BUFFER SOLUTION
DEPARTMENT OF MEDICAL
CHEMISTRY
Yongxiang Hu
1,What is a buffer solution
Example:
In 1 liter pure water add 0.01 mol of
hydrochloric acid,Calculate this solution‘s pH
change value,And in 1 liter contain 0.1 mol
acetic acid and 0.1 acetate ion mixture solution
add 0.01mol of hydrochloric acid,Calculate this
solution‘s pH change value.
Notice,Square bracket will instead of equilibrium
concentration in following time,Such as [H+] instead
hydrogen ion equilibrium concentration.
Step 1,Calculate the pure water‘s pH change value
The pure water‘s hydrogen ion exponent (pH)
value:
pH(pure water) = 7.00
When add the hydrochloric acid:
[H+] = 0.01mol/1 liter =0.01mol.L-1
pH = -log 0.01 = 2.00
pH = 7.00 – 2.00 = 5.00 (pH unit)
Step 2,Calculate contain acetic acid and its
conjugate base solution‘s pH value
In the solution acetic acid‘s concentration and
acetate ion‘s concentration
[HAc] = 0.10 mol.L-1
[Ac-] = 0.10 mol.L-1
Acetic acid equilibrium-constant
Ka = 1.70?10-5
In this solution has a acid-base equilibrium
HAc H+ + Ac-
Starting 0.10 0.10
Substitute the concentrations into the equilibrium-
constant equation
Ka = [H+][Ac-]/[HAc]
[H+] = Ka? [HAc]/[Ac-]
= 1.70? 10-5? 0.10/ 0.10
= 1.70? 10-5 (mol.L-1)
pH = 4.77
Step 3,Calculate solution‘s pH value when add
hydrochloric acid
HAc H+ + Ac-
Starting 0.10 0.10
Change +0.01 -0.01
Equilibrium 0.10+0.01 0.10-0.01
Step 4,Substitute the equilibrium concentration
into the equilibrium-constant equation
[H+] = Ka? [HAc]/[Ac-]
= 1.70? 10-5? 0.11/ 0.09
= 2.07? 10-5
pH = 4.68
pH = 4.77–4.68= 0.09 (pH unit)
Definition of buffer solution,
A buffer solution is a solution
characterized by the ability to
resist changes in pH when limited
amounts of strong acid or strong
base are added to it.
2,Composition of Buffer Solution
Buffer solution consist of weak acid
and its conjugate base or weak base and its
conjugate acid,The conjugate acid-base pair
are called buffer system or buffer pair.
Important buffer pair such as:
HAc ---- Ac-
NH4+ ---- NH3
H2CO3 ---- HCO3-
H3PO4 ---- H2PO4-
H2PO4- ---- HPO42-
HPO42- ---- PO43-
3,Calculate the pH value of Buffer
Solution
Henderson-Hasselbalch Equation:
A buffer solution made up of a weak acid HA and
its conjugate base A-,The acid-ionization equilib-
rium is
HA(aq) H+(aq) + A-(aq)
and the acid-ionization constant is
Ka = [H+][A-]/[HA]
According to the constant equation,we can get an
equation for the H+ concentration.
[H+] = Ka? [HA]/[A-]
Take the negative logarithm of both sides of the
equation,That is,
-log[H+] = -log{Ka? [HA]/[A-]}
= -log Ka – log{ [HA]/[A-]}
The left side equals the pH,The pKa of a weak
acid is defined in a manner similar to pH and pOH.
pKa = -log Ka
The previous equation can be written
pH = pKa - log{ [HA]/[A-]}
= pKa + log{ [A-]/[HA]}
More generally,we can write
[base]
pH = pKa + log
[acid]
This is known as the
Henderson-Hasselbalch equation.
Buffer ratio,[Base/acid]
Henderson—Hasselbalch equation change form:
pH＝ pKa+lg ＝ pKa+lg
]HB[
]B[?
c(conjugate base)
c(conjugate acid)
pH＝ pKa+lg ＝ pKa+lg
Vn
Vn
/)HB(
/)B(?
)HB(
)B(
n
n?
pH＝ pKa+lg ＝ pKa+lg
)HB()HB(
)B()B(
Vc
Vc
)HB(
)B(
V
V?
Buffer Capacity and Buffer Effective Range
1,Buffer capacity(?)
is definited as the amount of substance of
strong acid or strong base when the pH
value change one unit of pH unit in 1 liter
of buffer solution.
