chapter 2

Solutions
Overview
Upon completion of this chapter, you should be able to:
■■ prepare a solution from first principles
■■ select an appropriate container in which to package a solution
■■ prepare an appropriate label for a solution
■■ understand the differing calculations used in the preparation of solutions, including:
Basic strength calculations
Tailored strength calculations
Percentage calculations
Parts calculations
Millimolar calculations.

Introduction and overview of solutions

Solutions are traditionally one of the oldest dosage forms used in the treatment
of patients and afford rapid and high absorption of soluble medicinal products.
Therefore, the compounding of solutions retains an important place in therapeutics
today. Owing to the simplicity and therefore speed of preparation of an ad hoc
formulation, they are of particular use for individuals who have difficulty in
swallowing solid dosage forms (for example, paediatric, geriatric, intensive care
and psychiatric patients), where compliance needs to be checked on administration
(for example, in prisons or psychiatric pharmacy) and in cases where precise,
individualised dosages are required.
Generally, water is chosen as the vehicle Definition
in which medicaments are dissolved, since it Essentially a solution is a
is non-toxic, non-irritant, tasteless, relatively homogeneous liquid preparation
cheap and many drugs are water-soluble. that contains one or more
Problems may be encountered where active dissolved medicaments. Since,
drugs are not particularly water-soluble or by definition, active ingredients
suffer from hydrolysis in aqueous solution. In are dissolved within the vehicle,
uniform doses by volume may
these cases it is often possible to formulate a
be obtained without any need
vehicle containing water mixed with a variety to shake the formulation. This
of other solvents. is an advantage over some
other formulation types, e.g.
British Pharmacopoeia (BP) definition suspensions (see Chapter 3).
(oral solutions)
Oral solutions are oral liquids containing one
or more active ingredients dissolved in a suitable vehicle.

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18 Pharmaceutical Compounding and Dispensing

KeyPoints General method

The following general method should be used
Advantages and disadvantages of in the preparation of a solution:
solutions as dosage forms
1. Write out the formula either from the
Advantages
■■ Drug available immediately for
prescription (unofficial) or from an official
absorption text (official).
■■ Flexible dosing 2. Calculate the quantities required for
■■ May be designed for any route each ingredient in the formula to produce
of administration the required final volume. Remember, it
■■ No need to shake container is not usual to calculate for an overage
■■ Facilitates swallowing in
of product in the case of solutions as it
difficult cases
is relatively easy to transfer the entire
Disadvantages final contents of the conical measure.
■■ Drug stability often reduced in Additionally, as far as is practically
solution possible, the product will be assembled
■■ Difficult to mask unpleasant in the final measure, thus reducing any
tastes transference losses.
■■ Bulky, difficult to transport and
3. Complete all sections of the product
prone to container breakages
■■ Technical accuracy needed
worksheet.
to measure dose on 4. Prepare a suitable label.
administration 5. Weigh all solids.
■■ Some drugs poorly soluble 6. Identify the soluble solids and calculate the
■■ Measuring device needed for quantity of vehicle required to dissolve the
administration solids fully. If more than one solid is to be
dissolved, they are dissolved one by one,
in order of solubility (i.e. the least soluble
first). In almost all cases, dissolution will
take place in a glass (or occasionally plastic)
KeyPoints beaker, not a conical measure. Remember
Dissolution will normally take that the solubility of the soluble solids will
place in a glass beaker, not a be dependent on the vehicle used.
conical measure, for a number of 7. Transfer the appropriate amount of vehicle
reasons. to a glass beaker.
■■ Firstly, owing to the shape of 8. If necessary, transfer the solid to a
the conical measure, any solid
glass mortar and use the glass pestle to
added to a conical measure
will tend to cake at the bottom
reduce particle size to aid dissolution
of the measure and hamper (Figure 2.1).
any attempt to stir the solid 9. Transfer the solid to the beaker and stir
around with the stirring rod to aid dissolution. If a mortar and pestle
which aids dissolution. have been used to reduce particle size,
■■ Secondly, the action of the ensure that the mortar is rinsed with a little
stirring rod may scratch the
vehicle to ensure complete transfer of the
inside of the glass conical
measure, permanently altering
powders.
the internal volume of the 10.  When all the solid(s) has/have dissolved,
measure. transfer the solution to the conical measure
that will be used to hold the final solution.

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 Solutions 19

KeyPoints
During the dissolution phase,
solutions should be stirred
gently and uniformly to avoid air
entrapment which may result in
foaming of the solution. If available,
automatic stirring devices may be
useful in assisting the production
of a uniform product and can be
time-saving. If stirring devices are
used to assist dissolution (e.g. rod,
magnetic stirrers), remember to
remove them before adjusting to
final volumes.

Figure 2-1.  A porcelain mortar and pestle and a smaller

glass mortar and pestle.
Tips
It is best to stir continuously
when combining ingredients into
11. Rinse out the beaker in which the a solution (either liquid or solid
ingredients). By stirring continually
solution was made with a portion of the
during incorporation, high
vehicle and transfer the rinsings to the concentration planes within the
conical measure. fluid body, which might increase
12. Add any remaining liquid ingredients to the likelihood of incompatibilities,
the conical measure and stir. will be avoided.
13. Make up to final volume with remaining
vehicle.
14. Transfer to a suitable container, label and
Tips
Further considerations during
dispense to the patient. the preparation of a solution:
 See Solutions video for a demonstration 1. To aid dissolution, high-viscosity
of the preparation of a solution.  liquid components should be
added to those of lower viscosity.
2. Completely dissolve salts in a
Worked examples small amount of water prior to
the addition of other solvent
Example 2.1 elements.
The preparation of Alkaline Gentian 3. In complex solutions, organic
components should be
Mixture BP
dissolved in alcoholic solvents
You receive a prescription in your pharmacy and water-soluble components
with the following details: dissolved in aqueous solvents.
4. Aqueous solutions should be
added to alcoholic solutions
Patient:  r David Shaw,
M
with stirring to maintain the
5 Longmeadow, Astonbury
alcohol concentration as high
Age: 56
as possible – the reverse may
Prescription: Mist Gent Alk
Directions: 10 mL tds ac ex aq
result in separation of any
Mitte: 150 mL dissolved components.

