Download Buffers and weak acids

SURAH AL-FATIHAH
Buffers and weak acids
 Buffers
are solutions that resist
change in pH when acid or base is
added to it.
 It consist of weak acid and its salt
(acetic acid and sodium acetate )or a
weak base and its salt (ammonium
hydro oxide and ammonium chloride
)
Mechanism of buffer action
 Added
H+ ions = in the form of
strong acid ,combine with anions A-(
largely form the salt component of
buffer), to form the weakely
dissociable HA, so that pH does
become as acid as it would be in
the absence of the buffer.
Mechanism cont..
 Acetic
acid freely ionizable
 Sodium acetate to a large extent
 CH3COOH=CH3COO+H+
 CH3COONa=CH3COO +Na+
 H Cl is added acetate will combine
with H+ to form acetic acid and
NaCl is formed
Mechanism contd..
 When
NaOH is added the H+ of
the buffer (acetic acid) combine
with OH- to form water ,which is
weakely dissociated .
 Thus pH change due to base
addition is also prevented by buffer

OH+H+ =H2O
BUFFERING
CAPACITY
The efficiency of a buffer in maintaining
a constant pH on the addition of acid
or base is referred to as buffering
capacity.
 The capacity to combine with added acid
remains so long as there is supply of the
buffer salt in the medium.
 OH can be buffered as long as some of
the acid HA remains to supply the H+.

H-H EQUATION
 The
quantitative relationship
between the concentrate of weak
(HA) and its conjugate (A-) is
Henderson equation.
 HA=weak acid =H+ +A H+ =proton
H-H EQUATION
 Is
important for under standing
buffer action and acid –base balance
in the blood and tissue.
 Restating the expression for
dissociation constant of an acid
 The pH of a solution of a weak acid
(or base) and its salt is given by
 pH=pKa _log [HA/A-]
H-H EQUATION
 Ke
=equilibrium constant
 K e =ionization constant
 Or
dissociation constant of an
acid or Ka
 Stronger acids :phosphoric acid,
carbonic acid ,acetic acid have
larger dissociation constant
Cont..
 Weaker
acids mono hydrogen PO4
have smaller dissociation constant
 Pka=log 1/Ka
 pKa =-logka
 The stronger the tendency to
dissociate a proton ,the stronger is
the acid lower the pKa
H-H eq…
 Ka=[H+][A-]/HA
 First
solve for [H+]=Ka [HA]/[A-]
 Take neg log on both side
-log [H+]=-log Ka –log [HA]/A Putting values pH=pKa -log[HA]
 pH=pKa+log[A-]/[HA]
 pH=pKa
Determination of pH
 Take
three test tubes
 To one add 1ml sodium
acetate+acetic acid=(log 1/10)=-1
 10 ml both(log 10/10)=0
 10ml Na acetate +1ml acid
(log10/1)=1
Buffers of the body fluids
 Intracellular
buffers;phosphate
buffers cossist of disodium hydrogen
phosphate and sodium dihydrogen
phosphate.
 This has pka close to physiological
ph
 Pka=6.8
Contd.
 Protein
buffers :depend on ionizable
side chain
 Histine imidazole group pka=6.1
 16 histidine in albumin
 38 histidine in haemoglobin
Buffers …
 Extracellular
;comprise43%(intracellular 57%)
 65%=bicarbonate buffers
 30%=haemoglobin
 4%=protein
 1%=phosphate buffers
 Buffers are first line of defence
against acid load
Bicarbonate buffer system
 Consist
of HCO3 and H2CO3
 Most significant
 65% of plasma buffering
 40% of buffering of body
 CO2 andHCO3 can diffuse easily
across membranes
 HCO3 (Metabolic component)
regulated by kidney and co2 by
respiration.
Buffers ..
HCO3 22-26 mmol/lt (24mmol)
 Pka=6.1
 H2CO3=(CO2 in forms
dissolved)
 pCO2=40
 Solubility coeffeciant=0.3
 pH=Pka+logHCO3/H2CO3
 7.4=6.1+LOg24/1.2
 7.4=6.1+LOg20
 7.4=6.1+1.3

Body buffers
 Three
mechanism: to regulate pH
and acid base balance and maintain
the blood pH (around 7.4)
 1.blood buffers
 2.respiratory mechanism
 3.renal mechanism
EFFECTS OF ACID BASE
DISTURBANCES
H+increased= acidosis,depression of
C.N.S, Disorientation, death in coma.
 H+decreased= alkalosis, overexcitability
of C.N.S,convulsions.
First in peripheral Nerves than C.N.S
Sensory effects,Tingling(pins and needle
sensation
Motor effects, muscle twitches, spasm
Extreme alkalosis- spasm of respiratory
muscles, death

EFFECTS…..
2. CHANGES IN ENZYMES ACTIVITY
Altering shape and activity of protein
molecule.
Some reactions are accelerated and
some are depressed.
EFFECTS…..
3. CHANGES IN CELLULAR pH
Reduced contractility of actin and
myosin in muscles.
CHANGES IN POTASSIUM LEVEL
H+ ions enters the cells for sodium
and potassium.
H+ ions are eliminated more than
potassium so hyperkalemia, cardic
disfunction.
Blood buffers
 Bicarbonate
buffer system:
NaHCO/H2CO3
 H2CO3 >H+ +HCO3
 pH=pKa +log [salt]/[acid]
 20:1 is the ratio
 Alkali reserve : responsible for
effective buffering of H+,generated
in the body.
Blood buffers.
 Bicarbonate
buffers ;is index to
understand the disturbances.
 Phosphate buffers; this is important
buffer intracellular .
 Sodium di hydrogen phosphate
and disodium hydrogen phosphate
 Ratio of base to acid for phophate
buffer is 4:1
 Pk
is 6.8

Tsunami disaster….
The end