Tinctures are alcoholic or hydroalcoholic solutions prepared
from vegetable or chemical substances. The concentration of solute varies
up to 50%, e.g. Vanilla Tincture USP, Iodine Tincture USP. Tinctures are
alcoholic solutions of nonvolatile substances which are generally extracted
by maceration or percolation. Tinctures of potent drugs represent the activity
of 10 g of the drug in each 100 ml of the tincture; they are 10% tinctures.
With a few exceptions, nonpotent tinctures represent 20 g of the drug per
100 ml of tincture.
Tinctures are prepared chiefly by percolation and maceration. Percolation
is the procedure of choice when the crude drugs are cellular in structure;
plant exudates tend to become impacted in the percolator and stop the flow
so that maceration is preferred in such preparations. Moderately coarse
powders are preferred, because coarse powders are slowly penetrated by
the menstruum and fine powders tend to clog the percolator.
Usually alcohol or a hydroalcoholic menstruum is employed. The choice
of menstruum depends on the solubility, stability, and ease of removal
of the desired constituent. Other inactive constituents are extracted,
but if the material is not objectionable, it is allowed to remain.
In the process of percolation, the drug is dampened with the menstruum
and allowed to stand for a short period before packing the percolator so
that the drug may expand as the menstruum is absorbed. If the drug is packed
into the percolator and moistened, the swelling would pack the drug so
firmly that the percolate could not flow.
The menstruum is then added to cover the drug and the lower opening
is closed when the liquid is about to drip from the percolator. This permits
the air between the particles to escape as the menstruum descends. Maceration
for a prescribed time permits saturation of the menstruum in contact with
the drug, assuring a more nearly complete extraction.
The menstruum is then allowed to flow or percolate at a definite rate.
Normally the percolate collected is assayed before final volume is reached,
and then it is adjusted to the proper strength.
In the process of maceration the drug is soaked with the menstruum in
a closed container. The closed container prevents the loss of volatile
constituents and evaporation of the menstruum. The mixture is agitated
frequently so the menstruum at the bottom of the container does not become
saturated and incapable of extracting further drug. Circulatory maceration
is an efficient modification which eliminates the need for agitation. When
heat is employed in maceration, the process is known as digestion. The
mixture is then transferred to a filter, and the residue is washed with
sufficient menstruum to bring the tincture to final volume.