1. Calcium dobesilate
Systematic name: Calcium 2,5-dihydroxybenzenesulfonate
In industry, it is prepared by redox-substitution reaction of 1,4-benzoquinone with calcium
hygrogensulfite:
O
O
OH
OH
S
O
O
O
2
Ca
2+
2 + Ca(HSO3)2
Calcium hydrogensulfite is not commercially available due to its instability and is prepared by
reaction of some suitable calcium(II) compound with sulfurous oxide in aqueous medium
immediately before usage.
For purposes of our practical courses, an „alternative“ preparation procedure is more
suitable. It consists of sulfonation of hydroquinone with sulfuric acid followed by neutralisation of
thus formed 2,5-dihydroxybenzensulfonic acid with calcium carbonate:
Scheme of preparation:
OH
OH
OH
OH
S
O
O
OH
OH
OH
S
O
O
OH
OH
OH
S
O
O
O
2
Ca
2+
+ H2SO4
2 + CaCO3 + CO2 + H2O
Chemicals:
hydroquinone [110.11] 1.0 g 0.009 mol
sulfuric acid 96% [98.08] 4 ml 0.075 mol
calcium carbonate [100.09] 12,5 g 0.124 mol
Procedure: Hydroquinone is added under stirring on a magnetic stirrer into a 50 ml flask with 96%
sulfuric acid at 45°C in a bath. Stirring is continued until all the amount of hydroquinone is
dissolved (about 3 hours). After cooling to room temperature, the reaction mixture is poured under
stirring into 120 ml of water in a beaker. Then calcium carbonate is gradually added under
continuous stirring (slowly and carefully, a foam is formed!). The mixture is then shortly refluxed
and still warm filtered by means of lowered pressure on a Büchner funnel (rounded paper filter of
suitable diameter needed!). The filtrate is evaporated on a rotary vacuum evaporator until dryness.
The residuum is then suspended in 25 ml of 96% ethanol and filtered through a normal paper filter
(without support of vacuum). The filtrate is again evaporated until dryness. The residuum is the
final product. The adequate portion of the residuum (approx. 0.2 g) is directly used for identity and
purity confirmation by measurement of UV spectrum on a spectrophotometer HP 8453 according to
the article Calcium dobesilate monohydrate (Calcii dobesilas monohydricus) of the actual European
Pharmacopoeia. The specific absorption A1%
1cm is calculated according to the formula
A1%
1cm= 40A / m,
where A1%
1cm is specific absorption, A is absorbance measured in a maximum near 301 nm and m
is the exact weight of calcium dobesilate in grams. The value of A1%
1cm should range between 174-
181. If the product is stored until the next practical lesson, it must be kept in a suitable well
closed vessel, protected against air moisture. It is also suitable to dry it under reduced pressure at
40°C to the constant weight.