Buffer capacity MANIK

preparation of buffers, buffers and isotonic systems. Methods for
adjustment of tonicity of solutions. Buffers in pharmaceutical and biological systems.

2. An unbufferedsolution
or a buffered solution
acid added
base added
acid added base added
Md. Imran Nur Manik

3. A buffer solutionis a solution which resists changes in pH when a
small amount of acidor baseis added.
The resistive action is the result of equilibrium between the weak acid
(HA) and its conjugate base (A
-
).
HA
(aq) + H
2O
(l) → H
3O
+
(aq) + A
-
(aq)
Typically a mixture of a weak acid and a salt of its conjugate base or weak base and a salt of its
conjugate acid.Md. Imran Nur Manik

4. Two types :
ACIDIC BUFFERS –
Solutionofamixtureofaweakacidandasaltofthis
weakacidwithastrongbase.
e.g. CH
3COOH + CH
3COONa
(weak acid ) (Salt )
BASIC BUFFERS –
Solutionofamixtureofa anda
e.g +
( Weak base) ( Salt)
Md. Imran Nur Manik

5. HOWBUFFERSWORK
Equilibrium between acid and base.
Example:ACETATEBUFFER
CH
3COOH⇌CH
3COO
⎯
+H
+
CH
3COONa⇌CH
3COO
⎯
+Na
+
IfmoreH
+
isaddedtothissolution,itsimplyshiftstheequilibrium
totheleft,absorbingH
+
,sothe[H
+
]remainsunchanged.
IfH
+
isremoved(e.g.byaddingOH
⎯
)thentheequilibriumshiftsto
theright,releasingH
+
tokeepthepHconstant
Md. Imran Nur Manik

6. Equilibrium between acid and base.
Example:BasicBuffer
NH
4OH⇌NH
4
+
+OH
⎯
NH
4Cl⇌NH
4
+
+Cl
⎯
IfmoreOH
⎯
isaddedtothissolution,itsimplyshiftstheequilibrium
totheleft,absorbingOH
⎯
,sothe[OH
⎯
]remainsunchanged.
IfOH
⎯
isremoved(e.g.byaddingH
+
)thentheequilibriumshiftsto
theright,releasingOH
⎯
tokeepthepHconstant
HOWBUFFERSWORK(CONT..)
Md. Imran Nur Manik

7. HANDERSONHASSELBALCHEQUATION
LawrenceJosephHendersonwrotean
equation,in1908,describingtheuse
ofcarbonicacidasabuffersolution.
KarlAlbertHasselbalchlaterre-expressed
thatformulainlogarithmicterms,resultingin
theHenderson–Hasselbalchequation.
Md. Imran Nur Manik

8. K
a=
[H
+
] [A
-
]
[HA]
Take the -log on both sides
The Henderson-HasselbalchEquation derivation
-log K
a= -log [H
+
] -log
[A
-
]
[HA]
pH =pK
a+ log
[A
-
]
[HA]
= pK
a+ log
[Salt]
[Acid]
HA H
+
+ A
-
pK
a= pH-log
[A
-
]
[HA]
Apply p(x) = -log(x)
And finally solve for pH…
Md. Imran Nur Manik

9. Problem:FindthepHofabuffersolutioncontaining0.20moleperlitreCH
3COONa
and0.15moleperlitreCH
3COOH.Kaforaceticacidis1.810
–5
.
Problem:Calculatetheconcentrationofaceticacidtobeaddedtoa0.1Msolutionof
sodiumacetatetogiveabufferofpH5(pKaofaceticacid=4.66).
Problem:TheKaofpropionicacidis1.34×10
–5
.WhatisthepHofasolutioncontaining
0.5Mpropionicacid,C
2H
5COOH,and0.5sodiumpropionate,C
2H
5COONa.What
happenstothepHofthissolutionwhenvolumeisdoubledbytheadditionofwater?
Problem:Abuffersolutioncontains0.015moleofammoniumhydroxideand0.025mole
ofammoniumchloride.CalculatethepHvalueofthesolution.Dissociationconstantof
NH
4OHattheroomtemperatureis1.80×10
–5
Problem:EstimatethepHat25°Ccontaining0.10Msodiumacetateand0.03M
aceticacidpKaforCH
3COOH=4.57.

