The document discusses the aliquot method for weighing and measuring solids and liquids when the desired quantity is below the measurement device's limits. It involves weighing a known quantity of the material and diluting it with an inert substance to obtain the desired amount. Examples are provided for weighing specific amounts of various drugs using this method. Calculations for adjusting doses based on factors like age, weight, and surface area are also described.

1. ALIQUOT METHOD WEIGHING AND
MEASURING

2. ALIQUOT METHOD FOR
SOLIDS

3. • If the quantity needed is greater the balance’s
capacity, the material may be weighed in
portions.
• For example, if 160gm of an ointment is to be
measured on the class A prescription balance
(which has an upper limit of 120gm)
–80gm of the ointment can be weighed twice
–40gm of the ointment can be weighed four
times

4. • Generally the potential error should be 5% or
less
• When the desired quantity of ingredients is
below the lower limits of class A prescription
balance the aliquot methods of weighing is
preferred
• An aliquot is a whole number part of a given
quantity
• For example,
– Five is an aliquot of ten, to be more specific, five is the
second aliquot of ten, two is the fifth aliquot of ten

5. • The minimum weighable amount (least
weighable quantity or LWQ) of that compound
is weighed and mixed with a certain quantity
of an inert material to obtain a stock mixture

6. • After thoroughly mixing the drug and the inert
material by employing the technique of
geometrical dilution.
• Weigh an aliquot of the stock mixture which
provides the desired quantity of the material
• Geometric dilution or mixing technique is
useful for uniformly mixing ingredients of
unequal amount
• For example: mixing of 2gm of Asprin with
14gm of lactose

7. • Mixing should be performed by geometric
dilution:
–Mix 2gm of aspirin with approximately 2gm
of lactose
–To this 4gm of mixture add 4gm of lactose
and mix thoroughly
–To this 8gm of mixture add the remaining
8gm of lactose and mix thoroughly

8. Examples of aliquot method of
weighing
Example 1: if it is desired to weigh 60mg of
salicylic acid on a class A prescription
balance with sensitivity requirement of 6
mg, and potential error of not more than
5%, explain how you would perform the
weighing.

9. a. Figure out the LWQ on the given balance
The LWQ of salicylic acid on the class A
prescription balance
5%=6mg*100/LWQ
LWQ=600/5=120mg
b. Weigh 120mg of salicylic acid. Some multiple
of this quantity can also be weighed but to
avoid the waste. LWQ approach is preferred

10. c. Weigh a suitable amount of lactose or some
other compatible inert material. The amount
should be equal to or in some multiple of the
LWQ. In this example, an amount of 120mg
lactose is chosen. Therefore, weigh this
amount.

11. d. Mix the salicylic acid and lactose thoroughly.
The total mixture now is 240mg. From this
mixture, weigh an aliquot which provides
60mg of the salicylic acid
By method proportion
120mg of SA/240mg of the mix= 60mg
of SA/Xmg of the mix
X=240*60/120=120mg of the mixture
• Instead of weighing 60mg of salicylic acid
directly, 120mg of the mixture of salicylic acid
and lactose should be weighed

12. Example two: explain how you would weigh gr
ss of atropine sulfate on a prescription balance
with a sensitivity requirement of 10mg
Hint: 1gr=64.8mg

13. Example 3: show how would you weigh 20mg of
menthol on a class A prescription balance.

14. Liquid measurements
• Parallax error is one of the main source of
error in liquid measurements.
• The liquid surface in a container is not even,
and a ‘’miniscus’’ is formed. For most liquids
the surface is concave.
• The uneven surface is due to surface tension,
and if not read properly, may result in an error
commonly known as parallax error

16. Aliquot method for liquids
• Measurement of volumes less than that of the
lower limitations of the measuring device
requires an aliquot approach.
Example 1: how would you measure 0.025ml of
a concentrate which is to be diluted to 60ml
following the measurement? You are provided
with a 5ml pippet with making in units of 1ml,
a measuring cylinder, and a container to
dispense the final product.

17. Example 2:
FD & C dye………………………….0.25mg
Puried water qs. Ad…………………..90ml
0.5% W/V OF FD &C dye stock solution is
provided explain how you would make this
product.

