_Pyrogallic Acid_ (Vol. vi., p. 612.).--In answer to the Query of your
correspondent E. S., I beg to give the following method of preparing
pyrogallic acid (first published by Dr. Stenhouse), which I have tried and
found perfectly successful.
Make a strong aqueous infusion of powdered galls; pour it off from the
undissolved residue, and carefully evaporate to dryness by a gentle heat:
towards the conclusion of the process the extract is very liable to burn;
this is best prevented by continued stirring with a glass or porcelain
spatula. Next, procure a flat-bottomed iron pan, about ten inches diameter
and five inches deep. Make a hat of cartridge paper pasted together, about
seven inches high, to slip over and accurately fit the top of the iron pan.
Strew the bottom of the pan with the gall extract to the depth of
three-quarters of an inch; over the top stretch and tie a piece of bibulous
paper pierced with numerous pin-holes; over this place the hat, and tie it
also tightly round the top of the pan.
The whole apparatus is now to be placed in a sand-bath, and heat cautiously
applied. It is convenient to place a glass thermometer in the sand-bath as
near the iron pan as possible. The heat is to be continued about an hour,
and to be kept as near 420° Fah. as possible; on no account is it to exceed
450°. The vapour of the acid condenses in the hat, and the crystals are
prevented from falling back into the pan by the bibulous paper diaphragm.
When it is supposed that the whole of the acid is sublimed, the strings are
to be untied, and the hat and diaphragm cautiously taken off together; the
crystals will be found in considerable quantity, and should be removed into
a stoppered bottle; they should be very brilliant and perfectly white; if
there is any yellow tinge, the heat has been too great.
I believe that close attention to the above details will ensure success to
any one who chooses to try the process, but at the same time I must remind
your correspondents that scarcely any operation in chemistry is perfectly
successful the first time of trial.