Download Weights and measures Lab1-Fall 2008

PHAROS UNIVERSITY
Faculty of Pharmacy & Drug Manufacturing
Lab 1
WEIGHTS AND MEASURES
OBJECTIVE:
Two of the most crucial steps in compounding any pharmaceutical product are the accurate weighing
and measuring of the component ingredients of the formulation. In order to carry out these critical
functions, the pharmacist must have a good knowledge of the three systems of measurement used
worldwide: the Metric system, the Apothecary system, and the Avoirdupois system. The relation
between the three systems should also be well understood.
INTRODUCTION
Weight is a measure of the gravitational force acting on a body and is directly proportional to the body’s
mass.
The mechanical device designed for weighing is the “balance” and “weights” are used to keep the body
in equilibrium.
Measure is the determination of the volume of a body. Temperature and pressure have a pronounced
effect on the volume, especially that of gases or liquids.
SYSTEMS OF WEIGHT AND MEASUREMENT
I. THE METRIC SYSTEM:
The metric system is the preferred and most frequently used system of measurement in pharmacy. Since
it is a decimal system, other denominations of measure in the system are easily and quickly generated as
a 10th multiple at the basic unit. The advantages of the metric or decimal system are its simplicity and
adaptability to everyday use.
In many pharmaceutical procedures, very small and sometimes very large quantities of weight and
volume are measured. To avoid the use of numbers with many zeros in such cases, the National Institute
of Standards and Technology (NIST) recognizes prefixes to be used to express fractions or multiples of
the International System of Units (SI).
The recognized prefixes, which are adjoined to an appropriate unit (as for example nanogram, Pico
mole, femtosecond or megavolt), are listed in Table1.
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Table1: Recognized prefixes,
Fraction
10-1
10-2
10-3
10-6
10-9
10-12
10-15
10-18
Prefix
deci
centi
milli
micro
nano
pico
femto
atto
Symbol
d
c
m
µ
n
p
f
a
Multiple
10
102
103
106
109
1012
1015
1018
Prefix
deka
hecto
kilo
mega
giga
tera
peta
exa
Symbol
da
h
k
M
G
T
P
E
A. Weights in Metric system:
The gram (g) is the unit of weight in the metric system.
Unit
Symbol
Kilogram
Kg
Hectogram
Decagram
Gram
Decigram
Centigram
Milligram
Microgram
Nanogram
hg
dag
g
dg
cg
mg
µg
ng
Equivalent
1,000,000 mg =
1,000 g
106 mg
100 g
10 g
1.0 g
1,000 mg
0.1 g
100 mg
0.01 g
10 mg
0.001 g
1 mg
-6
0.000 001 g= 10 g 0.001 mg = 10-3 mg
10-9 g
10-6 mg
One gram = 1,000,000 µg = 106 µg
= 1,000 mg =103 mg
= 0.001 Kg = 10-3 Kg
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B. Measures in Metric system :
The Liter (L) is the unit of volume in metric system.
Unit
Kilogliter
Symbol
K<
1,000 <
Equivalent
1,000,000 m< =
106 m<
Hectoliter
Decaliter
Liter
Deciliter
Centiliter
Milliliter
Microliter
Nanoliter
h<
da<
<
d<
c<
m<
µ<
n<
100 <
10 <
1.0 <
0.1 <
0.01 <
0.001 <
0.000 001< = 10-6 <
10-9 <
1,000 m<
100 m<
10 m<
1 m<
0.001 m <= 10-3 m<
10-6 m<
1 liter ( < ) = 1,000,000 µ< = 106 µ<
= 1000 m< = 103 m<
= 100 c< =102 c<
= 10 d<
= 0.001 k<
1ml = 103 µ<
=====================================================
II. THE ENGLISH (COMMON) SYSTEMS
In the United States both the Apothecary and Avoirdupois Systems of Weight and Measurement are
still sometimes used in handling of medicines (both weight and volume).
1-THE APOTHECARY SYSTEM:
The Apothecary system was commonly used in the past by pharmacists and physicians as the system of
weights and measures for prescribing and dispensing medications. Although it has largely been replaced
by the less difficult metric system, the pharmacist still encounters these symbols in his/her routine
practice.
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A-Weights in Apothecary System
The abbreviations of the denominations of apothecary weight are represented by the signs ounce,
drachm, scruple, 8 and grain, gr.
These long have been in use but possibly may be mistaken for one another in rapid or careless writing.
Unit
1 grain
1 scruple
1 drachm
1 ounce
1 pound
Symbol
Gr
€
z
z
lb
Equivalent
20 gr
60 gr
480 gr
5760 gr
3€
8z
12 z
B-Measures in Apothecary System
Indeed, the apothecary system of fluid measure is still commonly used in a variety of products, both
pharmaceutical and non-pharmaceutical, and everyone should be familiar with the gallon, pint,
fluidounce, fluid drachm and minim.
