Improvements relating to the Manufacture of Salts of Pyrocatechin- disulpho-acid, and of Pyrocatechin therefrom.
Year: 1895

page_text: Date of Application, 2nd Jan., 1895-Accepted, 9th Feb., 1895
Improvements relating to the Manufacture of Salts of Pyrocatechin-
disulpho-acid, and of Pyrocatechin therefrom.
I, Dr. GEORG TOBIAS, of 12 Waterlooufer, Berlin, S.W., in the Empire of
Germany, Chemist, do hereby declare the nature of this invention, and in what
manner the same is to be performed, to be particularly described and ascertained,
in and by the following statement :-
My invention relates to the manufacture of salts of pyrocatechindisulpho-acid,
and of pyrocatechin therefrom.
The method which is simplest and therefore most frequently used in the
industry for the introduction of a new phenol group, the melting of the
corresponding sulpho-acid with an alkali, gives, when applied to phenolortho-
sulpho-acid, a very small yield of pyrocatechin by reason of the high temperature
required for the transformation (P. Degener, Journ. für prakt. Chemie 20
page 300).
An incomparably greater yield of pyrocatechinmonosulpho-acid is obtained from
phenol-a-disulpho-acid, but only when melting with the more expensive caustic
potash at about 300 degrees Centigrade, while the employment of caustic
soda at 300 to 310 degrees Centigrade gives a considerably smaller vield and a
much more impure product (Barth und Schmidt, Berichte der Deutschen
Chemischen Gesellschaft 12. 1260).
I have found that phenoltrisulpho-acid, which may be easily maintained by
the action of a sufficient quantity of smoking sulphuric acid upon phenol, with
cooling and ensuing heating at 100 to 110 degrees Centigrade for some time, gives,
when melted with caustic potash at about 220 degrees Centigrade, and also with
caustic soda even at 240 to 260 degrees Centigrade, a good yield of pyrocatechin-
disulpho-acid. This disulpho-acid, like the disulpho-acid obtained by Cousin
(Compt. rend. 117 page 113) by sulphurating pyrocatechin, is characterized by its
somewhat sparingly soluble barium salt. The sodium and potassium salts of the
disulpho-acid are easily soluble in water. The acid itself was obtained in the
evaporation of its aqueous solution, produced by decomposing the barium salt with
an equivalent quantity of dilute sulphuric acid, as a syrupy mass which does not
crystallize. Its neutral and weak acid solution is coloured intensely bluish green
by ferric chloride; this colour as in the case of monosulpho-acid, in consequence
of the addition of sodium carbonate, changes through blue and violet to red.
A further essential part of my invention consists in the mode of splitting off
perfectly pyrocatechin from the pyrocatechindisulpho-acid thus produced. It
takes place by heating the aqueous solution of this acid for several hours to about
160 degrees Centigrade in an autoclave provided with a lining capable of
withstanding the action of acid. If it is desired to obviate the production of
pyrocatechindisulpho-acid and to subject its salts directly to hydrolysis it is
sufficient and expedient, while completely dispensing with the strong mineral acids
such as hydrochloric acid, sulphuric acid, phosphoric acid, large quantities of which
have heretofore generally been used in similar cases, to heat the concentrated
aqueous solution of the salts of the pyrocatechindisulpho-acid in autoclaves for a
comparatively long time to about 200 to 220 degrees Centigrade. A slight
addition of acid is here uninjurious, but as before stated is completely dispensed
with.
In the pressure of considerable quantities of the before-mentioned strong acids,
especially of sulphuric acid, the bydrolysis of the said salts will indeed take place

