Unformatted text preview: PHCY 6101- Dose Form Lab Fall 2010 LABORATORY 2
Introduction to the use of prescription balance class III (formerly class A)
Learn how to determine the sensitivity requirement of a prescription balance
Practice the use of the prescription balance and use of the Aliquot method
Practice measurements of liquid chemicals: correct use of graduate cylinders,
pipettes, and calibration of a medicine dropper.
Remington's (18th edition): pp 69-82.
Pharmaceutical Dosage Forms and Drug Delivery Systems: H. Ansel, N. Popovich
and L. Allen, 8th edition (2005), Appendix C, pp 685-695.
Measurement of Solids
CHARACTERISTICS OF THE PRESCRIPTION BALANCE
Although the mortar and pestle are commonly used as a symbol of pharmacy,
the ever-present (required by law) prescription balance could as appropriately
represent pharmacy. As the prescription balance is used daily to weigh vital medicinal
agents, it is imperative that the pharmacist completely understands its characteristics
and correct usage.
Prescription balances are termed class III (formerly class A) balances, which
meet the prescribed standards of the National Institute of Standards and Technology.
The sensitivity of a balance is usually referred to as the sensitivity requirement (SR),
which is defined as the maximum change in load that will cause a specified change,
one subdivision on the index plate, in the position of rest of the indicating element of
the balance. Prescription balances have a SR of 6 mg with no load and with a load of
10 g on each pan. The smaller the weight required moving the indicator one
subdivision, the more sensitive is the balance. Most class III prescription balances have
a maximum capacity of 120 g and bear a statement to that effect. If no information is
given, the nominal capacity of a prescription balance is assumed to be 15.5 g. The
prescription balance class III comes with a set of analytical weights, which meet the
National Bureau of Standards’ specifications for analytical weights. These weights LAB 2: Pharmaceutical measurements 1 PHCY 6101- Dose Form Lab Fall 2010 consist of one 50-g, one 20-g, two 10-g, one 5-g, two 2-g, one 1-g, one 500-mg, two
200-mg, one 100-mg, two 20-mg, one 10 -mg, and one 5-mg weight.
Balances with similar SR of a prescription balance should be used for all
weighing operations required in prescription compounding. The USP directs that to
avoid weighing errors of 5% or greater, which may be due to the limit of accuracy of
the prescription balance, one must weigh a minimum of 120 mg of any substance in
each weighing (5% of 120 mg being the 6 mg SR, or error inherent with the balance). If
a smaller weight of material is desired, it is directed that the pharmacist mix a larger
calculated weight of the ingredient (120 mg or more), dilute it with a known weight of
an inert dry diluent (as lactose), mix the two uniformly, and weigh an aliquot portion of
the mixture (again 120 mg or more) calculated to contain the desired amount of
agent. The class III balance with a capacity of 120 g should be used for all weighing
required in prescription compounding.
Electronic balances are available in various sensitivities. The one most
commonly used in prescription compounding has a readability of 0.001 g;
consequently, the least amount that can be weighed is 20 times that, or 20 mg.
Electronic balances are much faster and easier to use than prescription balances. The
digital readout is easy to read and the balance is quite versatile and easy to clean,
and it has a relatively small footprint.
LEVELING OF THE BALANCE
The prescription balance should always be kept in a well-lighted location,
placed on a firm, level counter approximately waist-high to the operator. The area
should be as free from dust as possible and in an area that is draft-free. When not in
use, the balance should always be kept with the weights off and the beam in the fixed
or locked (arrested) position.
Before weighing the balance must be made level. This is accomplished with the
leveling screws on the bottom of the balance. The arrest knob must be released
(unlocked) and the balance adjusted to obtain equal swings of the pointer level to the
right and left or up and down (depending on the type of balance) or to obtain
stoppage or rest with exact alignment of the pointer. The rider on the graduated
beam or the indicator of the dial on the front panel (depending on the type of
balance) must be at its zero position at all times during operations.
