Lab 5
Pharmaceutical Measurement
Percentage of error
 In
pharmacy its important to keep
measurement as accurate as possible.
 Pharmacist must know the limitations of
the measuring instruments.
 When using a torsion prescription balance
the percentage error must be calculated
to determine whether the error is allowed
or not.
Percentage of error
 Percentage
error : it’s the maximum
potential error multiplied by 100 and
divided by the quantity desired.
𝐄𝐫𝐫𝐨𝐫 × 𝟏𝟎𝟎%
= 𝐏𝐞𝐫𝐜𝐞𝐧𝐭𝐚𝐠𝐞 𝐨𝐟 𝐞𝐫𝐫𝐨𝐫
𝐐𝐮𝐚𝐧𝐭𝐢𝐭𝐲 𝐝𝐞𝐬𝐢𝐫𝐞𝐝
 This
formula is valid only if the error and
the quantity desired are expressed in the
same denomination.
Percentage of error
 Example:
when the maximum potential error is ± 4
mg in a total of 100 mg, what is the
percentage error?
𝟒 ×𝟏𝟎𝟎%
𝟏𝟎𝟎
= 𝟒%
Percentage of error
 If
certain % of error is not to be exceeded,
and the maximum potential error of an
instrument is known, its possible to calculate
the smallest quantity that can be
measured.
𝐁𝐚𝐥𝐚𝐧𝐜𝐞 𝐬𝐞𝐧𝐬𝐢𝐭𝐢𝐯𝐢𝐭𝐲(𝐒𝐑)
= 𝐒𝐦𝐚𝐥𝐥𝐞𝐬𝐭 𝐪𝐮𝐚𝐧𝐭𝐢𝐭𝐲
𝐏𝐞𝐫𝐜𝐞𝐧𝐭𝐚𝐠𝐞 𝐨𝐟 𝐞𝐫𝐫𝐨𝐫
Percentage of error
 Example
what is the smallest quantity that
can be weighed with a potential error of
not more than 5 % on a balance sensitive
to 6 mg?
𝟏𝟎𝟎 × 𝟔 𝐦𝐠
= 𝟏𝟐𝟎 𝐦𝐠
𝟓
Aliquot method of measuring
weighing
 When a degree of precision in measurement
is required that is beyond the capacity of the
instrument in hand, the pharmacist may use
the aliquot method of measuring.
 Aliquot part: its any part that is contained a
whole number of times in a quantity. Thus, 2 is
an aliquot part of 10; and since 10 / 2 = 5, 2 is
called the fifth aliquot of 10. Again, 4 is an
aliquot part of 16 “the 4th aliquot of 16”.
Aliquot method of measuring
Procedure:
Step 1 Select some multiple of the desired quantity
that can be weighed with the required precision.
This is done by calculating the smallest quantity of
the substance that can be weighed with the
required precision.
Step 2 Dilute the multiple quantity with an inert
substance (diluent) that is compatible with the
given preparation.
Step 3 Weigh the aliquot part of the dilution that
contains the desired quantity.
Aliquot method of measuring
Aliquot method of measuring
Aliquot method of measuring
Measuring volume
 Its identical in principle to the aliquot method
of weighing.
Procedure
Step 1 select multiple of the desired quantity that
can be measured with required precision.
Step 2 dilute the multiple quantity with compatible
diluent to an amount evenly divisible by the
multiple selected.
Step 3 measure the aliquot of the dilution that
contains the quantity originally desired.
Aliquot method of measuring
Least weighable quantity
method
 This
method may be used as an
alternative to the aliquot method of
weighing to obtain small quantities of a
drug substance.
Least weighable quantity
method
Procedure:
Step1 weigh an amount of the drug
substance that is equal to or greater than the
least weighable quantity.
Step 2 dilute the drug substance with a
calculated quantity of inert diluent.
Step 3 weigh the predetermined quantity of
the drug- diluent mixture will contain the
desired quantity of drug.
Least weighable quantity
method

Example: if 20 mg of a drug substance are
needed to fill a prescription, how you would
obtain this amount of drug with an accuracy of
±5% using balance having a sensitivity
requirement of 6 mg. use lactose.

=


The smallest amount to be weighed on this
balance
𝐒𝐑
𝐄𝐫𝐫𝐨𝐫
𝟔
× 𝟏𝟎𝟎 = × 𝟏𝟎𝟎 = 𝟏𝟐𝟎𝐦𝐠
𝟓
120 mg of drug substance is weighed.
Drug-diluent mixture must be equal 120 mg or
greater. ( 150 mg of drug-diluent mixture is
selected)
Least weighable quantity
method
 The
total amount of diluent to use is:
20 mg → 150 mg (drug-diluent mixture)
120 mg → X (total amount of drug-diluent mixture)
X = 900 mg
900 – 120 = 780 mg of diluent (lactose ) to use.
Practice problems
1.
2.
A pharmacist attempts to weigh 0.375 gm of
morphine on a balance of dubious
accuracy. When checked on highly
accurate balance, the weight is found to be
0.4 gm. Calculate the percentage of error in
the first weighing.
A prescription balance has a sensitivity
requirement of 0.006 gm. Explain how you
would weigh 0.012 gm of atropine sulfate
with an error not greater than 5%, using
lactose as the diluent.
Practice problems
3.
A 10 mL graduate weighs 42.745 grams.
When 5 mL distilled water are measured
in it, the combined weight of graduate
and water is 47.675 grams. By definition,
5 mL of water should weigh 5 grams.
Calculate the weight of the measured
water and express the deviation from 5
grams as percentage error.
Practice problems
4.
A pharmacist failed to place the
balance in equilibrium before weighing
three grains of codeine. Later he
discovered that the balance was out of
equilibrium and that 20% error was
incurred. If the balance pan on which he
placed the codeine was heavy, how
many grains of codeine did he actually
weigh?
Practice problems
5.
6.
On a prescription balance having a
sensitivity requirement of 0.012 gm, what
is the smallest amount that can be
weighed with a maximum potential error
of not more than 5%?
A prescription balance has a sensitivity
requirement of 6.5 mg. Explain how you
would weigh 20 mg of substance with an
error not greater than 2%.
Practice problems
7.
A prescription calls for 0.2 mL of clove oil.
Using a 5 mL graduated calibrated in
units of 0.5 mL, how would you obtain
the required amount of clove oil using
the aliquot method and alcohol as the
diluent?
Home work
1.
2.
A torsion prescription balance has a
sensitivity requirement of 0.002 gram.
Explain how you would weigh 0.008
gram of substance with an error not
greater than 5 %
In preparing a certain ointment, a
pharmacist used 28.35 gram of zinc
oxide instead of 32.3 grams called for.
Calculate the percentage of error on
the basis of the desired quantity.