10. Buffercapacityisameasureoftheefficiencyofabuffer,inresisting
changesinpH.Thebuffercapacityisdefinedas
Itisalsoknownasbufferefficiency,
bufferindex,andbuffervalue.
Conventionally,thebuffercapacity(β)isexpressedas
AbuffersolutioncanresistasmallamountofchangeofpHonaddingacid
oralkalitothesolution.Buffercapacitiesrangingfrom0.01-0.1areusually
adequateformostpharmaceuticalsolutions.

11. In1922,VanSlykefirstintroducedanapproximateequationtodeterminethebuffercapacityby
thefollowingequation:
Inwhichβ=Buffercapacity,deltaΔ=afinitechange,andΔB=thesmallincrementingram
equivalents(gEq)/litreofstrongbaseaddedtothebuffersolutiontoproduceapHchangeofΔpH.
Accordingtoequation,thebuffercapacityofasolutionhasavalueof1whentheadditionof1g
Eqofstrongbase(oracid)to1litreofthebuffersolutionresultsinachangeof1pHunit.
ThehigherthebuffercapacitythelessthebuffersolutionchangesitspH.
Amoreexactequationforbuffercapacity:Thebuffercapacitycalculatedfromabove
equationisonlyapproximate.Itgivestheaveragebuffercapacityovertheincrementofbaseadded.
KoppelandSpiroandVanSlykedevelopedamoreexactequation,
Where,C=thetotalbufferconcentration(i.e.thesumofthemolarconcentrationsofacidand
salt).

12. BodyfluidscontainbufferingagentsandbuffersystemsthatmaintainpH
atornearpH=7.4.Importantendogenous(natural)buffersystemsinclude
carbonicacid/sodiumbicarbonateandsodiumphosphateinthe
plasmaandhaemoglobin,andpotassiumphosphateinthecells.
AninvivovalueofpH<6.9orpH>7.8canbelifethreatening.
Pharmaceuticalsolutionsgenerallyhavealowbuffercapacityinorderto
preventoverwhelmingthebody’sownbuffersystemsandsignificantly
changingthepHofthebodyfluids.Bufferconcentrationsofbetween0.05
and0.5Mandbuffercapacitiesbetween0.01to0.1areusuallysufficient
forpharmaceuticalsolutions.

13. Components pH range
HCl,Sodiumcitrate 1–5
Citricacid,Sodiumcitrate2.5-5.6
Aceticacid,Sodiumacetate3.7-5.6
K
2HPO
4,KH
2PO
4 5.8-8
Na
2HPO
4,NaH
2PO
4 6-7.5
Borax,Sodiumhydroxide9.2–11

14. H
2CO
3⇌H
+
+HCO
3
–
ThepHofbloodiscontrolledbyabicarbonate(H
2CO
3/HCO
3
–)
buffersystem.WhenthepHgetstoo
high(highOHconcentration),theOH
–
reactswithcarbonicacid(H
2CO
3)toformHCO
3
–)
andH
2O.
WhenthepHgetstoolow(highH
+
concentration),theH
+
reactswithHCO
3
–
toformH
2CO
3.Because
H
2CO
3isaweakacid,theH
+
staysassociatedwiththeH
2CO
3.SincepHisanimportantfactorin
manyphysiologicalprocesses,achangeinthebloodpHisapotentiallylifethreateningcondition
requiringimmediateregulation.
ThepHofblood:
TheHCO
3
-
/H
2CO
3buffersystemispresentinbloodingreatestconcentrationandisveryimportantin
maintainingthepHofbloodwithinnormallimit.TheconcentrationofHCO
3
-
andH
2CO
3inbloodare
0.02Mand0.00125Mrespectivelyandhencethe[HCO
3
-
]/[H
2CO
3]ratiois20/1.Inblood,thepKa
valueforfirstionizationstageatbodytemperatureis6.1.
pH=pK
a+Log[salt]/[Acid]
=6.1+Log20/1=6.1+1.2=7.4
Thephosphoricandproteinbuffersofplasmaareofrelativelylittleimportantascomparedwith
bicarbonatebufferinregulatingpH.