18. Procedure:
a. Calibrate the final container 90ml. Determine
the amount of stock solution which contains
0.25mg of the dye
X=0.05ml
500mg of the dye
100ml of the stock
solution
=
0.25mg of the dye
X ml of stock
solution

19. b. One can either measure the 0.05ml by a
medicinal dropper after calibrating it
• Or, the stock solution can be diluted such that
the diluted stock solution would provide a
measurable quantity containing 0.25mg of the
dye.
• For dilution, measure one ml of the stock
solution and add a sufficient quantity of
purified water to obtain 100ml of diluted stock
solution.
C1V1=C2V2

20. c. Figure out the amount of diluted solution
which provide the required 0.25mg of the dye
X=5 ml
Measure 5ml of the diluted stock solution which
contains the required 0.25mg of the drug
=
5mg of the dye
100ml dil stock
soln
0.25mg of the dye
X ml dil stock
soln

21. d. Transfer 5ml of the diluted stock solution or
0.05ml of the original stock solution to the
final container and add a sufficient quantity of
water to fill up to the calibrated 90ml mark

22. Example 3:
Rx
Gentian violet……………………0.03g
Purified water qs ad………………floz ii
Show how would you prepare this prescription
using a class A prescription balance.

23. Procedure:
a. Calibrate the final container to 60ml. Since
0.03 is not weighable on class A prescription
balance, the aliquot method is required. Weigh
120mg of gentian violet using the balance.

24. b. Make a stock solution such that an aliquot of
stock solution will provide 0.03g of gentian
violet. Dissolve 0.12g of gentian violet in
purified water to make 20ml (this number is
arbitrarily chosen ) of the stock solution.
c. From the stock solution, measure an aliquot
that contains 0.03g of the gentian violet
0.03g of gentian violet
X ml of the stock soln
0.12g of gentian violet
20ml of the stock
solution
=

25. d. Transfer 5 ml of the stock solution into the
final container, and add a sufficient quantity of
purified water to make up to the calibrated
mark of 60ml

26. Practical problems
• How would you prepare the following
prescription? Show stepwise procedure and all
the calculations involved.
Rx
Propranolol HCL……………………2mg
D5w solution qs……………………..fƷ iv

27. • How do you obtain 10mg of codeine using a
prescription balance with sensitivity
requirement of 5mg? The potential error
should no be greater than 8%

28. How you would prepare the following
prescription?
Rx
L.C.D…………………………………0.005ml
Water qs ad………………………………50ml

29. • Explain how would you weigh gr iss of
pseudoephedrine on a prescription balance
with sensitivity requirement of 6mg and
potential error of 2% or less

30. • If a pharmacist needs 0.6ml of a drug and has a
10ml graduated cylinder with markings from 2
to 10ml in units of 1ml, explain how you
would obtain the required quantity of 0.6ml.
Use water as a diluent.

31. • If 0.75ml of a drug is needed and a pharmacist
has a 10ml graduated cylinder with markings
from 1 to 10ml in units of 1ml, explain how
the required quantity of 0.75ml can be
measured. Use water as diluent.

32. • Explain how to weigh one grain of
acetaminophen on a prescription balance
having sensitivity requirement of ¼ grain.

33. • A prescription requires 0.015ml of a drug
concentrate. Using a pipet with markings from
one to ten in units of 1ml and a 100ml
graduated cylinder, explain how you would
obtain the required quantity of drug
concentrate? Use water as diluent.

34. • Explain how one can obtain 2 minims of a
liquid concentrate using a 5ml pipet with
graduations from 1-5ml in units of 0.5ml, and
100ml measuring cylinder. Use water as a
diluent.

35. Practical problems
• Explain how to weigh 30mg of codeine with
an error not greater than 5%, using lactose as a
diluent. The balance has a sensitivity
requirement of 6mg. You only have 1gm
lactose left in stock.

36. • A prescription calls for 50mg of
chlorpheniramine maleate. Using a balance
with sensitivity requirement of 6mg, explain
how you would obtain the amount required of
chlopheniramine maleate with an error not
greater than 5%

37. • Explain how to weigh 20mg of drug with an
error less than 2.5% using lactose as diluent.
The balance has a sensitivity requirement of
6mg.

38. Calculation Of Doses And
Formulas

39. Calculations Of Doses
• There are many drugs for which there are no
standard doses, and for these drugs calculation
of the dose required is dependent on a patient
characteristic, such as body weight or surface
area.

