Part 3: Metrology &
Pharmaceutical Calculations
References:
1- Remington 2006, Chapter 11, Meterology & Pharmaceutical
Calculations.
2- Ansel H.C., Mitchell J.S., 11th Edt., Pharmaceutical
Calculations, 2001
Roman numbers
ss = 1/2
I or i = 1 II or ii = 2
V or v = 5 X or x = 10
L or l = 50 C or c = 100
D or d = 500 M or m = 1000
LX = 60 XL = 40
2- Avoirdupois system
The basic unit of weight in the
avoirdupois system is the grain
(gr). The larger units are the
ounce (oz) and the pound (lb).
WEIGHT: AVOIRDUPOIS SYSTEM
a. You must know the following relationships
between the avoirdupois units:
1 lb = 16 oz =7000 gr
b. To convert from the avoirdupois system to the
metric system, you must know these conversion
factors:
1 kg = 2.2 lb
1 lb = 454 g
3- Apothecary system
Apothecary weight units
1 pound (lb) = 12 oz
1 pound (lb) = 5760 grain = 373 g
1 oz Apot = 31.1 g
The units of volume in the apothecary
system are Minim, fluid dram (fζ), fluid
ounce (fξ), pint (pt), quart (qt), and gallon
(gal).
VOLUME: APOTHECARY SYSTEM
Know the following relationships between apothecary units:
1 gal = 4 qt
1 qt = 2 pt
1 pt = 128 fζ (fl dram)
1 fl dram = 60 minim
To convert from the apothecary system to the metric system, you
must know the following factors:
1 gal = 3785 ml
Length Uinits
Inch
Foot
1ft = 12 inch =30 cm
1ft = 0.30480m
1 inch=2.51 cm
Length units
1 meter 39.37 inch
1 inch 2.54 cm
1 foot 0.305 m
Different Kinds of Balances
Single-beam equal arm balance
Unequal arm balances
Preferred in laboratory works when large amounts are to be
weighed
Electronic Balances
Single pan balances, much more sensitive than the traditional
ones, easier & quicker to use, but usually more expensive.
Single-beam equal arm balance
Different Kinds of Balances
Single-beam equal arm balance
Unequal arm balances
Preferred in laboratory works when large amounts are to be
weighed
Electronic Balances
Single pan balances, much more sensitive than the traditional
ones, easier & quicker to use, but usually more expensive.
Unequal arm balances
Prescription Balance
Electronic Balances
NIST (National Institute of Standards and
Technology)
Weight & Measure Division:
Discuss about the specifications and standards
used primarily in weights and measures and
commercial applications.
Sensitivity (readability) of a balance:
Minimum weight that causes the indicator of
balance change one unit.
If addition of 6mg weight to balance causes one unit
movement of indicator , sensitivity of balance is 6mg.
Class 3 balances:
Minimum sensitivity 6mg up to 60g.
Electronic divices have usually a sensitivity less than
10mg.
Accuracy of a balance
if a known calibration standard weight of
100.000 grams was placed on a balance and
the display shows 100.002 grams we could say
the accuracy of the balance is 0.002 grams or 2
milligrams.
Accuracy tells how close a balance gets to the
real value.
Different terms related to balances
Capacity (maximum weight can be weight by
balance)
Readability minimum weight can be weight by
the balance
Repeatability ( variation in different
measurements of one object)
Are usually mentioned by the manufacturer for
electronic devices.
E.g. Capacity 60g, readability 6mg, repeatability ±1mg
Protection & Maintenance of a Balance
Position chosen for balance should be flat.
Should be far from dampness, dust & corrosive
vapor
Analytical balances are kept in glass cases.
Some balances can be locked & protect from
vibration when they are not in use.
Should be cleaned just by a clean clothe.
Should be checked & calibrated periodically.
Protection & Maintenance of a Balance
For compounding, accurate sets of weights
from 10mg to 10g should be kept precisely in a
closed box.
For analytical purposes, weights from 1mg to
100g should be available.
The weight should not be handled by finger.
Torsion Prescription
Balance
Checking a balance periodically
Calibration:
Weighing a standard mass on balance to see
how much it is different from the real weight.
It tells us accuracy (how much the balance
work accurate).
Periodically checking of a prescription
balance
Sensitivity requirement
Arm Ratio test
Shift test
Minimum Weighable Quantity (MWQ)
The USP allows a maximum error of 5% in a single
weighing operation
The percent of the error depend on the amount of
drug weighed & sensitivity of balance according to
the formula below:
The percent of error will increase as the amount of
drug decreases
Examples
Percent of Error
Aliquot Method for weighing &
measuring
A torsion prescription method has a sensitivity
requirement of 6 milligram. Explain how you
would weight 4 milligrams of atropine sulfate
with an accuracy of ± 5%, using lactose as the
diluent?
Measuring (for liquids)
In pharmacy, measuring
usually refers to the exact
determination of a definite
volume of liquid.
Reading should be made at
the bottom of this meniscus.
Measuring (for liquids)
Glassware are preferred.
Metal devices some times are used but should be preferably stainless steel
or copper not iron.
If there are materials susceptible to oxidation in the formulation. Copper
should not be used.
There are two types of Graduated glassware for measuring:
Conical (less accurate)
Cylindrical (more accurate)
For volumes less than 1.5 mL, a graduated pipette, burette or
dropper should be used.
Volume of drop can be differ by many factors, density,
temperature, viscosity, surface tension, size & shape of orifice.
When drugs are potent, a calibrated dropper should be used.
Unmarked portion of graduated
glassware
To improve accuracy, the lower portion of
graduates should not be used.
Alligation Alternate
Method
Potency of a Drug
The Potency of some antibiotics, hormones, vitamins
& Biologics (Vaccines) is based on their activity &
expressed in terms of Units or Micrograms per
milliliters (µg/mL).
Insulin & Penicillin antibiotics
These preparation should obey USP standard
reference,
Penicillin G 1590 Units/mg are USP reference
standard
1500 U/mg < Penicillin G <1750 U/mg is acceptable
(Normogram method)
(Normogram method)
The nomogram method is the best method:
because it is formulated on the patient's size.
It takes into consideration the person's body
surface area in meters square.
1.73 M2 is the surface area of the
average adult (150-154 lbs.).
(Normogram method)
Child's surface area in m2
----------------------------------- × Adult dose = Child's dose
1.73 m2
فرمول محاسبه مساحت سطح بدن
BSA
BSA= W0.425 x H0.725 x 71.84
W; kg
H; cm
BSA; Square Centimeter
Fried's Rule
Child's age in months × Adult dose
Child's dose = -------------------------------------------------
150
c. Young's Rule:
Child's age in years × Adult dose
Child's dose = ---------------------------------------------
(Child's age in years + 12)
d. Clark's Rule:
Child's weight in lbs × Adult dose
Child's dose = ------------------------------------------
150
Examples
Answers
Aliquot Method for weighing &
measuring
A torsion prescription method has a sensitivity
requirement of 6 milligram. Explain how you
would weight 4 milligrams of atropine sulfate
with an accuracy of ± 5%, using lactose as the
diluent?
Aliquot Method for weighing &
measuring
A prescription calls for 0.2 mL of olive oil,
using a 5 mL graduate calibrated in units of 0.5
mL, how would you obtain the required
amount of olive oil using the aliquot method
& alcohol, as the diluent?
