The GHP in its methods 2 and 3 uses the moisture (H) as unit of
drug strength (UDS) in order to calculate the amount of alcohol
necessary to prepare the MT.
In method 2 we add one part of solvent for each part of
moisture. So if the moisture has the value H, we must add H
parts of solvent to obtain 2 H volumes of MT (although we should
say aprox. 2 H, because of the substances soluble in water which
are extracted during the preparation and which are not taken
into account by the GHP). The drug strength (DS) is the
fraction that indicates how much parts of the MT contain one
unit of drug strength (UDS).
Since each UDS is contained is 2 parts of MT, the DS of this
method is 1/2.
The WSP considers as UDS, not the moisture of the plant, but the
juice. Therefore in method 2 we must add one part of solvent for
each part of juice, and using a similar reasoning that each UDS
is contained in 2 parts of MT, the DS of this MT is also 1/2.
Now when we have a MT prepared following the GHP, seen form the
point of view of the WSP, we have a DS related to the juice
(J), and therefore we must express this DS as parts of MT which
contain one part of juice (J), while we consider to have a
final volume of
In this case each UDS is contained in x parts of MT, and the
DS from the point of view of the WSP is 1/x. Since x is always
smaller than 2, 1/x will always be greater than 1/2 which shows
that the MT prepared following the GHP, but seen from the point
of view of the WSP will always be more concentrated. This
difference will be larger as the DR of the juice is higher.
Lets give an example. Suppose we have a plant with 70% of
moisture (H) and 10% of dry residue (DR). We prepare two MT
following method 2 of each pharmacopoeia. Then to prepare our
first potency by WSP, we take 2 parts of the MT for each part
of juice we need and proceed to add solvent up to 10 or 100
parts (for the decimal or centesimal scale resp.) Now, if we use
the MT prepared by the GHP, and if we want to obtain the same
concentration for our first potency, we should take the same
amount of juice (which corresponds to 1 part of UDS) and which
is contained in x parts of MT.
However the DS given in both pharmacopoeias is the same, and in
theory we should also have taken 2 parts.
The percentual difference (d%) can be calculated based on the
quantity we should use to have first potencies with the same
concentration, and the quantity we normally take following the
DS of the pharmacopoeia we use.
To obtain a general formula we can state that


theorical part we should use to obtain equal concentration


parts we really use following the pharmacopoeia


difference in absolute value (=d)
if
then
Comparing the 2 pharmacopoeias in method 2 we can replace
by x' (=2) and
by x
, then we have:
Should we use a MT prepared by method 3 we have
and
then
We obtain exactly the same formula which proves that the
difference is independent of the final weight or volume of the
MT and solely depends on the UDS considered in each case.
This formula enables us to quantify the percentual difference
between the quantity of MT we should use, and the one we really
use to prepare our first potency according to the DS of each
pharmacopoeia.