40. • Example 1: what dose of salbutamol would
you recommend for a 10 year old child, weight
30kg, when the recommended dosage is
100mcg/kg?

41. • Example 2: what dose of vincristine is required
for a 92kg patient with a body surface area
(BSA) of 2.0m2 and a recommended dosage
of 1.4mg/m2 ?

42. • In some instances it is more appropriate to use
the patient’s ideal body weight (IBW)
• Calculation of IBW is necessary for adult
patients whose body weight is more than either
30% above or below the average adult weight
of 70kg (i.e. for obese or emaciated patients)

43. • Ideal body weight (kg) is calculated from the
patient’s height (H, expressed in centimetres)
using the following equation:
–Males IBW=(0.9x H)-88
–Females IBW=(0.9x H)-92

44. • BSA=?
Example 3: the dosage of chemotherapeutic
agent cyclophosphamide can be expressed in
mg/m2 in some regimens and in mg/kg in
other regimens. If the doses required are either
60mg/kg or 800mg/m2 for a male adult patient
who weighs 100kg and measures 1.8 metres,
give the three alternative doses to be
administered based on ABW, IBW, and BSA.

45. Drug dosage based on age
• Age of a patient is one of the most important
considerations for drug dosage modification
• Some of the important pharmacokinetic
parameters change with age.
• In general, the drug elimination which is
comprised of drug metabolism and excretion is
less functional in newborns
• Improves with age as they grow into healthy
adult individuals

46. • As they grow further to an age of 65 or above,
the elimination declines
• It is important to remember that age is not the
only valid criterion for dose modifications.
• Young’s equation: preferably from 1 to 12
years of age
Dose of child=(age of child in years)/(age in
child years + 12)*adult dose

47. • Cowling’s equation:
Dose of child=age of child in years at
next birthday/24*adult dose
• Fried’s equation: preferably from birth to one
year of age
Approx. child dose=age in months/150*adults
dose

48. • Example 1:an adult dose of drug is 500mg,
what is the dose for a 2 year old child?
• According to young’s formula
Dose of child=(age of child in years)/(age of
child in years + 12)*adult dose
=2/14*500mg
=71.42mg of drug

49. • Example 3: calculate the dose of valium© of a
13month old child by the young’s method,if
the adult dose is 10mg. valium© is available as
a 5mg/5ml oral solution. How would you
administer this medication
• Dose of child=(age of child in years)/(age in
child years + 12)*adult dose
• 0.83mg
• 5mg/5ml=0.83mg in 0.83ml

50. • Example 4: product information on lorabid©
shows that its usual adult dose is 200-400mg
Q12h. What would be the range of this
medication for a patient born on February 9,
1993. how would you administer the drug from
one of the available lorabid© medications?
Assume that the prescription was written on
may 1996.

51. • The patient is a child and would be four years
old on the following birthday.
• Therefore cowling’s equation will be used
• Cowling’s equation:
Dose of child=age of child in years
at next birthday/24*adult dose
• 4/24*200=33.33mg
• 4/24*400=66.66mg

52. Enlarging And Reducing
Formulas
• When a pharmaceutical product is to be
prepared extemporaneously, a reference
formula is usually required.
• These formulae can be found in the
pharmaceutical reference sources such as:
– British pharmacopia
– United state pharmacopia
– Indian pharmacopia etc.

53. • Frequently, the weight or volume of the
preparation given in the reference formula will
not be the same as that which must be
prepared, in which case the quantities of each
ingredient must be increased or reduced.

54. • Example 1: you are asked to prepare 300ml of
single chloroform water. The formula is given
below:
Concentrated chloroform water……………25ml
Purified water..………………………..to 1000ml

55. • Example 2: You are asked to prepare 5000g
zinc cream BP. The formula is given is below:
Zinc oxide---------------------------------------320g
Calcium hydroxide----------------------------0.45g
Oleic acid----------------------------------------5ml
Arachis oil--------------------------------------320ml
Wool fat -----------------------------------------80g
Purified water to produce-------------------1000g

56. • You are required to dispense 50g of hydrous
ointment BP. The formula is given below:
Wool alcohols ointment……………………500g
Phenoxyethanol……………………………10g
Dried magnesium sulphate…………….…..5g
Purified water……………………………485g