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20 Pharmaceutical Compounding and Dispensing

1. Use of the product

Gentian is used as a bitter to stimulate appetite (British
Pharmacopoeia 2007, p 949).
2. Is it safe and suitable for the intended purpose?
This is an official preparation, therefore the formula is safe and
suitable for purpose. Note that if the dose of an oral liquid is
specified as 5 mL or 10 mL, the dose regimen would be 5 mL or
10 mL three or four times daily by convention.
3. Calculation of formula for preparation
Prepare 150 mL of Alkaline Gentian Mixture BP.

Product formula
(from the British Pharmacopoeia 2007, p 2616):

Master 100 mL 50 mL 150 mL

Concentrated Compound 100 mL 10 mL 5 mL 15 mL
Gentian Infusion BP
Sodium Bicarbonate BP 50 g 5 g 2.5 g 7.5 g
Double Strength Chloroform 500 mL 50 mL 25 mL 75 mL
Water BP
Potable water to 1000 mL to 100 mL to 50 mL to 150 mL

Interim formula for Double Strewngth Chloroform Water BP

Concentrated Chloroform Water BPC 1959 5 mL
Potable water to 100 mL

4. Method of preparation
a. Solubility where applicable
Sodium Bicarbonate BP is soluble 1 in 11 in water (British
Pharmacopoeia 1988, p 509). Therefore to dissolve 7.5 g
Sodium Bicarbonate BP, a minimum of 7.5 × 11 = 82.5 mL of
water would be required. As this is greater than 50% of the
mixture, a solution of Double Strength Chloroform Water BP
and water would be used for dissolution.
b. Vehicle/diluent
Double Strength Chloroform Water BP and potable water
would be used as the vehicle as per the product formula.
c. Preservative
Double Strength Chloroform Water BP is included in this
product as the preservative as per the product formula.
d. Flavouring when appropriate
No extra flavouring is required. In addition to preservative
action Double Strength Chloroform Water BP will give
some flavouring and act as a sweetener, therefore helping to
counteract the bitter taste of the Concentrated Compound
Gentian Infusion BP

Sample chapter from FASTtrack: Pharmaceutical Compounding and Dispensing, 2nd edition
 Solutions 21

The following method would be used to

prepare 150 mL of Alkaline Gentian Mixture BP
Tips
from the formula above: Rather than attempt the
conversion from 1000 mL to
1. Using the master formula from the
150 mL in one stage, it may be
British Pharmacopoeia for 1000 mL of simpler to take the calculation
final product, calculate the quantity of through a number of stages. In
ingredients required to produce the final the example given above, the
volume needed (150 mL). quantities in the master formula
2. Calculate the composition of a convenient are first divided by 10 to give a
quantity of Double Strength Chloroform product with a final volume of
100 mL. These quantities are
Water BP, sufficient to satisfy the formula
then halved to give a product
requirements but also enabling simple, with a final volume of 50 mL. The
accurate measurement of the concentrated quantities in the 100 mL product
component. and 50 mL product are then added
together to give the quantities of
Method of compounding for Double ingredients in a product with a final
Strength Chloroform Water BP volume of 150 mL. By using this
method, the compounder is less
a. In this case, 75 mL of Double Strength
likely to make a calculation error.
Chloroform Water BP is required and so it
would be sensible to prepare 100 mL. To
prepare 100 mL Double Strength Chloroform Water BP, measure
5 mL of Concentrated Chloroform water BPC 1959 accurately
using a 5 mL conical measure.
b. Add approximately 90 mL of potable water to a 100 mL conical
measure (i.e. sufficient water to enable dissolution of the
concentrated chloroform component without reaching the final
volume of the product).
c. Add the measured Concentrated Chloroform Water BPC 1959
to the water in the conical measure.
d. Stir gently and then accurately make up to volume with
potable water.
e. Visually check that no undissolved chloroform remains at the
bottom of the measure.

Noting that Sodium Bicarbonate BP is soluble 1 in 11 with

water, a minimum of 11 mL of water would be required to
dissolve 1 g of Sodium Bicarbonate BP.
The final volume of Alkaline Gentian Mixture BP required
(150 mL) will contain 7.5 g of Sodium Bicarbonate BP. As 1 g of
Sodium Bicarbonate BP is soluble in 11 mL, 7.5 g is soluble in
82.5 mL (7.5 × 11 = 82.5 mL).
Therefore a minimum of 82.5 mL of vehicle would be
required to dissolve the 7.5 g of Sodium Bicarbonate BP in
this example. For ease of compounding, choose a convenient
volume of vehicle, say 90 mL, in which to dissolve the solute
initially. When choosing the amount of vehicle to use for

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22 Pharmaceutical Compounding and Dispensing

