Buffer Capacity[1]

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Solution propertiesSolution properties

pHpH AcidAcid BaseBase

Ionization of waterIonization of water pH scalepH scale significancesignificance

pKapKa

Weak acid, baseWeak acid, base Henderson-HaslebachHenderson-Haslebachequationequation

significancesignificance

Buffered solutionBuffered solution DefinitionDefinition ActionAction

Buffer capacityBuffer capacity Depend on ratioDepend on ratio

salt/acidsalt/acid Depend onDepend on

concentration salt +concentration salt +acidacid

Depend on amount ofDepend on amount ofadded a, badded a, b

Maximum bufferMaximum buffercapacitycapacity

PreparationPreparation

examplesexamples


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Acid

a substance which donates a proton(or hydrogen ion)

the addition of an acid to water willincrease hydrogen ion concentration(more than 10-7 mol/l)


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Base

a substance that accepts protons

the addition of a base will decrease

the concentration of hydrogen ions.


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dissociation of water

The dissociation of water can berepresented by:

H2O H+ + OH- In pure water the concentrations of

H+ and OH- ions are equal and at 25C

both have the values of 1 x 10-7 mol/l.


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dissociation of water

The hydrogen ion concentrationrange is from 1 mol/l for a strong

acid down to 1 x 10-14 mol/l for astrong base.

To avoid the use of such low values

pH has been introduced as a moreconvenient measure of hydrogen ionconcentration.


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pH scalepH scale


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pH

the negative logarithm of thehydrogen Ion concentration [H+]

pH = -log10

[H+]


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pH

pH of a neutral solution like purewater is 7, why?

because the conc. of H+ ions (andOH-) ions = 1 x 10-7 mol/l


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pH

pHs of acidic solutions will be lessthan 7

pHs of alkaline solutions will begreater than 7


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pH has several importantapplications in pharmaceutical

practice.

Affect the solubilities of drugs thatare weak acids or bases

Affect the stabilities of many drugs Affect the ease of absorption of

drugs from the GIT


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AcidAcid

BaseBase

Ionization of waterIonization of water pHpH


pKapKa Henderson-Henderson-

HaslebachHaslebach

equationequation

Buffer solutionBuffer solution DefinitionDefinition

ActionAction Buffer capacityBuffer capacity


examplesexamples


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Dissociation (or ionization)constants and pKa:

In solutions of weak acids or weakbases equilibria exist between

undissociated molecules and theirions.


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In a solution of a weakly acidic drugHA the equilibrium may be

represented by:

HA H+ + A-


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The protonation of a weakly basicdrug B can be represented by :

B + H+ BH+


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Such equilibrium do not occur insolutions of strong acids or bases in

water, why?

because they are completely ionized.


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The ionization constant (dissociation constant) Ka of a weak acid

can be obtained by applying the Law of Mass Action:

[H+] [A-]K

a=

[HA]

pKa

= the negative logarithm of Ka

pH = the negative logarithm of the hydrogen ion conc. [H+]

[HA]

pKa= pH + log

[A-]

Henderson-Hasselbalch equation:A general equation that is applicable to any acidic drug with one

ionizable group where:

Cu

= conc. of the unionized ; Ci

= conc. of the ionized species

Cu= +


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The ionization constant (dissociation constant) Ka of a protonated

weak base is given by

[H+] [B]K

a=

[BH+]

Taking the negative log of this equation:

[BH+]

pKa= pH + log

[B]

Henderson-Hasselbalch equation:

A general equation that is applicable to any weak basic drug with

one ionizable group where:

Ci= conc. of protonated ; C

u= conc. of the unionized species

CipK = pH + log

f f


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The degree of ionization of a drug in a solution can be

calculated from the Henderson-Hasselbalch equations for

weak acids and bases if the pKa of the drug and the pH of the

solution are known

Such calculations are useful in determining the degree of

ionization of drugs in various parts of the GIT and in the plasma

Examples

1. The pKa

of the weakly acidic drug sulphapyridine is about 8.0

and if the pH of the intestinal contents is 5.0 then the ratio of

unionized:ionized drug is given by:

Cu

log = pKa

- pH = 8 5 = 3C

i

C :C = antilo 3 = 103 : 1

Th K l f i i hi h i k id i b t 3 5 d


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The pKa value of aspirin, which is a weak acid, is about 3.5, and

thepH of the gastric contents is 2.0

Cu

log = pKa - pH = 3.5 - 2.0 = 1.5Ci

the ratio of the conc. of unionized acetylsalicyclic acid to

acetylsalicylate anion is given by:

Cu:Ci = antilog 1.5 = 31.62 :1

The pH of plasma is 7.4 so that the ratio of unionized:ionized

aspirin in this medium is given by:

Culog = pKa - pH = 3.5 7.4 = -3.9

Ci

Cu:Ci = antilog -3.9 = 1.259 x 10-4: 1


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AcidAcid

BaseBase

Ionization of waterIonization of water pHpH


pKapKa Henderson-Henderson-

HaslebachHaslebach

equationequation

Buffer solutionBuffer solution DefinitionDefinition

ActionAction Buffer capacityBuffer capacity


examplesexamples


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Buffered Solutions

A buffered solution or buffer is a

solution that resists a change in pHupon addition of small amounts ofstrong acid or strong base.