57. Formulae in part or percentage
• This type of formula is usually written by a
prescriber requesting special ointments or creams.
• Example 1: prepare 30g of the following ointment
Hydrocortisone ointment…………………..…..25%
White soft paraffin……………………………..50%
Liquid paraffin……………...………………….25%

58. • Example 2: prepare 400g of the following
cream
Betamethasone cream……………………1part
Aqueous cream………………….……….3parts

59. Practical problems
Acid gentian mixture BP
Conc. Compound gentian infusion….…100ml
Diluted hydrochloric acid……………….50ml
Double strength chloroform water….…500ml
Water to ……………………………...1000ml
Send:250ml

60. Potassium citrate mixture BP
–Potassium citrate…………………….300g
–Citric acid monohydrate………………..50g
–Lemon spirit…………………………….5ml
–Quillaia tincture…………………..…10ml
–Syrup…………………………………..25ml
–Double strength chloroform water….300ml
–Water…………………………to 1000ml
Prepare: 150ml

61. Chloral elixir paediatric BP
–Chloral hydrate ……………………..200mg
–Water………………………………….0.1ml
–Blackcurrant syrup…………………….1ml
–Syrup………………………………to 5ml

62. Paediatric ferrous sulphate mixture BP
–Ferrous sulphate………………………60mg
–Ascorbic acid…………………………10mg
–Orange syrup…………………………0.5ml
–Double strength choloform water……2.5ml
–Water to………………………………..5ml
Send:200ml

63. Coal tar and zinc ointment
–Strong coal tar solution…..……………100g
–Zinc oxide……………………………..300g
–Yellow soft paraffin…………..……….600g
Send: 50g

64. Zinc and salicylic acid paste BP
–Zinc oxide….………………………..24%
–Salicylic acid………………………….2%
–Starch…………………………………24%
–White soft paraffin……………………..50%

65. Magnesium hydroxide mixture BP
• Magnesium
sulphate……………………47.5g
• Sodium
hydroxide………………………..15g
• Light magnesium
oxide………………..52.5g
• Chloroform
…………………………….2.5ml
• Water

66. Prepare 30g of the following ointment
• Fluocinolone acetonide cream……………10%
• Aqueous cream……………………….to 100%

67. Prepare 75g of the following cream
• Betamethasone cream…………..………1part
• Aqueous cream………………………….4parts

68. Prepare 80g of the following ointment
• Dithranol ointment……………………..1part
• White soft paraffin………………to 4parts

69. PERCENTAGE
CALCULATIONS

70. Percentage
• The word percent means hundredths of a whole and is
represented by the symbol
• The word percentage indicates ‘’ rate per hundred’’
and indicates parts per 100 parts.
• 1% is the same as the fraction 1/100 or the decimal
fraction 0.01.
• to express a percent as a decimal, note that percent
means division by 100.
• To change a fraction to a percent, first change the
fraction to a decimal and then multiply it by 100.

71. Percent concentration expressions
• The concentration of a solution may be
expressed in terms of the quantity of solute in
definite volume of solution or as the quantity
of solute in a definite weight of solution.
• The quantity (or amount ) is an absolute value
(eg. 10ml, 5gm etc), whereas concentration is
the quantity of a substance in relation to a
definite volume or weight of other substance
(eg., 2g/5g, 4ml/5ml, etc)

72. • Percent can be
1. Percent weight in volume, %w/v: number of
grams of a constituent (solute) in 100ml of
liquid preparation (solution)
2. Percent weight in weight (percent by weight),
%w/w: number of grams of a
constituent(solute) in 100gm of preparation
(solution)

73. 3. Percent volume in volume (percent by
volume), %V/V: number of millilitres of a
constituent (solute) in 100ml of preparation
(solution)

74. Example 1:
If 4g of sucrose are dissolved in enough water
to make 250ml of solution, what is the
concentration in terms of 1% w/v of the
solution?
By the method of proportion:
4gm/250ml=Xgm/100ml
Solving for X, we get:
X=(100*4)/250=1.6g
Ans: 1.6gm in 100ml is 1.6% w/v