Arrest knob turned right = locked or arrested position
Arrest knob turned left = unlocked or released position
DETERMINATION OF THE SENSITIVITY REQUIREMENT
The National Formulary (NF) calls for a sensitivity requirement of 6 mg for a
prescription balance. This could be determined by the following procedure:
1. Level the balance and determine the rest point. The balance lid is to be closed to
prevent drafts from affecting the oscillations.
2. Place a 6-mg weight on the right pan. The rest point should not shift less than one
division on the indicator scale.
3. Level the balance and determine the rest point with a 10-g weight in the center of
4. Place a 6-mg weight on the right pan. The rest point should not be shifted less than
Since few, if any, set of weights include a mass of less than 10 mg, the following
LAB 2: Pharmaceutical measurements 2 PHCY 6101- Dose Form Lab Fall 2010 practical procedure is suggested to determine the sensitivity requirement in the
1. Level the balance and determine the rest point. The balance lid is to be closed to
prevent drafts from affecting the oscillations.
2. Record the number of scale divisions the index pointer is shifted by a 20-mg and a
10-mg weight. In some balances you may have to use a combination of 10, and
3. Plot the weight against the number of scale divisions shifted by each weight on a
4. Draw a straight line through the points, and from the straight line read the weight
that will cause a shift of one scale division.
5. Repeat with a 10-g weight on each pan.
USE OF THE PRESCRIPTION BALANCE
The pharmacist must not only maintain a proper prescription balance, but he
must employ the correct weighing technique and be conscious of the limitations of
the balance for the particular quantity of drug to be weighed.
Medicinal substances are weighed on powder papers or weighing
dishes/boats. The paper/dish protects the pans from chemical action and eliminates
the need for repeated washing of the pans. A new paper/dish for each item prevents
contamination. Another advantage to utilizing a paper/dish is that it serves as a
Weighing papers should have a glazed surface or should be made of
aluminum foil so that no appreciable amount of the drug will adhere to the paper. This
is especially important when small quantities are to be weighed. A paper should be
chosen so it is of reasonable size giving a maximum weighing area without touching
any part of the balance except the pan. The papers should be creased diagonally
from one corner to the opposite corner or the edges turned up (aluminum foil) before
placing on the pans. If desired, tared watch glasses or small beakers may be used on
the balance pans instead of papers.
In using a prescription balance, neither the weights nor the substance to be
weighed should be placed on the balance while the beam is free to oscillate.
Usual guidelines to weighing technique are:
1. Before weighing, powder papers of equal size should be placed on both pans of
the balance and the equilibrium of the balance tested by releasing the arresting
knob. If the balance is off because of differences in the weight of the powder
papers, small tearings of powder paper must be added to the light pan. Powder
papers taken from the same box can vary in weight by as much as 65 mg. If
balance equilibrium is not established after the papers are placed on the pans, an
error of more than 30% can result in weighing 200 mg of material.
2. When balanced (index pointer is at zero), the balance is placed in the arrested
position, the lid is opened and the desired weights added to the right-hand pan
and/or the weigh beam.
3. An amount of substance considered to be approximately the desired weight is LAB 2: Pharmaceutical measurements 3 PHCY 6101- Dose Form Lab 4.
5. Fall 2010 carefully placed on the left hand pan with a spatula and the lid closed.
The arrest knob should then be slowly released to check the equilibrium. The
arresting knob is most conveniently operated by the left hand.