Tips dissolution, it is important to consider

the total amount of each liquid ingredient
As discussed above, in this in the preparation to ensure that only the
example 90 mL of vehicle is
correct amounts are added or the final
required to dissolve the Sodium
Bicarbonate BP. It is important to product does not go over volume.
consider the total amount of each
liquid ingredient in the product 3.  eigh 7.5 g Sodium Bicarbonate BP on a
W
to ensure that only the correct Class II (Figure 2.2) or electronic balance.
amounts are added. 4. Accurately measure 75 mL Double
In this example, it would be Strength Chloroform Water BP
incorrect to dissolve the Sodium
using a 100 mL measure. To this add
Bicarbonate BP in 90 mL of Double
Strength Chloroform Water BP as approximately 15 mL potable water in
the final volume of the preparation order to produce 90 mL of vehicle which
only contains 75 mL. Equally, it should be poured into a beaker (in order to
would also be incorrect to dissolve produce sufficient volume to dissolve the
the Sodium Bicarbonate BP in 7.5 g Sodium Bicarbonate BP).
90 mL of water as the final volume 5. The Sodium Bicarbonate BP (7.5 g) should
of the preparation will contain less
be added to the vehicle, thus following the
than 75 mL.
In this case, all the Double principle of adding solutes to solvents.
Strength Chloroform Water BP is 6. Stir to aid dissolution.
used (75 mL) along with enough 7. Transfer the solution to a 250 mL conical
potable water to reach the desired measure.
volume (approximately 15 mL). 8. Rinse the beaker with potable water,
adding the rinsings to the Sodium
Bicarbonate BP solution.
9. Accurately measure 15 mL of Concentrated
Compound Gentian Infusion BP in an
appropriately sized conical measure
and add to the Sodium Bicarbonate BP
solution in the 250 mL measure. Rinse out

Figure 2-2. 
A Class II balance.

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 Solutions 23

the small conical measure with potable water and add the
rinsings to the mixture.
10. Make up to volume (150 mL) accurately with potable water
and stir.
11. Transfer the solution to a 150 mL amber flat medical bottle
with a child-resistant closure and label.

5. Choice of container
A plain amber bottle with a child-resistant closure would be
most suitable as the preparation is a solution for internal use.
6. Labelling considerations
a. Title
The product is official, therefore the following title would be
suitable: ‘Alkaline Gentian Mixture BP’.
b. Quantitative particulars
Quantitative particulars are not required as the product is
official.
c. Product-specific cautions (or additional labelling requirements)
Not applicable.
d. Directions to patient – interpretation of Latin abbreviations
where necessary
‘Take TWO 5 mL spoonfuls THREE times a day before food in
water’.
e. Recommended British National Formulary cautions when
suitable
Not applicable.
f. Discard date
The British Pharmacopoeia (2004, p 2453) states that this
product should be recently prepared, therefore it will attract a
4-week discard date. Alternatively, a 2-week discard date could
be assigned as the product also contains an infusion.
g. Sample label (you can assume that the name and address of the
pharmacy and the words ‘Keep out of the reach and sight of
children’ are pre-printed on the label):

Alkaline Gentian Mixture BP 150 mL

Take TWO 5 mL spoonfuls THREE times a day before food in water.
Do not use after (2 weeks)
Mr David Shaw Date of dispensing

7. Advice to patient
The patient would be advised to mix two 5 mL spoonfuls with
an equal volume of water to dilute the bitter taste three times
a day before food. In addition, the discard date would be
highlighted to the patient.

Sample chapter from FASTtrack: Pharmaceutical Compounding and Dispensing, 2nd edition
24 Pharmaceutical Compounding and Dispensing

Example 2.2
The preparation of Ammonium Chloride Mixture BP
You receive a prescription in your pharmacy with the following
details:

Patient: Mr James Watson, 4 Arrow Ave, Astonbury

Age: 42
Prescription: Ammonium Chloride Mixture BP
Directions: 10 mL tds prn
Mitte: 50 mL

1. Use of the product

Used as an expectorant to treat chesty coughs (Martindale 35th
edn, p 1399).
2. Is it safe and suitable for the intended purpose?
This is an official preparation, therefore the formula is safe
and suitable for purpose. The dose of 10 mL three times a day
when required is consistent with the adult standard dosage
for official oral liquids (of 10 mL) three to four times a day by
convention.
3. Calculation of formula for preparation
Prepare 50 mL of Ammonium Chloride Mixture BP.

Product formula
(from the British Pharmacopoeia 2007, p 2317)

Master 500 mL 50 mL

Ammonium Chloride BP 100 g 50 g 5g
Aromatic Ammonia Solution BP 50 mL 25 mL 2.5 mL
Liquorice Liquid Extract BP 100 mL 50 mL 5 mL
Potable water to 1000 mL to 500 mL to 50 mL

4. Method of preparation
a. Solubility where applicable
Ammonium Chloride BP is soluble 1 in 2.7 in water (British
Pharmacopoeia 1988, p 36). Therefore to dissolve 5 g of
ammonium chloride a minimum of 5 × 2.7 = 13.5 mL of water
would be required.
b. Vehicle/diluent
Potable water would be used as the vehicle as per the product
formula.
c. Preservative
No additional preservative is required as per the product
formula.
d. Flavouring when appropriate

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 Solutions 25

Liquorice Liquid Extract BP, although a

mild expectorant, is mainly included for
Tips
its flavouring and sweetening properties As Ammonium Chloride BP is
soluble 1 part in 2.7 parts of
and its ability to disguise the taste of the
water, the 5 g required for this
Ammonium Chloride BP. product would only dissolve in a
minimum initial volume of 13.5 mL
Method of compounding aqueous vehicle. Therefore we
1. Calculate the quantity of ingredients should choose a convenient
required to produce the final volume volume of vehicle to dissolve the
needed. As with Example 2.1, this solute, for example, 15 mL.
calculation is best attempted in stages.
2. Weigh 5 g Ammonium Chloride BP
accurately on a Class II or electronic Tips
balance. The solution should be gently
3. Measure approximately 15 mL potable stirred to reduce the likelihood of
water and transfer to a beaker. frothing that can occur upon the
4. Add the Ammonium Chloride BP to addition of Liquorice Liquid Extract
the water in the beaker and stir until BP. Any resultant frothing will make
the accurate reading of the final
dissolved.
meniscus difficult.
5. Transfer to a 50 mL conical measure
with rinsings.
6. Measure 5 mL Liquorice Liquid Extract
BP accurately in a 5 mL conical Tips
measure and add, with rinsings, to the Note that the syringe will not need
50 mL measure containing ammonium rinsing as it is designed to deliver
chloride solution. a measured volume.
7. Measure 2.5 mL Aromatic Ammonia
Solution BP accurately in a syringe and transfer to the
50 mL measure containing the composite solution.
8. Make up to the final volume of 50 mL with potable water
and stir.
9. Pack into a 50 mL amber flat medicine bottle and label.
5. Choice of container
A plain amber bottle with a child-resistant closure would be
the most suitable as the preparation is a solution intended for
internal use.
6. Labelling considerations
a. Title
The product is official, therefore the following title would be
suitable: ‘Ammonium Chloride Mixture BP’.
b. Quantitative particulars
Quantitative particulars are not required as the product is
official.
c. Product-specific cautions (or additional labelling requirements)
No product-specific cautions are applicable, although the
patient could be advised to take the product in water (to dilute

