Transcript
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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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Solution propertiesSolution properties
AcidAcid
BaseBase
Ionization of waterIonization of water pHpH
significancesignificance
pKapKa Henderson-Henderson-
HaslebachHaslebach
equationequation
Buffer solutionBuffer solution DefinitionDefinition
ActionAction Buffer capacityBuffer capacity
PreparationPreparation
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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Dissociation (or ionization)constants and pKa:
In a solution of a weakly acidic drugHA the equilibrium may be
represented by:
HA H+ + A-
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Dissociation (or ionization)constants and pKa:
The protonation of a weakly basicdrug B can be represented by :
B + H+ BH+
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Dissociation (or ionization)constants and pKa:
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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Solution propertiesSolution properties
AcidAcid
BaseBase
Ionization of waterIonization of water pHpH
significancesignificance
pKapKa Henderson-Henderson-
HaslebachHaslebach
equationequation
Buffer solutionBuffer solution DefinitionDefinition
ActionAction Buffer capacityBuffer capacity
PreparationPreparation
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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Solution propertiesSolution properties
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
PreparationPreparation
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
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