If the substance is in excess, the balance is arrested again and a small portion of
the substance removed with the spatula. The same procedure is used if more
substance is needed. A pharmacist should always remember that when a stated quantity of a
substance was weighed, this includes not only the true weight but also the possible
excessive or deficient weight by virtue of the sensitivity of the balance. Knowing the
sensitivity of the balance, one may calculate the percent of possible error for a given
amount to be weighed. For example, assume that 5 mg produce a change of one
index plate division in the rest point and 200 mg of drug is to be weighed, the actual
weight could be between 195-205 mg. The percentage of possible error is calculated
by the ratio, 5 mg
producing a value of X = 2.5%. Thus, the inherent percentage of error for any given
situation may be expressed as, Percentage of Error = Sensitivity Requirement X 100
Quantity Desired The same relationship may be used to calculate the smallest amount that can
be weighed within a particular permissible percentage of error. Assuming the
sensitivity requirement of a balance is 5 mg and the permissible percentage of error is
5%, one may calculate the smallest quantity that can be weighed within this limit or
error. A ratio is set up as before, 5mg
and will produce a value of X = 100 mg. Thus, the smallest amount that can be
weighed with a certain permissible percent or error may be expressed as
Smallest quantity to be weighed = _____SR
Permissible Percent of Error In order to achieve 99% accuracy, that is, to permit a 1% error, the amount
weighed must be at least 100 times as great as the sensitivity requirement of the
balance. To achieve 90% accuracy or to allow a 10% error, one must weigh at least 10
times as much as the sensitivity requirement of the balance. LAB 2: Pharmaceutical measurements 4 PHCY 6101- Dose Form Lab Fall 2010 ALIQUOT METHOD
When the amount of drug is too small to be weighed directly on the
prescription balance (<120 mg), the aliquot method of weighing is used. An aliquot
part is defined as any part that is contained a whole number of times in a quantity;
e.g. 2 is an aliquot of 10, since 10/2 = 5, 2 is referred to as the fifth aliquot of 10. Also, 2
mg of drug uniformly dispersed in exactly 98 mg of diluent represents a fifty-fold
3. To use the aliquot method in compounding, follow three steps:
Weigh a multiple of the quantity desired (prescription balance).
Dilute the multiple in a suitable manner.
Weigh or measure an aliquot of the final mixture (dilution). Example 1:
Suppose the smallest amount of drug that we can weigh on a Prescription
Balance is 100 mg, but we need only 20 mg. Procedure is to weigh out 100 mg of drug
(which is multiple of 20 mg). We then dilute it with 900 mg of lactose to make a total of
1000 mg (1 g). This 1000 mg of mixture contains 20 mg of drug in each 200 mg. We
can now weigh out 200 mg of mixture (fifth aliquot of 1000 mg) and it will contain 20
mg of the drug.
A pharmacist needs 60 ml of silver nitrate solution 1:5000.
For the above preparation we need 1000 mg
x 60 ml = 12 mg AgNO 3
The procedure is to weigh out 120 mg AgNO3 and dissolve it in distilled water.
The volume is made up to 100 ml. This will give a 1.2 mg/ml solution. 10 ml of this
solution should be measured and diluted to 60 ml.
Measurement of Liquids
In pharmacy, measuring of liquids should be done by selecting the measuring
device most appropriate to the volume of liquid to be measured and the desired
degree of accuracy. With liquids, the more narrow the column of liquid, the more
accurate is likely to be the measurement. Many types of devices are used in this
operation, depending on the kind and quantity of liquid to be measured. The three
most important devices used in liquid measurement in pharmacy are graduated
cylinders (or cylindrical graduates), pipettes (pipets) and droppers.
Cylindrical graduates are generally calibrated in metric units and are available
in a wide variety of capacities, ranging from 5 to 1000 mL or more. In measuring small
volumes, less than 1.5 mL, the pharmacist should use a pipet and a pipet filler (pipet
helper), which will also allow accurate delivery of the liquid. The pharmacopeial LAB 2: Pharmaceutical measurements 5 PHCY 6101- Dose Form Lab Fall 2010 medicine dropper is used for measuring small quantity of liquid. The dropper is
constricted at the delivery end to a round opening having an external diameter of 3
mm. A vertically positioned pharmacopeial medicine dropper delivers a drop of
water weighing between 45 and 55 mg. In using droppers the pharmacist must realize
that a drop is not a unit of volume as the volume of a drop delivered from the same
dropper depends on different factors. For a given dropper, the volume of a drop
varies as a function of the pressure applied and the temperature, and as the function
of the density, viscosity, and surface tension of the liquid delivered. To maintain
accuracy, one must remember the position of the dropper while pouring the drop.