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26 Pharmaceutical Compounding and Dispensing

the taste) or in warm water (to dilute the taste but also to
improve the expectorant action by hastening the release of the
ammonia, which is the main expectorant in the mixture).
d. Directions to patient – interpretation of Latin abbreviations
where necessary
‘Take TWO 5 mL spoonfuls THREE times a day when required’.
e. Recommended British National Formulary cautions when
suitable
Not applicable.
f. Discard date
The British Pharmacopoeia (2004, p 2181) states that this
product should be recently prepared, therefore it will attract a
4-week discard date.
g. Sample label (you can assume that the name and address of the
pharmacy and the words ‘Keep out of the reach and sight of
children’ are pre-printed on the label):

Ammonium Chloride Mixture BP 50 mL

Take TWO 5 mL spoonfuls THREE times a day when required.
Do not use after (4 weeks)
Mr James Watson Date of dispensing

7. Advice to patient
The patient would be advised to take two 5 mL spoonfuls three
times a day when required. In addition, the discard date and
the fact that the solution may be taken diluted with an equal
volume of warm water would be highlighted.

Example 2.3
The preparation of Sodium Chloride Compound
Mouthwash BP
You receive a prescription in your pharmacy with the following
details:

Patient: Mrs Avril Asker, 21 Station Road, Astonbury

Age: 36
Prescription: Sodium Chloride Compound Mouthwash BP
Directions: Use 20 mL qqh prn
Mitte: 150 mL

1. Use of the product

Used as a mouthwash to cleanse and freshen the mouth,
suitable for treatment of superficial infections if used
frequently (British National Formulary 61st edn, p 697).

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 Solutions 27

2. Is it safe and suitable for the intended purpose?

This is an official preparation, therefore the formula is safe and
suitable for purpose. The dose of 20 mL every 4 hours when
required is consistent with the recommended dosage (British
National Formulary 61st edn, p 698).
3. Calculation of formula for preparation
Prepare 150 mL of Sodium Chloride Compound Mouth
wash BP.

Product formula
(from the British Pharmacopoeia 2007, p 2911)

Master 100 mL 50 mL 150 mL

Sodium Bicarbonate BP 10 g 1 g 500 mg 1.5 g
Sodium Chloride BP 15 g 1.5 g 750 mg 2.25 g
Concentrated Peppermint 25 mL 2.5 mL 1.25 mL 3.75 mL
Emulsion BP
Double Strength Chloroform 500 mL 50 mL 25 mL 75 mL
Water BP
Potable water to 1000 mL to 100 mL to 50 mL to 150 mL

Interim formula for Double Strength Chloroform Water BP

Concentrated Chloroform Water BPC 1959 5 mL
Potable water to 100 mL

4. Method of preparation
a. Solubility where applicable
Sodium Bicarbonate BP is soluble 1 in 11 parts of water
(British Pharmacopoeia 1988, p 509). Therefore to dissolve
1.5 g of Sodium Bicarbonate BP a minimum of 1.5 × 11 =
16.5 mL water would be required. Sodium Chloride BP is
soluble 1 in 3 parts of water (British Pharmacopoeia 1988, p
512). Therefore to dissolve 2.25 g of Sodium Chloride BP a
minimum of 2.25 × 3 = 6.75 mL water would be required.
b. Vehicle/diluent
Double Strength Chloroform Water BP and potable water
would be used as the vehicle as per the product formula.
c. Preservative
Double Strength Chloroform Water BP is included in this
product as the preservative as per the product formula.
d. Flavouring when appropriate
Concentrated Peppermint Emulsion BP is added to this
product as a flavouring as per the product formula. In addition
the Double Strength Chloroform Water BP will also sweeten
and flavour the product. The following method would be used

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28 Pharmaceutical Compounding and Dispensing

to prepare 150 mL of Sodium Chloride Compound Mouthwash

BP from the formula above:
1. Using the master formula from the British Pharmacopoeia for
1000 mL of final product, calculate the quantity of ingredients
required to produce the final volume needed (150 mL).
2. Calculate the composition of a convenient quantity of Double
Strength Chloroform Water BP, sufficient to satisfy the formula
requirements but also enabling simple, accurate measurement
of the concentrated component.

Method of compounding for Double Strength

Chloroform Water BP
a. In this case, 75 mL of Double Strength Chloroform Water BP
is required and so it would be sensible to prepare 100 mL. To
prepare 100 mL Double Strength Chloroform Water BP, measure
5 mL of Concentrated Chloroform Water BPC 1959 accurately
using a 5 mL conical measure.
b. Add approximately 90 mL of potable water to a 100 mL conical
measure (i.e. sufficient water to enable dissolution of the
concentrated chloroform component without reaching the final
volume of the product).
c. Add the measured Concentrated Chloroform Water BPC 1959
to the water in the conical measure.
d. Stir gently and then accurately make up to volume with
potable water.
e. Visually check that no undissolved chloroform remains at the
bottom of the measure.

