less soluble than magnesium
sulphate). The residue insoluble in acetic acid typically contained oxides of iron,
silicon and aluminium, occasionally also barium and manganese. Iron and aluminium oxides are soluble in hydrochloric acid, leaving the silica undissolved.
From the acid extract, iron could be precipitated with potassium hexacyanoferrate
(II) and aluminia could be precipitated with alkali.
The second part of the publication makes extensive use of stoichiometry:
The weight of the precipitate may often, indeed, be of considerable use even in that view, as
shall presently be shewn, though it has not yet been employed for that purpose” [30].
For example, mineral waters frequently contain calcium and magnesium carbonates which are soluble in the presence of carbon dioxide. Bergman dissolved
them in sulphuric acid and separated the soluble magnesium sulphate from the
sparingly soluble calcium sulphate. To determine the amount of the corresponding
carbonates that had originally been present in the water, the sulphates could be
dissolved in water and precipitated with alkali carbonate, but “this tedious process
may be avoided, if we recollect that 100 parts of gypsum [calcium sulphate dihydrate] contain about 34 [should be 33] of pure lime [calcium oxide], which are
equivalent to nearly 62 [should be 58] of aerated lime [calcium carbonate]. [31]
This was the birth of gravimetric analysis and given the pioneering nature of this
work, the accuracy is quite impressive.
To help the analyst, Bergman gave a description of all salts that frequently occur in
natural waters, including their quantitative composition. It is noteworthy that all of
Bergman’s salts contain water, even salts such as sodium chloride and potassium
23.3 Water Analysis
317