Unit
1 minim
1 fluid drachm
1 fluidounce
1 pint
1 gallon
Symbol
m
flz
flz
pt or O
gal
Equivalent
8 fluid drachm
16 fluidounce
8 pints
60 minim
480 minim
7680 minim
61440 minim
2- THE AVOIRDUPOIS SYSTEM:
The Avoirdupois system is a system of weight measurement only. Its basic unit, the grain, is the same
as in the Apothecary system. The Avoirdupois ounce and pound differ in weight and symbols from those
in the Apothecary system. The Avoirdupois pound is the pound to which we are all accustomed in our
daily lives. It is also the weight measure in which bulk chemicals and over the counter pharmaceuticals
are bought and sold. It is important to make this distinction from weights in the Apothecary systems,
which are used only in the prescription or medication order.
The abbreviations or signs of avoirdupois weight differ from those of apothecary weight, and care
should be used to confound them. They are lb, pound; oz, ounce and gr; grain.
Weights in Avoirdupois System
Pounds
1=
Ounces
16=
1=
Grains
7000
437.5
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Relationships of Weights and Measures
Tables of weights and measures and a table of practical equivalents should be kept in the prescription
department
The following equivalents should be well memorized.
Equivalents of weight:
Equivalents of volume:
1 g =15.23 gr
1 grain (gr)
1/15.4 gm
D 0.065 gm
1 grain (gr)
64.8 mg
1 drachm (z)
4 gm
1 ounce (z )
31.1 D 30 gm
1 pound (Ib)
Avoir du pois
454 gm
1 pound (Ib)
apothecary
373 gm
1 minim (m)
1/ 16.23 ml D 0.06 ml
1 fluid drachm
(fl.z)
4 ml
1 fluid ounce
(fl.z)
1 pint (pt)
5
29.57 ml D30 ml
473 ml
D500 ml
1 gallon (gal)
3785 ml D 4 L
1 liter (L)
1 mililter (ml)
0.25 gal
16.2 m
Roman Numerals
The Apothecaries' system rationally uses Roman numerals in place of Arabic numbers.
In roman numerals, we put the unit of measurement before the numeral.
e.g. 5 grain would be written gr V.
3 grain would be written gr III
Values of single Numerals:
ss = 1/2
L or l = 50
I or i or j = 1
C or c = 100
V or v = 5
D or d = 500
X or x = 10
M or m = 1000
Values of combined Numerals:
a] When a smaller value or the same value numeral follows another numeral; the numerals are to be
added.
1. II = 1+1 2
2. VI = 5+1= 6
3. XXIII = 10+10+1+1+1 = 23
b] When a larger value numeral follows another numeral, the smaller value is to be subtracted from the
larger value.
1. I V = 5-1 = 4
2. I X = 10-1 = 9
3. V C = 100 – 5 = 95
4. L I V = 50 + 4 = 54
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Work Sheet
Student Name:......................................
ID Number:..........................
Section:.................................................
•
Convert the following Roman numerals to Arabic numerals:
CCXXXII...............................
IVss........................................
XLIII......................................
XII..........................................
XL..........................................
XIV........................................
XIX........................................
XCIV......................................
CIII........................................
LVI.........................................
• Add the following and express the total in mg.
1. 5 g + 250 mg + 1.600 µg = .....................................
2. 1.4 g + 0.45 g + 180 µg +0.24 mg =..........................
•
Add the following and express the total in liters.
60 ml + 0.5 L + 600 ml =.............................
•
Solve the appropriate equivalents for each of the following:
1. 480 ml = ...................fl.z
2. 90 mg = ....................gr
3. 15 ml = .....................fl.z
4. 10 ml =......................tsp
5. 45 ml = .....................Tbs
•
•
The doctor prescribes 10gr = gr X of a medical for a patient.
How many milligrams of medical should the patient receive?
..................................................................................................................................................
Calculate the metric equivalents of weight and measures in the following:
-Ammonium carbonate
- Menthol
gr v .............gm
gr i ............... gm
-Thymol gr iss............. gm
- Oil of lavender fl.z ss ............ ml
-Eucalyptol fl.z ii............. ml
- Alcohol fl.z ii.............. ml
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Household measures (Domestic measures)
In appropriating doses for a patient, the practitioner is usually compelled to order the liquid medicine to
be administered in certain quantities that have been established by custom and are estimated as follows:
Teaspoonful (tsp) = 5 ml
Dessertspoonful = 10 ml
Tablespoonful (Tbs) = 15 ml
Coffee cupful = 30 ml
Tea cupful = 120 ml
Cupful = 240 ml
In almost all cases, careful tests have found that modern teacups, tablespoons, dessertspoons and
teaspoons to average 25% greater capacity than the theoretical quantities.
