NTA Level 5 • Semester 2 • PST05208

Determination of Isotonicity by Freezing Point

Pharmaceutics Theory and Compounding • Source Session/Topic 12

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Session 12: Determination of Isotonicity by Freezing Point

Method

Total Session Time: 120 minutes + 6 hours of Practices

Prerequisites

• None

Learning Tasks

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

• Define freezing point depression

• Determine the isotonicity by freezing point depression

Resources Needed:

• Flip charts, marker pens, and masking tape

• Black/white board, chalk and whiteboard markers

• LCD projector and computer

SESSION OVERVIEW

Activity/

Step Time Content

Method

1 05 Minutes Presentation Introduction, Learning Tasks

2 45 Minutes Presentation Introduction to Freezing Point Depression

Presentation Determining the Isotonicity by freezing Point

3 60 Minutes Small Group Depression

Discussion

4 05 Minutes Presentation Key Points

5 05 Minutes Presentation Evaluation

SESSION CONTENTS

STEP1: 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: Freezing Point Depression (45 minutes)

• The freezing point of body fluid is at – 0.520C.

• Any solution freezing at – 0.520C will have the same osmotic pressure as body fluids

• Hypotonic solution will have a higher freezing-point and consequently need the addition of

adjusting substance to reach a freezing point of – 0.520C

• The amount of adjusting substance which is often sodium chloride, is calculated as shown in the

following examples.

o Solution A freezes at – 0.390C. How much sodium has to be added to obtain an isotonic

solution?

o The adjusting substance sodium chloride has to decrease the freezing point by 0.52 – 0.39 =

0.130C

o A 1 % w/v sodium chloride solution freezes at – 0.5760C; so the amount needed is found by

the proportion:

o 1 == x x == 1 x 0.13 == 0.2%w/v

0.576 0.13 0.576

• Therefore, solution A will need the addition of 0.2%w/v sodium chloride in order to become

isotonic with body fluids; e.g. 50mL of this solution require 0.1g sodium chloride.

• From the above used proportion, the following equation has been derived:

o W == 0.52 – a

o Where:

 w== concentration %w/v of adjusting substance in the final solution

 a == freezing point of unadjusted solution

 b==freezing point of a 1%w/v solution of adjusting substance

• The freezing points of 1%w/v aqueous solution of various substances are obtain from the table

below; “a „ is calculated by multiplying the freezing depression of a 1%w/v solution by the

concentration of unadjusted solution expressed as %w/v

• Example:

o Adjust 1000mL of a 2%w/v solution of anhydrous dextrose isotonic with body fluids;

adjusting substance is sodium chloride

 Freezing point of 1%w/v anhydrous dextrose solution : – 0.1010C

 Freezing point of unadjusted solution: – 0.101 x 2 == – 0 .2020C

 Freezing point of 1%w/v sodium chloride solution = -0.5760C

 The last two values are substituting “ a “ and “b” in the formula

o W = 0.52 – 0.202 = 0.318 = 0.55%w/v

0.576 0.576

o Therefore, a 2%w/v anhydrous dextrose solution requires the addition of 0.55%w/v

sodium chloride in order to become isotonic with blood serum and tears; for 1000mL

solution 5.5g are needed

STEP 3: Determining the Isotonicity by Freezing Point Depression (60 minutes)

To determine amount of ingredients required to make isotonic solution

Activity: Small Group Discussion ( 30 minutes)

DIVIDE students in small manageable groups

ASK students to discuss in groups on the following questions

1. Render the following solution iso – osmotic with blood and tears

Ephedrine hydrochloride…………………………0.1g

Chlorbutol…………………………………………0.1g

Water for preparation……………………….to 20mL

Adjusting substance is sodium chloride

REFER Students to Pharmaceutical Calculation. 13th Edition by HOWARD C.

ANSEL: Chapter 11, For reference

ALLOW students to discuss for 20 minutes

ALLOW each groups to present for 5 minutes

CLARIFY and SUMMARIZE by using the contents below

• The freezing point depression of 1%w/v solution is obtained from the table below.

• First the concentrations %w/v must be calculated for ephedrine hydrochloride and Chlorbutol by

proportion:

0.1 = xx =0.1 x100 x = 0.5%w/v

20 100 20

• A 0.5%w/v solution of ephedrine hydrochloride has a freezing point depression of 0.165 x 0.5 =

0.08250C

• A 0.5%w/v solution for Chlorbutol has a freezing point depression of 0.14 x 0.5 =0.070C

• Now the equation can be used:

W = 0.52 – 0.1525 ==0.3675 = 0.638%w/v =0.64%w/v

0.576 0.576

• Therefore, the unadjusted solution needs the addition of 0.64%w/v sodium chloride. The actual

amount for 20mL solution is found by simple proportion

0.638 = x x = 0.638 x 20 =0.1276gm of sodium chloride

100 20 100

• So the formula for the adjusted solution becomes:

Ephedrine hydrochloride…………………………0.1g

Chlorbutol………………………………………..0.1g

Sodium chloride………………………………….0.13g

Water for preparation……………………….to 20mL

STEP 4: Key Points (5 minutes)

• Freezing-point depression is the decrease of the freezing point of a solvent on addition of a non-

volatile solute

• The formula for determining the Isotonicity by freezing point depression is

w= 0.52 – a

STEP 5: Evaluation (5 minutes)

• What is freezing point depression in determining isotonicity?

• What is the formula for determining isotonicity by freezing point depression?

STEP 6: Take Home Assignment (15 minutes)

Activity: Take home Assignment (15 minutes)

ASK each individual student to do the following assignment

• Calculate the strength of sodium chloride solution which is iso – osmotic with blood

serum and tears

ALLOCATE time for students to do the assignment and submit

REFER students to recommended references

References

Aulton, M. E., & Taylor, K. (2013). Aulton's pharmaceutics : the design and manufacture of

medicines (4th ed.). Edinburgh: Churchill Livingstone/Elsevier.

Pharmaceutical Society of Great Britain., & Pharmaceutical Society of Great Britain. Department of

Pharmaceutical Sciences. (1994). The Pharmaceutical Podex, Principles and Practice of

Pharmaceutics (W. Lund Ed. 12th ed.). London,: Pharmaceutical P.

Senya, S. S., Mwasha, C. Y. S., Muyinga, A. M., Amiri, R. I., & Mauga, E. A. S. K. (2011).

Tanzania Pharmaceutical Handbook (2nd ed.). Dar es salaam: School of Pharmaceutical

Sciences, Institute of Allied Health Sciences, Muhimbili University of Health and Allied

Health Sciences.

Remington, J. P. (2005). Remington, the science and practice of pharmacy (T. David Ed. 21st ed.).

Easton, Pa. London, UK: Mack Pub. Co. Pharmaceutical Press.

Ansel, H. C., Stockton, S. J., & Bradley, W. T. (2017). Pharmaceutical calculations. Philadelphia:

Wolters Kluwer.

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