Note that Sodium Bicarbonate BP is soluble 1 in 11 with water,

a minimum of 11 mL of water would be required to dissolve 1 g
of Sodium Bicarbonate BP.
The final volume of Sodium Chloride Compound
Mouthwash BP required (150 mL) will contain 1.5 g of Sodium
Bicarbonate BP. As 1 g of Sodium Bicarbonate BP is soluble in
11 mL, 1.5 g is soluble in 16.5 mL (1.5 × 11 = 16.5 mL).
The Sodium Chloride BP is soluble 1 in 2.8 with water.
Therefore a minimum of 2.8 mL of water would be required to
dissolve 1 g of Sodium Chloride BP.
The final volume of Sodium Chloride Compound
Mouthwash BP required (150 mL) will contain 2.25 g of
Sodium Chloride BP. As 1 g of Sodium Chloride BP is soluble
in 2.8 mL, 2.25 g is soluble in 6.3 mL (2.25 × 2.8 = 6.3 mL).
Therefore a minimum of 16.5 mL of vehicle would be
required to dissolve the 1.5 g of Sodium Bicarbonate BP
and a minimum of 6.3 mL of vehicle would be required to
dissolve the 2.25 g of Sodium Chloride BP in this example.

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 Solutions 29

For ease of compounding, choose a convenient volume of

vehicle, for example, 30 mL, in which to dissolve the solute
initially.

3. Weigh 2.25 g Sodium Chloride BP on a Tips

Class II or electronic balance. When choosing the amount of
4. Weigh 1.5 g Sodium Bicarbonate BP on a vehicle to use for dissolution, it
Class II or electronic balance. is important to consider the total
5. Measure 75 mL of Double Strength amount of each liquid ingredient in
the preparation to ensure that only
Chloroform Water BP in a 100 mL conical
the correct amounts are added or
measure. the final product does not go over
6. Transfer approximately 30 mL of Double volume.
Strength Chloroform Water BP to a beaker;
add the Sodium Bicarbonate BP and stir to aid dissolution.
7. When the Sodium Bicarbonate BP has dissolved, add the
Sodium Chloride BP and stir to aid dissolution.
8. Transfer the solution from the beaker to a 250 mL conical
measure.
9. Rinse out the beaker with some Double Strength Chloroform
Water BP and add the rinsings to the conical measure.
10. Measure 3.75 mL of Concentrated Peppermint Emulsion BP
using a 5 mL and a 1 mL syringe.
11. Add the Concentrated Peppermint Emulsion BP to the conical
measure.
12. Make up to volume with the remaining Double Strength
Chloroform Water BP and potable water.
13. Transfer to a 200 mL amber fluted bottle and label.

5. Choice of container
As this is an extemporaneously prepared mouthwash, the
container of choice would be an amber fluted medical bottle
with child-resistant closure.
6. Labelling considerations
a. Title
The product is official, therefore the following title would be
suitable: ‘Sodium Chloride Compound Mouthwash BP’.
b. Quantitative particulars
Quantitative particulars are not required as the product is
official.
c. Product-specific cautions
As the product is a mouthwash, ‘Not to be taken’, ‘Do not
swallow in large amounts’ and ‘Dilute with an equal volume of
water before using’ would all be required on the product label.
d. Directions to patient – interpretation of Latin abbreviations
where necessary
‘Use 20 mL as a mouthwash every four hours when required’.

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30 Pharmaceutical Compounding and Dispensing

e. Recommended British National Formulary cautions when

suitable
The British National Formulary has no specific labelling
cautions for this product but does advise dilution with an
equal volume of warm water.
f. Discard date
The product is a mouthwash containing a preservative and so
will attract a 4-week discard date.
g. Sample label (you can assume that the name and address of the
pharmacy and the words ‘Keep out of the reach and sight of
children’ are pre-printed on the label):

Sodium Chloride Compound Mouthwash BP 150 mL

Use 20 mL as a mouthwash every four hours when required.
Dilute with an equal volume of water before use.
Do not swallow in large amounts
Not to be taken
Do not use after (4 weeks)
Mrs Avril Asker Date of dispensing

7. Advice to patient
The patient would be advised that 20 mL of mouthwash should
preferably be diluted with an equal volume of warm water and
used every 4 hours when required. In addition, the discard
date and the fact that, although the mouthwash should not be
swallowed, it is not harmful to swallow small amounts would
be highlighted.

Example 2.4
The preparation of a magistral formulation from a doctor’s
prescription
You receive a prescription in your pharmacy with the following
details:

Patient: Mr Gary Murray, 12 Bishop Road, Astonbury

Age: 49
Prescription: Potassium Permanganate Solution 0.2%
Directions: Dilute 1 in 20 and use as a wet dressing alt die
Mitte: 150 mL

1. Use of the product

Used for cleansing wounds and deodorising suppurating
eczematous reactions and wounds (British National Formulary
61st edn, p 744).

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2. Is it safe and suitable for the intended purpose?

It is commonly used as a solution at a strength of 1 in 10
000 (British National Formulary 61st edn, p 744). When the
prepared solution is diluted as indicated it will provide a 1 in
10 000 solution.
0.2% w/v solution is the same as a 1 in 500 solution
which, when diluted 20 times, becomes a 1 in 10 000 solution.
3. Calculation of formula for preparation
Prepare 150 mL of Potassium Permanganate Solution 0.2% w/v.
0.2% w/v is equal to 0.2 g in 100 mL. Therefore there is
200 mg of Potassium Permanganate BP in every 100 mL of
solution.

Product formula
Master 50 mL 150 mL
Potassium Permanganate BP 200 mg 100 mg 300 mg
Freshly boiled and cooled purified water to 100 mL to 50 mL to 150 mL

4. Method of preparation
Tips
a. Solubility where applicable Rather than attempt the above
conversion in one stage, it may
Potassium Permanganate BP is soluble 1
be simpler to take the calculation
in 16 in cold water and freely soluble in through a number of stages. In
boiling water (British Pharmacopoeia 1988, the example given above, the
p 455). quantities in the master formula
b. Vehicle/diluent are first divided by 2 to give a
Freshly boiled and cooled purified water product with a final volume of
would be used as the vehicle, as no 50 mL. The quantities in the
100 mL product and 50 mL product
preservative will be added and the solution
are then added together to give
is intended for application to a wound. the quantities of ingredients in
c. Preservative a product with a final volume of
No preservative is to be added to this 150 mL. Using this method, the
product. compounder is less likely to make
d. Flavouring when appropriate a calculation error.
Not applicable as the solution is for
external use.