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Composition and Action ofBuffered Solutions

A buffer consists of a mixture of aweak acid (HX) and its conjugate

base (X). HX(aq) H+(aq) + X(aq)

Thus a buffer contains both:

An acidic species (to neutralize OH)and

A basic species (to neutralize H+).


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B ff C it


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Buffer Capacity

A buffer counteracts the change in pH of a solution upon the

addition of a strong acid, a strong base, or other agents that tend

to alter the hydrogen ion concentration.

Buffer capacity :buffer efficiency, buffer index or buffer value

Is the resistance of a buffer to pH changes

upon the addition of a strong acid or base.Definition:

It can be defined as being equal to the amount of strong acid or

strong base , expressed as moles of H +or OH- ions, required to

change the pH of one litre of the buffer by one pH unite.Maximum buffer capacity (max) obtain when ratio of acid to salt =

1 i.e. pKa = pH


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Definition The ratio of increase of strong base (or acid) to small

change in pH brought about by this addition.

= B pH

B = the small increment in gram equiv./liter of strongbase

added to the buffer solution to produce a pH change pH = pH change


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The buffer capacity of the solution has a value of 1:

when the addition of 1 gram equiv. of strong base (or acid) to

1 liter of the buffer solution results in a change of 1 pH unit.

Ex:

Acetate buffer contains: 0.1 mole each of acetic acid & sodium acetate

in 1 liter of solution.a) 0.01 mole portions of NaOH is added

b) The conc. of Na acetate (the [salt] in buffer equation) by 0.01 mol/l

& the conc. of acetic acid [acid] by 0.01 mol/lbecause each increment of base converts 0.01 mole of acetic acid into

0.01 mole of sodium acetate according to the reaction.

HAc + NaOH NaAc + H2O0.1 0.01 0.01 0.1 + 0.01


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The changes in concentration of the salt and the acid by the

addition of a base are represented in the buffer equation by

using the modified form

pH = pKa + log [salt ] + [base]

[acid] - [base]

Before the addition of the first portion of NaOH, the pH ofthe buffer solution is

pH = 4.76 + log ( 0.1 + 0 ) = 4.76( 0.1 0 )


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Moles of

NaOH added

pH of

solution

0 4.76

0.01 4.85

0.02 4.94

0.03 5.03

0.04 5.13

0.05 5.24

0.06 5.36

Buffer

Capacity,

0.11

0.11

0.11

0.10

0.09

0.08

0.07


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The buffer capacity is not a fixed value for a given buffer

system, but depends on the amount of base added.

With the addition of more NaOH, the buffer capacity

decreases rapidly, and, when sufficient base has been

added the acid convert completely into sodium ions and

acetate ions

The buffer has its greatest capacity before any base is

added where [salt] / [acid] = 1, and according to equation,

pH =pKa.

The buffer capacity is influenced by an increase in the total

conc. of the buffer constituents since a greater conc. of salt

and acid provides a greater alkaline and acid reserve.

V Sl k d l d t ti f l l ti


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Van Slyke developed a more exact equation for calculation

of buffer capacity

Ka [H3O+] 2.3 C = [H3O

+])2+ Ka)

C = The total buffer concentration (the sum of the molarconcentrations of the acid and the salt).

Ka = dissociation constant

H3O+ = hydrogen ion concentration

The equation permits the calculation of the buffer capacity

at any hydrogen ion concentration, i.e. when no acid or base

has been added to the buffer

E l


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Example:

If hydrogen ion concentration is 1.75 x 10-5, pH = 4.76

what is the capacity of the buffer containing 0.10 mole of

each of acetic acid and sodium acetate per liter of solution?

The total concentration , C = [acid] + [salt], is 0.20 mol/l andthe dissociation constant Ka is 1.75 x 10-5

Ka [H3O+] 2.3 C =

(Ka + [H3O+])2

= 2.3 X 0.20 X (1.75x10-5) X (1.75 X 10-5) = 0.115

[(1.75x10-5) +(1.75 X 10-5)]2


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Prepare a buffer solution of pH 5having a capacity of 0.02.