75. Example 2:
 an injection contains 40mg pentobarbital
sodium in each millilitres of solution. What is
the concentration in terms of %w/v of the
solution?
by the method of proportion
40mg/1ml=xmg/100ml
Solving for X, we get:
X=(100x40)=4000mg or 4 g
Ans=4 g in 100ml is 4%w/v

76. Example 3:
How many grams of zinc chloride should be
used in preparing 5 L of the mouth wash
containing 1/10%w/v of zinc chloride?
1/10%=0.1%=0.4gm in 100ml
By the method of proportion:
0.1g/100ml=Xg/5000ml
X=(0.1x5000)/100
Ans:=5gm

77. Practice problems
1. A pharmacist dispenses 180 prescriptions a
day. How many more prescriptions does he
need to dispense each day to bring a 15%
increase?
2. If an ophthalmic solution contains 10mg of
pilocarpine in each millilitres of solution
would be needed to deliver 0.5mg of
pilocarpine?

78. 3. A pharmacist prepared a solution containing
10 million units of potassium penicillin per 20
ml. how many units of potassium penicillin
will a 0.5ml solution contain?
4. A cough syrup contains 5mg of
brompheniramine maleate in each 5ml dose.
How many milligrams of bromphniramine
maleate would be contained in a 120ml
container of the syrup?

79. 5 . What is the percentage strength, expressed as
% W/W, of a solution prepared by dissolving
60gm of potassium chloride in 150ml of
water?
6. How many grams of antipyrine should be
used in preparing 5% of a 60ml solution of
antipyrine?

80. 7.How many milligrams of a drug should be
used in preparing 5 L of a 0.01% drug
solution?
8. How many litters of 2%w/v iodine tincture
can be made from 108g of iodine?
9. How many millilitres of 0.9% sodium chloride
can be made from 325gm of sodium chloride?
10. If a physician order 25mg of a drug for a
patient, how many millilitres of a 2.5%W/V
solution of the drug should be used?

81. 11. A mouthwash contains 0.1%w/v
chlorhexidine gluconate. How much
chlorhexidine gluconate in grams is contained
in 250ml of the mouthwash?
12. What weight of miconazole is required to
make 40g of a cream containing 2%w/w of the
drug?
13. How much arachis oil is required to make
300ml of an emulsion containing 30%v/v of
arachis oil?

82. 14. A patient dissolves two tablets, each
containing 300mg of asprin, in 120ml of water.
What is the asprin concentration (%w/V) of
the solution?
15. Calculate the number of milligrams of
potassium permanganate in 90ml of a 1:500
w/v potassium permanganate solution.

83. Quantity per volume
• Quantity per volume expressions are used to
give the concentration of drugs in solution and
also for clinical laboratory test results.
Example 1: what weight of sodium bicarbonate
(in grams) is required to make 200ml of a 6g/L
solution ?

84. Example 2: a patient has a serum potassium level
of 4 mmole/L
a) How many millimoles of potassium are
present in a 20ml sample of the patient’s
serum?
b) How many milligrams of potassium are
present in this sample? (RMM of
potassium=40)

85. Ratio concentrations
• A ratio concentration is most commonly used
to express the concentration of very dilute
solutions.
• 1:5000 solution of a drug indicates that 1g of
the drug is dissolved in 5000ml (5L) of
solution.
Example 1: how many milligrams of adrenaline
are contained in 10ml of a 1:10000 solution

86. Example 2: what volume of a 1:20000 solution
of adrenaline would contain 50mg of the drug?
Example 3: a patient uses 200ml of a 1:8000
solution of an antiseptic, daily, for 10days.
How many grams of the antiseptic have been
used?

87. Parts as expressions of concentration
• This method of expressing concentration is
similar to ratio expressions except that the
convention is to replace the ratio symbol with
the word ‘in’
–1:1000 solution becomes a 1 in 1000,
but the meaning is unchanged, i.e. 1g
of a drug dissolved in 1000ml of a
solution

88. Example: a 10ml ampoule of a 1 in 200,000
solution of bupivacaine hydrochloride is
administered to a patient. How many
milligrams of bupivacaine hydrochloride does
the patient receive?

89. Converting between expressions of
concentration
• It is frequently necessary to convert between
the various expressions of concentration
Example 1: a solution contains 10mg of drug in
5ml of solution. Express this as a ratio
concentration.