The following method would be used to prepare 150 mL of

Potassium Permanganate BP 0.2% w/v from the formula above:
1. Weigh 300 mg Potassium Permanganate BP on a Class II or
electronic balance.
2. Transfer to a glass mortar as the Potassium Permanganate BP
is crystalline and for ease of dissolution needs to be ground
under water into a powder.
3. Transfer the solution to a 250 mL conical measure.

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32 Pharmaceutical Compounding and Dispensing

Tips 4. R inse the mortar with freshly boiled and cooled

purified water and add the rinsings to the conical
Potassium Permanganate BP is measure.
an oxidising substance, therefore
5. Make up to volume with freshly boiled and
there is risk of explosion. To
prevent this, add approximately cooled purified water.
20 mL of freshly boiled and cooled 6. Transfer the solution to a 150 mL amber fluted
purified water to a glass mortar medical bottle with a child-resistant closure and
(Potassium Permanganate BP label.
stains and so a porcelain mortar 5. Choice of container
would not be suitable) and grind A fluted amber bottle with a child-resistant
under water. In addition, a glass
closure would be most suitable as the preparation
mortar is always used for grinding
as crystalline substances can is a solution for external use.
scratch a porcelain mortar. 6. Labelling considerations
a. Title
The product is unofficial, therefore the following
title would be suitable: ‘Potassium Permanganate
Solution 0.2% w/v’.
b. Quantitative particulars
The product is unofficial, therefore it is necessary
to put the quantitative particulars on the label.
As the product is intended for external use, the
quantitative particulars would be expressed per
container.

This container contains:

Potassium Permanganate BP 0.2%
Freshly boiled and cooled purified water to 100%
or
This container contains:
Potassium Permanganate BP 300 mg
Freshly boiled and cooled purified water to 150 mL

The quantitative particulars for a product for external use may

be expressed in percentage terms instead of actual quantities;
either would be correct.
c. Product-specific cautions (or additional labelling
requirements).
‘For external use only.’
d. Directions to patient – interpretation of Latin abbreviations
where necessary
‘Dilute ONE capful with NINETEEN capfuls of water and use
as a wet dressing every other day.’
e. Recommended British National Formulary cautions when
suitable

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The British National Formulary has no specific labelling

cautions for this product but does advise that solutions of
Potassium Permanganate BP stain skin and clothing. Therefore,
a suitable caution could be added to the final label.
f. Discard date
The product would attract a 2-week discard date as there is
no preservative and the product is being applied to an open
wound.
g. Sample label (you can assume that the name and address of the
pharmacy and the words ‘Keep out of the reach and sight of
children’ are pre-printed on the label):

Potassium Permanganate Solution 0.2%w/v 150 mL

Dilute ONE capful with NINETEEN capfuls of water and use as
a wet dressing every other day.
Caution: Stains skin, hair and fabric
Do not use after (2 weeks)
For external use only
This solution contains:
Potassium Permanganate BP 0.2%
Freshly boiled and cooled purified water to 100%

Mr Gary Murray Date of dispensing

7. Advice to patient
The patient would be advised to dilute one capful of the
solution with 19 capfuls of water and use as a wet dressing
every other day. In addition, the discard date and the following
cautions would be highlighted to the patient:
For external use only.
The preparation may stain hair, skin and fabrics.
With prolonged use nails may also be stained.
Discontinue use if the skin becomes dry.

Self-assessment
Basic strength calculations
The simplest way to express the strength of a solution is to specify the amount of
solute to be dissolved in a stated amount of solvent.
If the solute is a solid dissolved in a liquid, the strength of the solution may
often be expressed as mg/mL, mg/100 mL, g/100 mL, mg/L or g/L. Similarly, if the
solute is a liquid, the strength could be expressed as mL/10 mL, mL/100 mL or
mL/L.

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34 Pharmaceutical Compounding and Dispensing

Example 2.5
You are asked to prepare a 100 mL solution containing Sodium Chloride BP 9 mg/mL
Sodium Chloride BP 9 mg 90 mg 900 mg
Potable water to 1 mL to 10 mL to 100 mL

Therefore the amount required would be 900 mg (= 0.9 g).

Similarly, the request could be to prepare 100 mL of a solution containing
Sodium Chloride BP 0.009 g/mL
Sodium Chloride BP 0.009 mg 0.09 mg 0.9 mg
Potable water to 1 mL to 10 mL to 100 mL

Therefore the amount required would be 0.9 g (= 900 mg).

Questions
1. How much solid would be required in order to produce 500 mL of a
15 mg/10 mL solution?
a. 75 mg
b. 150 mg
c. 750 mg
d. 1500 mg
e. 7500 mg

2. If 30 mg of an ingredient was dissolved in 1.5 mL of solvent, what would be the

strength of the resulting solution expressed as mg/mL?
a. mg/mL
b. 15 mg/mL
c. 20 mg/mL
d. 30 mg/mL
e. 200 mg/mL

3. A patient requires a dose of 1 mg of atropine sulphate. Ampoules containing

600 micrograms/mL are available. If a 2 mL syringe graduated to 0.1 mL is
available, which of the following provides the nearest dose?
a. 1.5 mL
b. 1.6 mL
c. 1.7 mL
d. 1.8 mL
e. 1.9 mL

4. A paediatric vitamin drop contains 0.25 mg of vitamin D in each millilitre. How

many micrograms of vitamin D are contained in 0.2 mL of this preparation?
a. 50 micrograms
b. 75 micrograms
c. 100 micrograms
d. 150 micrograms
e. 250 micrograms

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 Solutions 35

5. What weight of sodium bicarbonate (in grams) would be required to make

150 mL of a 6 g/L solution?
a. 0.5 g
b. 0.6 g
c. 0.75 g
d. 0.9 g
e. 1 g

Tailored strength calculations

Often this type of calculation is required if you are attempting to give a tailored
dose to a patient using existing pre-prepared stock mixtures.