Therefore, the physician and pharmacist should recommend the use of calibrated dropper, medication
cup and calibrated spoons.
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Experiment I;
Domestic Measures
Objective: to demonstrate variation in domestic measures.
Procedure:
1- Using suitable measuring cylinders, determine the actual capacity of the following
household measures:
a. A teaspoonful
b. A dessertspoonful
c. A tablespoonful
d. A coffee cupful
2- Report your results in tabular form and write your comments.
Domestic
measure
Actual capacity
Theoretical capacity
(ml)
(ml)
Teaspoonful
Dessertspoonful
Tablespoonful
Coffee cupful
Note:
% Error = Theoretical capacity - Actual capacity *100
Theoretical capacity
3- Compare your results with those of other students.
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Error %
Measurement of Pharmaceutical Liquids
To accurately and rapidly measure liquids, the pharmacist should have graduated cylinder, burette,
graduated pipette and dropper, from which to select the proper measuring vessel.
1. Measuring cylinders
Size of cylinder (c.c)
Minimum volume giving (5% error c.c)
10
25
50
2
6
8
100
250
500
10
18
21
2. Small volume of liquid is usually measured by graduated pipettes.
3. Volume of liquid less than 1 ml is usually measured by a dropper.
Experiment II
Measuring by means of droppers
Objective:
To illustrate the influence of some factors on the number of drops per unit volume.
Principle:
The USP medicine dropper is 3 mm in external diameter at its delivery end and when held
vertically, delivers 20 drops of water at 25 C, weighing 0.9 to 1.1 g. However, the volume of drop
depends on many factors, including density, temperature, surface tension, viscosity and the size and
nature of the orifice from which it is delivered.
Procedure:
1. Fill a dropper with one ml of the following liquids:
a. water
b. alcohol
c. glycerol.
2. Hold the dropper vertically and count the number of drops.
3. Repeat step 2 using each of other liquids.
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4. Record your results in a tabular form.
5. Comment on your results.
Liquid
Number of
under test
drops in 1-ml
Water
Alcohol
Glycerol
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Dispensing techniques
The process of a small scale manufacture of medicines from basic ingredients is called compounding or
dispensing.
In any dispensing process, the end product is going to be used or taken by a patient. Therefore it is
important that the medicine should be of highest achievable quality and this means that the highest
standards must be applied during preparation process.
Compounding and good practice requires:
Organization: The environment in which you work will have influence on your efficiency.
Therefore, it is important to develop a tidy and organized method of working.
Cleanliness: The bench, equipment, tools you use and the container holding the final product
must all be thoroughly clean.
Cleanliness minimizes microbial contamination.
Appearance: A clean white coat should be worn preventing outdoor clothes becoming stained if
spillages occur and any skin lesions covered with a dressing.
Documenting procedures and results: Records including the formula, ingredients, and
quantities used, their sources, batch numbers and expiry date must be kept. Also, records for a
prescribed item should include the patient, prescription and date of dispensing.
Dispensing techniques are classified into:
1. Weighing
2. Measuring liquids
3. Mixing and Grinding
4. Heat Sources
I. Weighing
Balances are the equipment used for weighing technique.
There are three types of balances: class I, class II and class III. All balances must be calibrated in
metric units and all new balances must be marked with both maximum and minimum weights that
can be weighed.
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Classes of balances with their weighing capabilities
Min.
Increment
Weight
weight
Class I
50 mg
1 mg
1 gm
Class II
100 mg
10 mg
50 gm
Class III
1 gm
100 mg
2 kg
Type
Max. weight
The balance most commonly used in dispensing is class II balance.
Pointer
scale
pan
Nowadays the move is to use. Electronic top-pan balances
Use of top pan balance:
1. Ensure the balance is level and working properly.
2. Place a weighing paper on the pan and use the auto-zero to cancel its weight.
3. Add the material to be weighed until the correct weight is shown on the display.
4. Carefully remove the weighed substance from the pan.
Sensitivity of Balance:
Is defined as the least weight which makes the pointer moves one scale division from the
original rest point.
The smaller the weight required, the more sensitive is the balance.
Percentage of error
The error in weighing should not exceed 5%
% error = sensitivity * 100
Weight
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Example: using the sensitivity of 10 mg,
what would be the % error in weighing 200 mg?
