NTA Level 5 • Semester 2 • PST05208

Calculations Involving Buffer Solutions

Pharmaceutics Theory and Compounding • Source Session/Topic 22

Full source-text version: all educational wording from the extracted learning source is retained; only presenter/tutor metadata and web-layout noise are removed, while formatting is improved for readability.

Session 22: Calculations Involving Buffer Solutions

Total Session Time: 60minutes + 6 hours of Practices

Prerequisites

• None

Learning Tasks

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

• Give overview of calculations involving buffer solutions

• Perform calculations involving buffer solution

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

45minutes Introduction to Calculations Involving Buffer

2 Presentation Solutions

60 minutes Presentation Performing Calculations Involving Buffer

3 Solutions

Demonstration

4 05 minutes Presentation Key Points

5 05 minutes Presentation Evaluation

150

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: Buffer Solution (45 minutes)

• When a minute trace of hydrochloric acid is added to pure water, a significant increase in

hydrogen – ion concentration occurs immediately

• In similar manner, when a minute trace of sodium chloride is added to pure water, it causes a

correspondingly large increase in in the hydroxyl – ion concentration

• These changes takes place because water alone cannot neutralize even traces of acid or base, that

is, it has no ability to resist changes in hydrogen – ion concentration or pH. A solution of a neutral

salt, such as sodium chloride, also lacks this ability. Therefore it is said to be unbuffered

• The presence of certainly substances or combination of substances in aqueous solution imparts to

the system the ability to maintain a desired pH at a relatively constant level, even with the

addition of materials that may be expected to change the hydrogen – ion concentration.

• These substances or combinations of substances are called buffers, their ability to resist changes

in PH is referred to as buffer action; their efficiency is measured by the function known as buffer

capacity; solutions of them are called buffer solutions

• By definition, then, a buffer solution is a system, usually an aqueous solution, that possesses the

property of resisting changes in PH with the addition of small amounts of a strong acid or base

• Buffers are used to establish and maintain an ion activity within rather narrow limits

• In pharmacy, the most common buffer systems are used in

o The preparation of such dosage forms as injections and ophthalmic solutions, which are

placed directly into pH sensitive body fluids;

o The manufacture of formulations in which the pH must be maintained at a relatively constant

level to ensure maximum product stability: and

o Pharmaceutical tests and assays requiring adjustment to or maintenance of a specific pH for

analytic purposes

• A buffer solution is usually composed of a weak acid and a salt of the acid, such as acetic acid

and sodium acetate, or weak base and a salt of the base, such as ammonium hydroxide and

ammonium chloride

• Typical buffer systems that may be used in pharmaceutical formulations include the following

pairs; acetic acid and sodium acetate, boric acid and sodium borate, and disodium phosphate and

sodium acid phosphate

151

• Formulas for standard buffer solutions for pharmaceutical analysis are given in the united States

Pharmacopeia

• In the selection of a buffer system, due consideration must be given to the dissociation constant of

the weak acid or base to ensure maximum buffer capacity. This dissociation constant, in the case

of an acid, is a measure of the strength of acid; the more readily the acid dissociates, the higher

its dissociation constant and the stronger the acid

• Selected dissociation constants, or ka value, the dissociation constant, or Ka value, of a weak acid

is given by the equation:

Ka = (H+) (A-) Where A- == salt

(HA) HA == acid

• Because the numerical values of most dissociation constants are small numbers and may vary

over many powers of 10, it is more convenient to express them as negative logarithms

pKa = – log Ka

When equation Ka = (H+) (A-) is expressed in logarithmic form, it is written:

(HA)

pKa = – log (H+) – log salt

acid

Since pH = – log (H ):

then pKa = pH – log salt

acid

and pH = pKa + log salt

acid

152

Buffer Equation:

• The equation just derived is the Henderson –Hasselbalch equation for weak acids, commonly

known as the buffer equation

• Similarly, the dissociation constant, or Kb value, of a weak base is given by the equation:

Kb = (B+) (OH -) in which B+ = salt

(BOH) and BOH = Base

And the buffer equation for weak bases, which is derived from this relationship, may be expressed as:

pH = pKw – pKb + log base

salt

• The buffer equation is useful for calculating

o The pH of a buffer system if its composition is known

o The molar ratio of the components of a buffer system required to give a solution of a desired

pH. The equation can also be used to calculate the change in pH of a buffered solution with

the addition of a given amount of acid or base

STEP 3: Performing Calculations Involving Buffer Solutions (60 Minutes)

Activity: Small Group Discussion ( 30 minutes)

DIVIDE students in small manageable groups

ASK students to discuss in groups on the following questions

• pKa Value of a Weak Acid with known Dissociation Constant

• Calculate the pKa value of a weak acid, given its dissociation constant, Ka.

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

153

The dissociation constant of acetic acid is 1.75 x 105- at 250C. Calculate its pKa value

Ka = 1.75 x 10-5

And log Ka = log 1.75 + 105-

= 0.2430 – 5 = – 4.757 or – 4.76

Because pKa = – log Ka

pKa = – (-4.76) = 4 .76

STEP 4: Key Points (5 minutes)

• Buffer solution is a system, usually an aqueous solution, that possesses the property of resisting

changes in pH with the addition of small amounts of a strong acid or base

• A buffer solution is usually composed of a weak acid and a salt of the acid, such as acetic acid

and sodium acetate, or weak base and a salt of the base, such as ammonium hydroxide and

ammonium chloride

• Typical buffer systems that may be used in pharmaceutical formulations include the following

pairs; acetic acid and sodium acetate, boric acid and sodium borate, and disodium phosphate and

sodium acid phosphate

STEP 5: Evaluation (5 minutes)

• What is a buffer solution?

• What are the applications of buffer solutions in pharmacy?

154

STEP 6: Take Home Assignment (15 minutes)

Activity: Take home Assignment (15 minutes)

• pH Value of a salt /Acid Buffer System

• Calculate the pH value:

• What is the pH of a buffer solution prepared with 0.05M sodium borate and 0.005M

boric acid?

• The pKa value of boric acid is 9.24 at 250C

Note that the ratio of the components of the buffer solution is given in molar

concentrations

ALLOCATE time for students to do the assignment and submit

REFER students to recommended references

• Using the buffer equation for weak acids:

pH = pKa + log salt

acid

= 9.24 + log 0.05

0.005

= 9.24 + log 10

= 9.24 +1

= 10 .24

155

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

156

← Previous Topic Next Topic →View all Pharmaceutics Theory and Compounding topics Open Complete Full Notes

PDF / OFFLINE NOTES

Unataka kutumiwa notes hizi kupitia WhatsApp?Kwa notes zilizopangiliwa vizuri kwa kusoma offline au PDF, bonyeza kitufe hapa chini. Ujumbe wenye Level, Semester, Module na Topic utaandaliwa moja kwa moja.TUMIWA NOTES WHATSAPP

WhatsApp: 255620339260