Errors in measuring may be made by the operator if he tilts the measuring
device or fails to align the graduation mark and the bottom of the meniscus (meniscus
= concave or crescent shaped surface of liquid that bulges downward). This is known
as parallax effect. Parallax is the apparent displacement of an object that follows a
change in the position from which it is viewed. This phenomenon may cause
measuring errors larger than 5%, which are unacceptable from a pharmaceutical
point of view. To avoid this error, the graduate/other device is raised and held straight
up so that the graduation point to be read is on a level with the eye, keeping the
brown surface (if any) towards the back, and the liquid is measured. LAB 2: Pharmaceutical measurements 6 PHCY 6101- Dose Form Lab Fall 2009 In using graduates, the pharmacist pours the liquid into the graduate slowly,
observing the level. In measuring viscous liquids, adequate time must be allowed for
the liquid to settle in the graduate, as some may run slowly down the inner sides of the
graduate. It is best to attempt to pour such liquids toward the center of the graduate,
avoiding contact with the sides. In emptying the graduate of its measured contents,
adequate drain time should be allowed.
When pouring liquids from bottles, good pharmaceutical technique is to keep
the label on the bottle facing up; this avoids the possibility of any errant liquid running
down over the label as the bottle is righted after use. The bottle orifice should be
wiped clean after each use.
Prescriptions consisting of several liquids should not be compounded in
prescription bottles, but they must be accurately measured in a suitable graduate and
then transferred into graduated or un-graduated prescription bottles. The graduations
in the prescription bottles are usually very inaccurate. LAB EXERCISES
Exercises using the torsion/prescription balance:
A. Obtain a box with a standard set of weights (in a drawer of the lab center
island) with a number that matches your balance number. Determine the
sensitivity requirement (SR) of your balance twice. 1)Using a 20-mg and a 10mg weight. Record the number of scale divisions the index pointer is shifted. Plot
the weight against the number of scale divisions shifted by each weight on a
graph paper (provided). Draw a straight line (as best as possible) through both
points, and from the straight line read the weight that will cause a shift of one
scale division, if needed. 2) Repeat the same procedure and graphing with a
10-g weight on each pan.
B. Fill one medium size weighing boat with charcoal and one with lactose and
bring to your bench. Using the aliquot method or the least weighable amount
method (consider USP-approved error and SR) prepare the following charcoal
dilution (trituration), have it checked by the instructor or TA and discard the
mixture into trash can. Return unused charcoal and lactose to original
containers. Do not mix spatulas and try to not contaminate one chemical with
Mix and make trituration Instructions:
1. Use glass mortar and pestle and plastic spatulas (to help fluffing the powders as
you mix them).
2. Prepare a dilution to allow an aliquot to contain the required amount of charcoal
in lactose, avoiding excessive waste of unused dilution (trituration).
3. In preparing the dilution to obtain the required aliquot, start with weighed amount
of charcoal. When mixing a small amount of drug (mg level) with a large amount LAB 2: Pharmaceutical measurements 7 PHCY 6101- Dose Form Lab 4.
6. Fall 2009 of a second ingredient, the drug is placed in the mortar with equal bulk of the
other ingredient. The two ingredients are triturated until intimately mixed. Then, an
equal bulk of the second ingredient is added to the mixture, and the powders are
triturated until intimately mixed. The material is triturated by geometric addition
until all gone. This method is known as geometric dilution.
Get dilution checked by instructor/TA for homogeneity.
Weigh out the aliquot and the additional lactose required to complete the Rx
After final Rx is completely mixed, get it checked by instructor/TA for homogeneity
and discard into trash cans. Exercises using electronic balances:
C. Perform the following weighing operations using electronic balances.
i. Weigh 0.5 g (gram) of dextrose. Ask instructor/TA to check the amount
weighed. Return weighed powder to original container.
ii. Weigh 50 mg of dextrose, by instructor/TA, return to container.
