NTA Level 4 • Semester 1 • PST04105

Calculations of Dilution and Concentration of Liquids

Pharmaceutical Calculations • Source Session/Topic 9

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Session 9: Calculations of Dilution and Concentration of Liquids

Total Session Time: 120 minutes

Prerequisites

None

Learning Tasks

By the end of this session students are expected to be able to:

• Define dilution and concentration of liquids

• Calculating dilution and concentration of liquids

Resources Needed:

• Flip charts, marker pens, and masking tape

• Black/white board and chalk/whiteboard markers

SESSION OVERVIEW

SESSION CONTENTS

STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)

READ or ASK students to read the learning tasks and clarify

ASK students if they have any questions before continuing.

STEP 2: Definition of Dilution and Concentration of Liquids (30 Minutes)

Dilution

If a mixture of a given percentage or ratio strength is diluted to twice

its original quantity, its

active ingredient will be contained in twice as many parts of the whole,

and its strength therefore will be reduced by one half

For example: if 50 mL of a solution containing 10 g of active ingredient

with a strength of 20% or 1:5 w/v are diluted to 100 mL, the original

volume is doubled, but the original strength is now reduced by one half to

10% or 1:10 w/v

Concentration

By contrast, if a mixture is concentrated by evaporation to one-half its

original quantity, the active ingredient (assuming that none was lost by

evaporation) will be contained in one half as many parts of the whole, and

the strength will be doubled

For example: : if 50 mL of a solution containing 10 g of active ingredient

with a strength of 20% or 1:5 w/v is reduced by evaporation to 25 mL or one

half the original quantity, the 10 g of the active ingredient will indicate

a strength of 40% or 1:2.5 w/v

Therefore, the amount of active ingredient remains constant, any change in

the quantity of a solution or mixture of solids is inversely proportional

to the percentage or ratio strength; that is, the percentage or ratio

strength decreases as the quantity increases, and conversely

This relationship is generally true for all mixtures except solutions

containing components

that contract when mixed together

This type of calculation is very important in the pharmaceutical field,

because most of the reagent/ chemicals used in the hospitals are received

in a concentrated form, and they will have to be diluted to desired

strength before use

Dilution of Liquids is done generally may be solved by the following

equation:

(1st Volume) x (1st Concentration)_= (2nd Volume) x (2nd Concentration),

or V1 X C1 = V2 X C2.

Wheres V1 = Volume of strong solution

C1 = concentration of strong solution

V2 = Volume of diluted solution

C2 = concentration of diluted solution

STEP 3: Dilution and Concentration of Liquids (50 Minutes)

|Activity: Small Group Discussion ( 20 minutes) |

| |

|DIVIDE students into small manageable groups |

| |

|ASK students to discuss on the following question |

|Exercise 1: Calculate the volume of alcohol 90% v/v needed to prepare |

|400 mL of alcohol 70% v/v |

|Exercise 2: How many mL of formalin 2% v/v will be prepared from 350 mL|

|of formalin 36% v/v |

|ALLOW students to discuss for 15 minutes |

| |

|ALLOW few groups to present and the rest to add points not mentioned |

| |

|CLARIFY and SUMMARIZE by using the contents below |

• The following are the solutions of the above exercises:

Exercise 1: Information given;

Concentration of strong solution (C1) = 90%

Volume of the strong solution (V1) = X?

Concentration of weak solution (C2) = 70%

Volume of the weak solution (V2) = 400 mL

Formula

C1V1= C2V2

V1= C2V2

C1

X = (70%×400 mL)/90%

= 311.11 mL of 90% alcohol will be needed to prepare 400 mL of 70%

alcohol.

Exercise 2: Information given;

Concentration of strong solution (C1) = 36%

Volume of the strong solution (V1) = 350 mL

Concentration of weak solution (C2) = 2%

Volume of the weak solution (V2) = X mL

Formula

C1V1= C2V2

V2= C1V1

C2

V2=( 36%×350 mL)/2

=6300 mL of 2% formalin solution will be prepared from 350 mL of 36%

formalin.

STEP 4: Key Points (10 minutes)

• Calculating dilution by using ratio; C1V1=C2V2

• This is the method of calculating dilution of a drug by using the

concentration of strong drug and concentration of diluted drug and their

volume.

• This type of calculation is very important in the pharmaceutical field as

most of pharmaceutical solutions needed to be diluted prior for use

• Dilution reduces the concentration of a solution

STEP 7: Evaluation (25 minutes)

• What is dilution of liquid?

• What is concentration of liquid?

• What is the volume of potassium permanganate 10%w/v needed to prepare

2000 mL of 1%w/v solution of potassium permanganate?

• What volume of dextrose 5% w/v solution will be prepared from 500 mL of

dextrose 25%w/v solution?

References

Ansel, H. C & Stocklosa, M. J. (2001). Pharmaceutical Calculations (11th

ed.). Philadelphia, United States: LIPPINCOTT WILLIAMS & WILKINS

Ansel, H. C (2010) Pharmaceutical Calculations (13rd ed.). Philadelphia,

United States: LIPPINCOTT WILLIAMS & WILKINS

Senya, S. S, Mwasha, C.Y, Muyinga, A. M, Amiri,R. I. and Mauga E.A.S.K.

(2011) Tanzania Pharmaceutical Handbook (2nd ed.). Dar eS Salaam,

Tanzania: School of Pharmaceutical Sciences.

Zatz, J.L and Teixeira, M.G. (2005). Pharmaceutical Calculation (4th ed.).

New Jersey: John Wiley & Sons, Inc

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