Example 2.6
A common dose seen in paediatric prescribing is 62.5 mg phenoxymethylpenicillin
four times a day. This is the recommended dose for a child 1 month–1 year. The
readily available mixture is 125 mg/5 mL. Therefore to provide a dose of 62.5 mg we
give 2.5 mL of a 125 mg/mL mixture.

strength required
Volume required = × volume of stock solution
stock strength
62.5
= × 5 ml
125
62.5 5 ml
= ×
125 1
312.5 ml
=
125
= 2.5 ml

Questions
6. A patient requires a dose of 5 mg of a drug. The available stock solution
contains 25 mg/5 mL. How much of this stock solution would be required to
deliver this dose?

7. A baby requires a dose of 37.5 mg chloroquine base each week to prevent

infection with malarial parasite. The solution available for you to dispense
contains 50 mg/5 mL chloroquine base. How much of this stock solution should
be given to the baby each week?

8. A 5-year-old child needs a dose of 125 mg cimetidine four times a day. The
stock solution of cimetidine available contains 200 mg/5 mL. How many
millilitres of this stock solution will be administered for each dose?

Percentage calculations
Percentages are also commonly used to express the strength of solutions. Usually
these solutions are not intended for the oral route of administration. As a

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36 Pharmaceutical Compounding and Dispensing

percentage this can have four different meanings and in order to make clear the
intention the following terms are used:
■■ % w/w percentage weight in weight. This expresses the amount in grams of
solute in 100 g of product.
■■ % w/v percentage weight in volume. This expresses the amount in grams of
solute in 100 mL of product.
■■ % v/v percentage volume in volume. This expresses the number of millilitres of
solute in 100 mL of product.
■■ % v/w percentage volume in weight. This expresses the number of millilitres of
solute in 100 g of product.

The strength of solutions of solids in liquids is usually expressed as % w/v,

whereas that of liquids in liquids is expressed as % v/v. When the type of
percentage is not specified by convention the above rule will apply. For example, %
solid in liquid is interpreted as % w/v.

Example 2.7
Prepare 50 mL potassium permanganate solution 2.8%.
As Potassium Permanganate BP is a solid this would mean: prepare a solution
containing potassium permanganate 2.8% w/v.
This means that there would be 2.8 g of Potassium Permanganate BP dissolved
in every 100 mL of solution.
Potassium Permanganate BP 2.8 g 1.4 g
Freshly boiled and cooled purified water to 100 mL to 50 mL

Questions
What quantities would be required for the following?
9. 500 mL of a 0.1% w/v solution using a 20% w/v solution.

10. 5 L of a 0.9% w/v solution.

11. 20 mL of a 5% solution.

12. How much solid would be required in order to produce 50 mL of a 0.2% w/v
solution?
a. 100 micrograms
b. 200 micrograms
c. 10 milligrams
d. 100 milligrams
e. 200 milligrams

13. How much solid would be required in order to produce 20 mL of a 5% w/v
solution?
a. 100 mg
b. 500 mg

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c. 1000 mg
d. 5000 mg
e. 10 000 mg

14. How much solid would be required in order to produce 300 mL of a 0.01% w/v
solution?
a. 3 micrograms
b. 3 milligrams
c. 30 milligrams
d. 300 milligrams
e. 3 grams

15. How much solid would be required in order to produce 750 mL of a 15% w/v
solution?
a. 75 mg
b. 125 mg
c. 1125 mg
d. 112.5 g
e. 750 g

16. How much solid would be required in order to produce 10 litres of a 0.45%
solution?
a. 45 mg
b. 4.5 g
c. 9 g
d. 45 g
e. 90 g

17. What is the percentage strength when 5 mL of disinfectant concentrate is made
up to 1 litre with water?
a. 0.05% v/v
b. 0.15% v/v
c. 0.5% v/v
d. 1.5% v/v
e. 5% v/v

18. You have been given the following prescription: Sodium Bicarbonate BP 5%
potable water to 10 mL. How much Sodium Bicarbonate BP (in grams) will be
needed to prepare the product?
a. 0.05 g
b. 0.15 g
c. 0.5 g
d. 1.5 g
e. 5 g

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38 Pharmaceutical Compounding and Dispensing

19. How much of a 20% w/v solution would be required to produce 250 mL of a
0.5% w/v solution?
a. 2.5 mL
b. 6.25 mL
c. 25 mL
d. 62.5 mL
e. 125 mL

20. How much of a 4% w/v solution would be required to prepare 150 mL of a 1%

w/v solution?
a. 2.6 mL
b. 3.75 mL
c. 26 mL
d. 37 mL
e. 37.5 mL

21. How much of a 25% w/v solution would be required to prepare 250 mL of a
0.5% w/v solution?
a. 0.5 mL
b. 1 mL
c. 2.5 mL
d. 5 mL
e. 10 mL

22. Calculate the amount of stock solution that would be required to make the
following solutions.
a. Half a litre of a 1% v/v solution using a 15% v/v solution
b. 250 mL of a 1% v/v solution using a 40% v/v solution
c. 500 mL of a 1% v/v solution using a 10% solution
d. 1 litre of a 0.5% v/v solution using a 15% solution
e. 1 litre of a 0.05% solution using a 4% solution

Parts calculations
The concentration of solutions may also be expressed in terms of ‘parts’. By this we
mean ‘parts’ of solute in ‘parts’ of product. This is interpreted as parts by weight
(grams) of a solid in parts by volume (millilitres) of the final solution or in parts by
volume (millilitres) of a liquid in parts by volume (millilitres) of the final solution.
Solubility of ingredients is often expressed in this way.