15. Bufferisveryimportantforbiologicalsystem.Someofthepicturesareasfollows
1.Buffermaintainsconstant[H
+
]inthebodyrequiredforoptimumcellularactivity.
2.ThepHofblood(around7.4)iscontrolledbyabicarbonate(H
2CO
3/HCO
3
–)
buffersystem.
3.Thephosphatebuffersystem(HPO
4
2-
/H
2PO
4
-
)playsaroleinplasmaanderythrocytes.
H
2PO
4
-
+H
2O⇌H
3O
+
+HPO
4
2-
M/A:Anyacidreactswithmonohydrogenphosphatetoformdihydrogenphosphate
HPO
4
2-
+H
3O
+
H
2PO
4
-
+H
2O
monohydrogenphosphate dihydrogenphosphate
Thebaseisneutralizedbydihydrogenphosphate
H
2PO
4
-
+OH
-
HPO
4
2-
+H
3O
+
dihydrogenphosphate monohydrogenphosphate
4.Proteinsasabuffer:Proteinscontain–COO
-
groups,which,likeacetateions(CH
3COO
-
),canactasproton
acceptors.Proteinsalsocontain–NH
3
+
groups,which,likeammoniumions(NH
4
+
),candonateprotons.
M/A:Ifacidcomesintoblood,hydroniumionscanbeneutralizedbythe–COO
-
groups
-COO
-
+H
3O
+
-COOH+H
2O
Ifbaseisadded,itcanbeneutralizedbythe–NH
3
+
groups
-NH
3
+
+OH
-
-NH
2+H
2O

16. PROTEIN BUFFER SYSTEM
Proteinsareverylarge,complexmoleculesin
comparisontothesizeandcomplexitiesofacidsor
bases
Proteinsaresurroundedbyamultitudeofnegative
chargesontheoutsideandnumerouspositivecharges
inthecrevicesofthemolecule
-
-
-
----
-
-
-
-
-
-
-
--
-----
-
---
-
-
-
-
- --
-
+
+
++
+
+
+
+
+
+
+
+
+
++ +
+
+
+
+
+
+
++
+
Md. Imran Nur Manik

17. PROTEIN BUFFER SYSTEM
H
+
ions are attracted to and held from chemical
interaction by the negative charges
-
-
-
----
-
-
-
-
-
-
-
--
-----
-
---
-
-
-
-
- --
-
+
+
++
+
+
+
+
+
+
+
+
+
++ +
+
+
+
+
+
+
++
+
H
+
H
+
H
+
H
+ H
+ H
+ H
+ H
+ H
+ H
+
H
+
H
+
H
+
H
+
H
+H
+
H
+
H
+
H
+
H
+
H
+
Md. Imran Nur Manik

18. PROTEIN BUFFER SYSTEM
OH
-
ions which are the basis of alkalosis are attracted
by the positive charges in the crevices of the protein
-
-
-
----
-
-
-
-
-
-
-
--
-----
-
---
-
-
-
-
- --
-
+
+
++
+
+
+
+
+
+
+
+
+
++ +
+
+
+
+
+
+
++
+
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
Md. Imran Nur Manik