page_text: and is to be carried out at a somewhat lower temperature; but no advantage is
gained at all either as regards the quality or the yield of the resulting pyrocatechin
by this method of working as compared with the method first described.
The mode of carrying my said invention into practice may be explained by the
following example:
The basic sodium phenoltrisulpho-salt to be employed is obtained by saturating
the dilute crude phenoltrisulpho-acid with lime, precipitating the filtrate with an
equivalent quantity of sodium carbonate, filtering, evaporating and drying at
100 degrees Centigrade.
10 parts of this previously ground salt are gradually introduced into 15 to 17
parts of caustic soda which is melted with 10 to 15 per cent. of water in a cast-iron
vessel provided with a stirrer and has reached a temperature of about 230 degrees
Centigrade which during the introduction of the salt is increased to 250 to
260 degrees Centigrade. After the introduction of the salt the fluid melt is
maintained for some time at the same temperature.
The cooled melt is dissolved in about twice its weight of water and the solution
is caused to pass slowly into a suitable quantity of 40 to 50 per cent. sulphuric acid
necessary for the neutralization and for the expulsion of the sulphurous acid, and
then the main quantity of the sodium sulphate is with proper cooling crystallized
out while stirring, the liquor being then drawn or thrown off as completely as
possible, then washing with small quantities of cold water until the deep colouring
of the liquor caused by some dissolved iron has become quite weak in the washing-
water. The first filtrate is evaporated a little for as complete a removal as
possible of the sodium sulphate, preferably cooled with ice, and the sulphate
separated is washed out like the first main quantity. The washing-water serves
for dissolving the next melt.
The ultimate liquor thus obtained, being substantially a concentrated solution of
sodium pyrocatechindisulpho-salt, may either be subjected directly to hydrolysis or
I obtain therefrom after further evaporation during the cooling the sodium salt in
a solid crystalline form. For obtaining the barium salt this ultimate liquor is
again diluted a little and by fractionated addition of barium chloride to the hot
solution the sulphuric acid present as sodium sulphate is removed, whereupon the
barium pyrocatechindisulpho-salt is precipitated in the crystalline form.
Instead of using sulphuric acid I may treat the solution of the malt with
carbonic acid or with washed products of combustion, preferably the waste gases
resulting from the burning of lime, and thus recover, during the strong cooling of
the solution, a great part of the caustic soda used in excess in the form of
crystallized sodium carbonate which may be washed out in the same manner as
the sodium sulphate.
Or the sufficiently reduced melt is digested with only so much water that say a
40 per cent. soda liquor is produced. Then the tetrasodium salt of the pyro-
catechindisulpho-acid together with sodium sulphite free from water remains in
the granular crystalline state undissolved, and by decantation, filtration and
pressing, preferably while heating by reason of the greater fluidity of the liquor, a
large portion of the caustic soda used may be recovered in the form of a
concentrated liquor.
In lieu of caustic soda, use may be made, as hereinbefore stated, of caustic
potash at a lower melting temperature; instead of in an open vessel the melt may
be treated in closed apparatus provided with stirrers at a pressure corresponding to
the quantity of water contained in the alkali and the melting temperature;
moreover the quantitative proportions and temperatures stated in the example
given may be varied within certain limits in the melting process.
For the production of pyrocatechin the ultimate liquor obtained as set forth in
the before-mentioned example is heated in an autoclave, provided with a lining
capable of withstanding the action of acid, for from 10 to 15 hours to between
200 and 220 degrees Centigrade. If in the case of the purer sodium pyro-
catechindisulpho-salt isolated by crystallizing out, the concentration of the solution

page_text: to be subjected to hydrolysis is such that after the splitting a bisulphate solution
saturated at the ordinary temperature is produced, the largest part of the pyro-
catechin formed will crystallize out after the cooling, this pyrocatechin being
sparingly soluble in a concentrated bisulphate solution in the cold state. The part
dissolved in one or the other case is withdrawn by ether in the well-known manner
and after distilling the latter is obtained in a solid crystalline form. The small
quantity of phenol which may still be contained therein can be easily removed by
separating the small portion of the product distilling below 240 degrees and not
congealing at once, or by once crystallizing the product distilled without fraction
from two parts of benzol.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is :-
:-
First. The manufacture of salts of the pyrocatechindisulpho-acid by heating salts
of the phenoltrisulpho-acid with caustic soda or caustic potash in open or closed
vessels to temperatures above 200 degrees Centigrade.
Second. The separation of basic sodium salt of the pyrocatechindisulpho-acid
and the recovery of a portion of the used caustic soda in the form of a concentrated
soda liquor by treating the soda melt obtained as set forth to the first claim with a
certain quantity of water.
Third. The recovery of caustic soda, which has been used in excess, in the form
of sodium carbonate by treating the solution of the soda melt obtained as set forth
in the first claim with carbonic acid or gases containing carbonic acid.
Fourth. The manufacture of pyrocatechin which consists in heating pyro-
catechindisulpho-acid obtainable from its alkaline salts produced as set forth in the
first claim, or salts of this acid, with water to which acids may or may not be
added, in closed vessels to temperatures above 140 degrees Centigrade.
Dated this 2nd day of January 1895
HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, W.C., Agents for the Applicant.