Example 2.8
Sodium Bicarbonate BP is soluble in 11 parts of water. This means that 1 g of
Sodium Bicarbonate BP will dissolve in 11 mL of water. Therefore if you had a
formula that required 4 g of Sodium Bicarbonate BP you would need a minimum of
4 × 11 = 44 mL of water in which to dissolve the 4 g of Sodium Bicarbonate BP.

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 Solutions 39

Questions
23. How many millilitres of potable water are required to dissolve 3 g of a solid
which is soluble in 2.5 parts of water?
a. 2.5 mL
b. 3 mL
c. 5 mL
d. 7.5 mL
e. 8 mL

24. How many millilitres of potable water are required to dissolve 7 g of a solid,
which is soluble 1 in 1.5 parts of water?
a. 1.5 mL
b. 15 mL
c. 7.5 mL
d. 10.5 mL
e. 75 mL

25. Sodium Bicarbonate BP is soluble 1 in 11. How much Double Strength

Chloroform Water BP is needed to dissolve 0.37 kg?
a. 3.7 L
b. 4.07 L
c. 4090 mL
d. 4.1 L
e. 4700 mL

26. How much of a 1 in 150 w/v solution would be required to produce 200 mL of a

0.2% solution?
a. 3 mL
b. 15 mL
c. 25 mL
d. 30 mL
e. 60 mL

27. Express 500 micrograms in 2 mL as ‘1 in x’.

a. 1 in 250
b. 1 in 400
c. 1 in 2000
d. 1 in 2500
e. 1 in 4000

28. How much Crystal Violet BP is required to prepare 5.4 litres of a 1 in 12 000
solution?
a. 450 micrograms
b. 540 micrograms
c. 4.5 milligrams
d. 450 milligrams
e. 540 milligrams

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40 Pharmaceutical Compounding and Dispensing

Millimolar calculations
The strength of active ingredient within a pharmaceutical preparation can be
expressed as the number of millimoles per unit volume or mass of product. The
mole is the unit of amount of substance and there are 1000 millimoles in a mole.
To calculate the number of millimoles of an ingredient in a medicinal product, you
will firstly need to know the molecular weight of the ingredient.
The number of moles of ingredient is the mass of ingredient divided by the
molecular mass:
Mass in grams
Number of moles =
Molecular mass

For example, the molecular weight quoted for Sodium Chloride BP is 58.44.
Therefore a molar solution of Sodium Chloride BP would contain 58.44 g of
Sodium Chloride BP in a litre.

Example 2.9
Prepare 100 mL of Sodium Chloride BP solution containing 1.5 mmol per mL.

1 ml contains 1.5 mmol

100 ml contains 150 mmol
1 mole (1000 mmol) of Sodium Chloride BP weighs 58.44 g
58.44 g
1 mmol of Sodium
m Chloride BP weighs
1000
150 mmol of Sodium Chloride
e BP weighs = 8.766 g (weigh 8.77 g)

Questions
29. The molecular weight of Sodium Chloride BP is 58.44. How many grams of
Sodium Chloride BP would be needed to prepare 1 litre of a molar solution?
a. 0.2922 g
b. 0.5844 g
c. 5.844 g
d. 29.22 g
e. 58.44 g

30. The molecular weight of Sodium Bicarbonate BP is 84. How many grams of
Sodium Bicarbonate BP would be required to produce 150 mL of 0.5 mmol/mL
solution?
a. 0.63 g
b. 0.84 g
c. 6.3 g
d. 8.4 g
e. 63 g

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 Solutions 41

31. The molecular weight of Sodium Chloride BP is 58.44. How many grams of
Sodium Chloride BP would be required to produce 100 mL of a 2 mmol/mL
solution?
a. 2.92 g
b. 5.84 g
c. 11.69 g
d. 29.20 g
e. 58.40 g

32. The molecular weight of Sodium Bicarbonate BP is 84. How many grams of
Sodium Bicarbonate BP would be required to produce 75 mL of 1 mmol/mL
solution?
a. 0.63 g
b. 4.2 g
c. 6.3 g
d. 8.4 g
e. 12.6 g

Formulation questions
This section contains details of extemporaneous products to be made in the same
way as the examples earlier in this chapter. For each example, provide answers
using the following sections:

1. Use of the product

2. Is it safe and suitable for the intended purpose?
3. Calculation of formula for preparation
4. Method of preparation
a. Solubility where applicable
b. Vehicle/diluent
c. Preservative
d. Flavouring when appropriate
5. Choice of container
6. Labelling considerations
a. Title
b. Quantitative particulars
c. Product-specific cautions (or additional labelling requirements)
d. Directions to patient – interpretation of Latin abbreviations where necessary
e. Recommended British National Formulary cautions when suitable
f. Discard date
g. Sample label (you can assume that the name and address of the pharmacy
and the words ‘Keep out of the reach and sight of children’ are pre-printed
on the label)
7. Advice to patient

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42 Pharmaceutical Compounding and Dispensing

33. You receive a prescription in your pharmacy with the following details.

Patient: Miss Julie Jordan, 21 Fair View, Astonbury

Age: 2 months
Prescription: Sodium Bicarbonate Solution 0.5 mmol/mL
Directions: 10 mL bd with feeds
Mitte: 200 mL

34. You receive a prescription in your pharmacy with the following details.

Patient: Mrs Sally Burns, 14 Netherton Grove, Astonbury

Age: 45
Prescription: Hibitane solution 0.05%
Directions: For cleansing the affected area
Mitte: 150 mL

Sample chapter from FASTtrack: Pharmaceutical Compounding and Dispensing, 2nd edition