19. PROTEIN BUFFER SYSTEM
-
-
-
----
-
-
-
-
-
-
-
--
-----
-
---
-
-
-
-
- --
-
+
+
++
+
+
+
+
+
+
+
+
+
++ +
+
+
+
+
+
+
++
+
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
OH
-
H
+
H
+
H
+
H
+ H
+ H
+ H
+ H
+ H
+ H
+
H
+
H
+
H
+
H
+
H
+H
+
H
+
H
+
H
+
H
+
H
+
Md. Imran Nur Manik

20. ToprepareapharmaceuticalbuffersolutionhavingdefinitepHandcapacity,thepharmacistshouldmaintainthe
followingsteps:
1.SelectaweakacidhavingapKaapproximatelyequaltothepHatwhichthebufferistobeused.Thiswill
ensuremaximumbuffercapacity.
2.Fromthebufferequation,calculatetheratioofsaltandweakacidrequiredtoobtainthedesiredpH.(4-10)
3.Considertheindividualconcentrationsofbuffersaltandacidneededtoobtainasuitablebuffercapacity.
(Aconcentrationof0.05to0.5Misusuallysufficient;andabuffercapacityof0.01to0.1isgenerallyadequate.)
4.Otherfactorsofsomeimportanceinthechoiceofapharmaceuticalbufferinclude-
•Availabilityofchemicals
•Sterilityofthefinalsolution
•Stabilityofthedrugandbufferonaging
•Costofmaterials
•Freedomfromtoxicity
e.g.aboratebuffer,becauseofitstoxiceffectscannotbeusedtostabilizeasolutiontobeadministeredorallyor
parenterally.
5.Finally,oneshoulddeterminethepHandbuffercapacityofthecompletedbuffersolutionusingareliablepH
meter.

21. Osmotic Pressure
Theflowofthesolventthroughasemipermeablemembrane
frompuresolventtosolutionorfromadilutesolutionto
concentratedsolutionistermedosmosis(GreekOsmosmeans
“topush”.)
Osmoticpressuremaybedefinedastheexternalpressure
appliedtothesolutioninordertostoptheosmosisofthesolvent
intothesolutionseparatedbyasemipermeablemembrane.
Amembranewhichispermeabletosolventandnottosoluteis
calledsemipermeablemembrane.
Animalandvegetablemembranesarenotcompletelysemipermeable.Cupric
ferrocyanide,Cu
2Fe(CN)
6,membranedepositedinthewallsofporouspotis
perfectlyasemipermeablemembrane.
Md. Imran Nur Manik

22. Theflowofthesolventthroughasemipermeablemembranefrompuresolventtosolutionor
fromadilutesolutiontoconcentratedsolutionistermedosmosis.
Osmoticpressure(π)maybedefinedastheexternalpressureappliedtothesolutionin
ordertostoptheosmosisofthesolventintothesolutionseparatedbyasemipermeable
membrane.
IsotonicSolutions
Solutionshavingthesameosmoticpressurearesaidtobeisotonic.Intermsofphysiological
fluids,thesolutionshavingosmoticpressureequaltotheosmoticpressureofintracellular
fluidiscalledisotonicsolution.
(π
soln= π
cell)
Inpharmacyandmedicalscience,isotonicsolutionisthatsolution;whichhaveequaltonicity
withbodyfluidi.e.blood,serum,plasmaorlacrimalfluid.0.9%NaClsolutionisalso
regardedasisotonicsolution.

23. Hypertonic Solutions
Ascomparedtothebloodplasmaifasolutionhashigherosmoticpressureissaidtobehypertonic
solution.(π
soln>π
cell).
Physiologicalsolutionshavingagreaterosmoticpressurethanthatofbodyfluidor0.9%NaCl
solutionisreferredtoashypertonicsolution.
Hypotonic Solutions
Ascomparedtothebloodplasmaifasolutionhaslowerosmoticpressureissaidtobehypotonic
solution.(π
soln<π
cell).
Physiologicalsolutionswithanosmoticpressurelowerthanthatofbodyfluidor0.9%NaClsolution
isreferredtoashypertonicsolution.
ParatonicSolutions
Thesolutionthatisnotisotonicthatmeansboththehypertonicandhypotonicsolutionsare
calledparatonicsolution.