% error = 10 * 100 = 5%
200
Smallest quantity that can be weighed on a given balance = sensitivity *100
% error
Notes:
In addition to use the balance correctly, there are some rules that should be considered when weighing to
ensure good dispensing practice:
1. If using a solid material which requires to be size reduced or sieved, this is carried out before
weighing the required quantity to avoid loss of the material .The best approach is to roughly
weigh an excess quantity. Grind or sieve it firstly, then accurately weigh off the required
quantity.
2. Never split quantities and do two weighing, this will increase the inaccuracy.
3. If a quantity less than the sensitivity of balance is needed, it is necessary to weigh the minimum
weight allowable and make an excess of the product or prepare it by Trituration ( Dilution of
potent powdered drug for weighing it, by mixing the drug with a suitable diluent in a definite
proportion by weight, e.g. for diluents; is Lactose).
III. Mixing & Grinding:
Are carried out using Mortar & Pestle.
Mortars & Pestles are used in:
1. Reducing size of powders.
2. Mixing powders.
3. Mixing powders & liquids.
4. Making emulsions & suspensions.
Two types of Mortars & Pestles are found, each in varying sizes:
1. Glass mortars & pestles:
The smooth surface of the glass reduces the friction which can be generated.
They are therefore not suitable for size reduction, except for friable materials such as crystals.
They are suitable for:
- Dissolving small quantities of ingredients.
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-
Mixing small quantities of fine powders.
Mixing of colored substances, such as dyes which can be absorbed by & stain porcelain mortar.
2. Porcelain mortars & pestles:
They have a rougher surface.
They are ideal for:
- Size reduction of solids.
- Mixing larger quantities of powders.
- Mixing solids & liquids as in the preparation of suspensions & emulsions.
Manipulative Techniques.
A. Mixing Solids with Solids:
Using the Mortar & Pestle,
1. Adequate mixing will only be achieved if there is sufficient space, thus
overfilling of the mortar should be avoided.
2. The pestle should be rotated in both right & left directions.
3. Undue pressure should not be used, as this will cause impaction of the powder
on the bottom of the mortar.
** As well as the correct use of a mortar & pestle, the amounts of material being mixed
together must be considered.
Where the quantity of material to be mixed is small and the proportions are
approximately the same: the materials can be added to an appropriately sized
mortar and effectively mixed.
Where a small quantity of powder has to be mixed with large quantity: in order
to achieve effective mixing, it must be done in stages as shown in Exp.1.
Exp.1: Mixing 0.5 g Charcoal with 4.5 g Lactose :
1. Charcoal 0.5 g is placed in a glass mortar.
2. A quantity of Lactose, approximately equal in volume to the 1st, is added and
carefully mixed, using the glass pestle, until uniformly mixed.
3. A further quantity of Lactose, approximately equal in volume to that mixture
present in mortar is now added & again mixed well.
4. This process known as "Doubling up'' or '' Geometric dilution'' method is
continued until all the powder has been added.
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B. Mixing Solids with Liquids:
Exp. 2 : Prepare the following solution:
Rx
Sodium bicarbonate
Glycerol
Water
to
350 mg
3ml
20ml
Note: - NaHCO3 is soluble in water.
- Glycerol is a viscous vehicle.
Procedure:
1. 350 mg of NaHCO3 is weighed & placed in a small beaker.
2. Add about 15 ml of water & stir the contents of the beaker with a glass rod until
bicarbonate is completely dissolved.
3. Transfer the solution to a 25 ml measuring cylinder & (not 20), & adjust volume to 17 ml
with water.
4. Glycerol is viscous & trying to pour 3ml from a cylinder is inaccurate (residual quantity
remain in cylinder) so the 3ml of Glycerol can now be added by pouring it into 17ml of
bicarbonate solution & carefully making the volume in the measuring cylinder up to
20ml, So we will be sure that 3ml of the Glycerol are taken.
** Viscous Liquids such as Glycerol can be measured by difference.
C. Weight & Volume Relationship:
* If we add 5ml of water to 95 ml of water, then we will have a total volume of 100 ml of
water.
* However, if we add 5g of salt to 95 ml of water & stir the mixture until the salt has
dissolved, then we will have a total volume of solution above 95 ml, but, may be less than
100 ml or may be greater.
The salt occupies a space & displaces some of the water such that the total volume of
solution is more than 95ml.
Displacement Volume: Is the quantity of solvent that is displaced by one gram of
solid during dissolution.
Exp. 3: Calculating the displaced volume:
Procedure:
1. Weight 5g of sodium chloride & put them in 25 ml measuring cylinder.
2. Add 20 ml of water & dissolve the salt using a glass rod.
3. Measure the total volume of solution.
4. Determine the Displacement volume / g.
Displacement volume = volume of solution – volume of water (20 ml)
Weight of salt (5g)
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