Exercises using glass cylinders:
D. Fill a large beaker available in your locker with tap water. Perform on your bench
all of the following measurement operations and get all measured volumes
checked at the same time by instructor/TA. Discard checked volumes in sink.
i. Using 10-mL cylinder measure 2.5 mL of water.
ii. Using 25-mL cylinder measure 12 mL of water.
iii. Using 50-mL cylinder measure 28 mL of water.
iv. Using 100-mL cylinder measure 56 mL of water.
Calibration of a prescription bottle:
E. Obtain one 3-ounce prescription bottle and fill with tap water to the 90-ml mark.
Transfer the contents of the bottle to a 100-ml cylindrical graduate. Record the
volume. Observe the difference between both measurements. Return the
prescription bottle separated from cap to the container on the counter.
Exercises using pipettes:
F. Using pipettes, pipette helpers, and liquids provided on top of center island in
lab, perform the following measuring operations and get the measured volumes
checked by instructor/TA.
i. Using a 5-mL pipette measure 2.7 mL of water, by instructor, discard into
large container provided.
ii. Using a 10-mL pipette measure 4.5 mL of water, by instructor, discard into
large container provided.
iii. Using a 10-mL pipette measure 5.7 mL of alcohol, by instructor, discard into
the bottle provided.
Exercises using droppers:
G. Obtain one disposable “dropper” (transfer pipette) and get your 10-mL
cylindrical graduate. Using droppers, and liquids provided on top of center
island in lab, perform the following measuring operations:
i. Holding a medicine dropper vertically, count and record the number of
LAB 2: Pharmaceutical measurements 8 PHCY 6101- Dose Form Lab Fall 2009 drops of water required to fill a 10-ml cylindrical graduate to the 3-ml mark.
ii. Repeat the previous measurement holding the dropper at a ~20-30° angle
with the horizontal line.
iii. Holding a medicine dropper vertically, count and record the number of
drops of alcohol required to fill a 10-ml cylindrical graduate to the 3-ml mark.
H. Measure glycerin using droppers attached to the glycerin containers. Holding a
medicine dropper with a slight inclination, count and record the number of
drops of glycerin required to fill a 10-ml cylindrical graduate to the 3-ml mark.
Discard into sink by rinsing the graduate thoroughly (glycerin is water-soluble and
I. Prepare a 1% solution (10 ml) of sodium lauryl sulfate (use mask or handle
powder slowly while weighing to prevent inhalation).
i. Holding the dropper vertically, count and record the number of drops
required to fill a 10-ml cylindrical graduate to the 3-ml mark.
ii. Heat the surfactant solution to 70° C (use thermometer to monitor the
temperature, not to stir the solution). Using this warm solution, repeat the
CALCULATIONS/RESULTS (to be submitted at the end of lab)
At the end of lab submit on a sheet of paper all results, calculations, graphs, and
performed measurements to the instructor/TA for grading (lab assignment = BONUS).
Include your name, section (TUE, WED, THU), locker number and balance number.
5. Submit the graph paper with SR determinations. Include your name, section (TUE,
WED, or THU), locker number and balance number.
Submit the measurements (number of drops = gtt) performed with droppers
(exercises 9, 10 and 11 above).
Calculate the percent of error that would be incurred in measuring 50 mg of
atropine sulfate using the balance on your work bench.
If the permissible error of your torsion balance were 15%, which would be the
minimum quantity of a drug that you could weigh accurately using it?
Show calculations for the following compounding situation. A pharmacist needs
0.15 ml of flavoring oil (orange oil) to prepare an oral liquid. He has alcohol to be
used as a solvent and only a 2 mL pipette with 0.2 ml graduations. How could this
volume of orange oil be obtained? LAB 2: Pharmaceutical measurements 9 ...
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- Dose Form Lab, prescription balance