24. Effect of tonicity on body( Injection on blood)
Thesolutionswhicharenotisotonicwithplasmamaybeharmfultouse.On
injectingthehypotonicsolutionsintobloodstream,itmayenterthebloodcells
inanattempttoproduceequilibrium,thecellsswellsrapidlyuntilthey
burstleadingtohemolysis.Asthisdamageisirreversiblemayleadtoserious
dangertoRBC.
Whenhypertonicsolutionisinjectedintothebloodstream,thewatercomes
outofthemembraneofRBCinordertoreachequilibrium.Thecellsshrink
leadingtocrenulationwhichisonlyatemporarydamage.Whentheosmotic
pressureoftwosolutionsbecomesequalthedamagedcellswillcometoits
originalposition.Hencehypertonicsolutionsmaythereforebeadministered
withoutpermanentdamagetothebloodcells.Theyshouldbeinjectedslowly
toensurerapiddilutionintothebloodstreamandtominimizethecrenulation
ofbloodcells.

25. Calculation for the Preparation of Isotonic Solutions
Forthepreparationofisonoticsolutions,thequantitiesofsubstancestobeaddedmay
becalculatedbythefollowingmethods:
1.Basedonthefreezingpointdata(Freezingpointdepression).
2.Basedonmolecularconcentration.
3.BasedonSodiumChlorideequivalents.
4.Graphicalmethodbasedonvaporpressureandfreezingpointdeterminations.
Method:Basedonfreezingpointdepression.
Thefreezingpointisacolligativepropertyoftenusedinthecalculationoftheisotonic
solutionasitcanbemeasuredeasilyandaccurately.Thetemperatureatwhichblood
plasmaandtears(Lacrhrymalsecretions)freezeis–0.52ºCwhichisthesamevalue
ofa0.9%solutionofNaCl.Allsolutionswhichfreezeat–0.52ºCwillbeisotonicwith
bloodplasmaandlachrymalfluid.
Thefreezingpointsareusuallyexpressedintermsof1%solutions.

26. Thequantityof neededformakingthesolutionsisotonicwith
bloodplasmamaybecalculatedfromthegeneralformulagivenbelow:
PercentageW/Vof needed=
Where,a=freezingpointdepressionofunadjustedsolution.
b=freezingpointdepressionof1%W/Vofthe .
Problem:Findouttheconcentrationof requiredtorenderormakea
1%solutionofcocainehydrochlorideisotonicwithbloodplasma.Thefreezingpointof1%
W/Vsolutionofcocainehydrochlorideis–0.090ºCandthatofNaClis
–0.576ºC.
Problem:Findouttheconcentrationof requiredtorenderormakea
1.5%solutionofproocainehydrochlorideisotonicwithbloodplasma.Thefreezing
pointof1%W/Vsolutionofproocainehydrochlorideis–0.122ºCandthatof is
–0.576ºC.

27. Method:Based on Sodium Chloride Equivalents
Thismethodhasgainedpopularity.NaClmethodisdefinedasthe
weightofsolutionchloridewhichwillproducethesameosmotic
effectas1gofthedrugtoprepareanisotonicsolution.
Formulla:PercentageofNaCl foradjustmentto
isotonicity=0.9–PercentstrengthofdrugsolutionNaCleqivalentof
thethedrug.
Problem:CalculatethepercentageofKNO
3requiredtomake
a0.5%isotonicsolutionofAgNO
3.TheNaClequivalentofKNO
3
is0.56andNaClequivalentofAgNO
3is0.33.
Hints:0.9-(0.50.33)=0.735÷0.56=1.313%
Md. Imran Nur Manik

28. Md. Imran Nur Manik