90. Conversions of concentration

91. Dilution and concentration
• The dilution of solutions is one of the most
frequently carried out calculations in
pharmacy.
• A stock solution, or concentrate, must often be
diluted to a particular strength for patient use
• When a solution of a given strength is diluted,
its strength will be reduced.

92. =
Mass of active
before dilution
Mass of active after
dilution
C1=M1
V1
=
C2=M2
V2
C1*V1
= M2=C2*V2
M1=C1*V1
C2*V2
=

93. • For this equation to hold, both concentrations
must be expressed in the same units and both
volumes must also be expressed in the same
units.

94. Practical Problems
• How many millilitres of a 1:5000 solution of
phenylmercuric nitrate can be made from
250ml of a 0.2%w/v solution of the
compound?

95. • How many millilitres of a 10%w/v solution of
an antiseptic must be used to make 4litres of
a 0.25%w/v solution?
• How many millilitres of water must be added
to 250ml of an 18%w/v stock solution of
sodium chloride to prepare a 0.9%w/v sodium
chloride solution?

96. • How many millilitres of a 0.2%w/v solution of
an antiseptic must be used to prepare 1 litre of
a 1:5000 solution?
• What volume of a 1:5000 solution of cetrimide
can be made from 100ml of a 4% solution of
cetrimide?

97. • How many millilitres of water must be added
to 50ml of 13%w/v aluminium acetate solution
to prepare a 0.65%w/v solution?
• If 5ml of a 20%w/v aqueous solution of
furosemide is diluted to 10ml, what will be the
final strength of furosemide?

98. • How many milliliters of 95%w/w sulfuric acid
having a specific gravity of 1.820 should be
used to make 2L of 10%w acid?
• How much water should be mixed with
5000ml of 85%v/v alcohol to make 50%v/v
alcohol?

99. • How many milliliters of water should be added
to 100ml of a 1:125w/v solution to make a
solution such that 25ml diluted to 100ml will
yield a 1:4000 dilution.

100. • Zephiran chloride solution(17%w/v)
purified water qs………………….480ml
sig: one tbsp. diluted to gallon with water to
make a 1:10,000 dilution.
How many milliliters of zephiran chloride
solution should be used in preparing the
prescription?

101. Dilution of solid preparations
• Occasionally, a prescriber may request the
dilution of active ingredients in solid
preparation.
The following formula should be used
=
C1M1 C2M2

102. Example 1: you are supplied with 50g of
salicylic acid ointment 2%w/w. what weight of
emulsifying ointment (diluent) should be
added to reduce the concentration of salicylic
acid to 0.5%w/w?

103. Example 2: how many grams of emulsifying
ointment must be added to 200g of 5%w/w
calamine in emulsifying ointment, in order to
reduce the calamine concentration to 2%w/w?

104. • We wish to dilute an ointment containing 14%
sulfur with petrolatum to make 60gm of an
ointment containing 10% sulfur. How many
grams of 14% sulfur ointment and how many
grams of petrolatum will be necessary to make
the dilution?

105. Concentration
• We can increase the concentration of a given
preparation either by increasing the active
(solute ) or if we can by evaporating the
solvent results in decreased the volume
(amount ) of the total preparation.

106. Example 1: what weight of coal tar extract must
be added to 100g of a cream containing
1%w/w coal tar extract to produce a cream
containing 25%w/w coal tar extract?

107. Example 2: what weight of coal tar extract must
be added to 200g of cream containing 1%w/w
coal tar extract to produce a cream containing
25%w/w coal tar extract?

108. Example 3: a cream (weight 30g) contains
0.1%w/w dithranol. What weight of dithranol
powder should be added to increase the
concentration to 1%w/w?
Example 4: salicylic acid ointment contains
2%w/w salicylic acid. What weight of salicylic
acid powder should be added to 50g of the
ointment to produce a 10%w/w ointment?

109. Example 5: a cream contains 10%w/w coal tar
solution. What weight of coal tar solution
should be incorporated into this cream to
produce 30g of cream containing 12%w/w coal
tar solution?

110. • Example 6: If phenobarbital elixir containing
4%w/v phenobarbital is evaporated to 90% of
its volume, what is the strength of
phenobarbital in the remaining solution?

111. • How many grams of pure coal tar should be
added to 36gm of 4% coal tar ointment to
make a 10% coal tar ointment?