1-One chooses a weak acidhaving a pKa close to the pH

desired. Acetic acid, pKa= 4.76is suitable in

this case


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2-The ratio of salt and acidrequired to produce a pH of 5

was found to be [salt] / [acid] =1.74/ 1

3]Th b ff it ti i d t bt i th t t l


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3]The buffer capacity equation is used to obtain the total

buffer concentration , C = [salt ] + [acid]

Ka[H3O+

] 2.3 C = [H3O

+])2+ Ka)

0.02=2.3X C X (1.75x10-5) X (1X 10-5)

[(1.75x10-5) + (1 X 10-5)]2

C= 3.75 x 10-2 mol/l

From (b): [salt] = 1.74 x [acid], andfrom (c): C = 1.74 x [acid] + [acid] = 3.75 X 10-2 mol/l

[acid] = 1.37 x 10-2 mol/l

[salt] = 1.74 X [acid] = 2.38 X 10-2 mol/l

Th i fl f t ti b ff it


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The influence of concentration on buffer capacity.

The buffer capacity is affected not only by the (salt)/(acid) ratio but also by

the total concentrations of acid and salt . As shown previously in the table.when

0.01 mole of base was added to a 0.1 molar acetate buffer, the pH increasedfrom 4.76 to 4.85 or a pH of 0.09

If the concentration of acetic acid and sodium acetate is raised to 1 molar

,the pH of the original buffer solution remains at about 4.76 , but now upon the

addition of 0.01 mole of base it becomes 4.77 and pH of only 0.01

Therefore, an increase in the concentration of the buffer components results ina greater buffer capacity or efficiency.

In summary , the buffer capacity depends on

(a) the value of the ratio salt/acid , increase as the value

approaches unity.

(b) the magnitude of the individual concentrations of the

buffer components, increase as C increased

ff


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Maximum buffer capacity.

The maximum buffer capacity occurs when pH = pKa or

when (H3O+

) = Ka

max = 2.303 C (H3O+) 2 = 0.576

2(H3O+) 2

max = 0.576 C

Where C is the total buffer concentration

Example:

What is the maximum buffer capacity of an acetate buffer

with a total concentration of 0.20 mol/l?

max = 0.576 C

=0.01152=0.01


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pHpH AcidAcid BaseBase

Ionization of waterIonization of water pH scalepH scale significancesignificance

pKapKa Weak acid, baseWeak acid, base Henderson-HaslebachHenderson-Haslebach

equationequation significancesignificance

Buffered solutionBuffered solution DefinitionDefinition ActionAction

Buffer capacityBuffer capacity Depend on ratioDepend on ratio

salt/acidsalt/acid Depend onDepend on

concentration salt +concentration salt +acidacid

Depend on amount ofDepend on amount ofadded a, badded a, b

Maximum bufferMaximum buffercapacitycapacity


examplesexamples


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I think you are referring to buffers. At theI think you are referring to buffers. At the

point of half neutralisation, the acid and itspoint of half neutralisation, the acid and its

conjugate base are at equalconjugate base are at equalconcentrationsconcentrations

Ka= [H+][A-]/[HA]Ka= [H+][A-]/[HA]

But when [HA]=[A-] half way through theBut when [HA]=[A-] half way through the

reacion, this cancels down to Ka=[H+]reacion, this cancels down to Ka=[H+]

=> pKa=pH=> pKa=pH


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Isoelectric pointIsoelectric point TheThe isoelectric pointisoelectric point ((pIpI), sometimes), sometimes

abbreviated toabbreviated to IEPIEP, is the pH at which a particular, is the pH at which a particularmolecule or surface carries no net electricalmolecule or surface carries no net electricalcharge.charge.

Amphoteric molecules called zwitterions containAmphoteric molecules called zwitterions contain

both positive and negative charges depending onboth positive and negative charges depending onthe functional groups present in the molecule.the functional groups present in the molecule.

The net charge on the molecule is affected by pHThe net charge on the molecule is affected by pHof their surrounding environment and canof their surrounding environment and canbecome more positively or negatively chargedbecome more positively or negatively charged

due to the loss or gain of protons (H+). The pI isdue to the loss or gain of protons (H+). The pI ispH value at which the molecule carries nopH value at which the molecule carries noelectrical charge or the negative and positiveelectrical charge or the negative and positivecharges are equal.charges are equal.

The pI value can affect the solubility of aThe pI value can affect the solubility of amolecule at a given pH. Such molecules havemolecule at a given pH. Such molecules have

i i l bili i l l ii i l bili i l l i

Buffer Capacity[1]

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