With differential coefficient:
β =
d p H
d )b(a
V
n
β＝ ＝ 2.303× [HB][B-]/ctotal
dp H
d )b(a
V
n
(1)
(2) 0.1mol.L-1 HAc+ NaOH
(3) 0.2mol.L-1 HAc+NaOH
(4)
pH
βmax＝ 2.303× (ct /2)(ct /2) /ct
＝ 0.576ct
∵ ct＝ [HB]+[B-]
β＝ 2.303× [HB][B-]/ct
＝ 2.303× ([HB] /ct )([B-]/ct )× ct
＝ 2.303× ([HB] /ct )(1-[HB-]/ct )× ct
2,Buffer effective range
When the buffer ratio is 1︰ 10 or 10︰ 1,
we cognize this buffer solution without
the buffer ability,So,we can definite the
buffer effective range as:
pH = pka± 1
Prepare the Buffer solution
Principle and Step
1,Choose correspondence of buffer system
2,The total concentration of buffer solution
3,Calculate the amount of buffer solution
4,Calibrate the buffer solution
Tris,trihydrixymethylaminomethane
(HOCH2)3CNH2
Standard of Buffer Solution
Potassium hydrotartrate (KHC4H4O6)
Potassium acid phthalate (KHC8H4O4)
Borax (Na2B4O7·10H2O)
Phosphate (KH2PO4-Na2HPO4)
Buffer System in Blood
In plasma:
H2CO3-HCO3-,H2PO4--HPO42-、
HnP-Hn-1P-(HnP instead of protein)
In red cell,
H2b-Hb- (H2b instead of haemoglobin)、
H2bO2-HbO2- (H2bO2 instead of
oxyhemoglobin),H2CO3-HCO3-、
H2PO4--HPO42-
CO2(solute)+H2O H2CO3 H++ HCO3-
pH＝ pKa,+lg [HCO3
-]
[CO2]solute
[HCO3-]
[CO2]solute＝ 6.10 + lg
In plasma:
[HCO3-] = 0.024mol·L-1
[CO2]solute= 0.001 2mol·L-1
pH＝ 6.10 + lg
＝ 6.10 + lg
＝ 7.40
1
1
Lm o l0 0 1 2.0
Lm o l0 2 4.0
1
20
SUMMA
1,Buffer solution is composed of a pair of conjugate acid-base
pair with enough concentration.The component of anti-acid is
conjugate base,The component of anti-base is conjugate acid,
When a few strong acid or base is added in to the buffer
solution.The component of anti-acid and anti -base will move
the equilibrium of dissociation to stabilize pH value.
2,We can use the Henderson-Hasselbalch‘s equation to calculate
the pH value of buffer solution:
pH＝ pKa+lg ＝ pKa+lg ＝ pKa+lg
]HB[
]B[? )HB( )B(cc
)HB(
)B(
n
n?
3,Buffer capacity is decided by pan-concentration
and buffer ratio,When the pan-concentration of the
buffer pair is settled,the buffer ratio is the more
close with to 1,the buffer capacity will be the more
large,When the buffer ratio is equal to 1,the buffer
capacity will attain the maximum?β max‘,When the
buffer ratio is equalization,the pan-concentration of
the buffer pair is the more large,the buffer
capacity will be the more large.
4,pH＝ pKa土 1 is stipulated the buffer effective
range,Over this range,the buffer solution will be
to lose the buffer ability too low of the buffer
capacity,Dispense the buffer solution,It may be
according to the table of this book,The methods
have three kinds,The first kind is ―weak acid add
its conjugate base‖,The second kind is ―weak acid
(overdose) add strong base‖,The third kind is
―strong acid add weak base (overdose)‖.
5,The buffer solution apply to medical science
about the buffer system and buffer action
principle in blood is important,In the human
blood,the major buffer pair have H2CO3-HCO3-、
H2PO4--HPO42-,HnP-Hn-1P-,In these buffer pair,
the carbonic acid buffer pair have highest
concentration and buffer ability is highest too
in blood,It have important affection to maintain
the pH value in 7.35~7.45 range of blood.
DEPARTMENT OF MEDICAL
CHEMISTRY
Yongxiang Hu
1,What is a buffer solution
Example:
In 1 liter pure water add 0.01 mol of
hydrochloric acid,Calculate this solution‘s pH
change value,And in 1 liter contain 0.1 mol
acetic acid and 0.1 acetate ion mixture solution
add 0.01mol of hydrochloric acid,Calculate this
solution‘s pH change value.
Notice,Square bracket will instead of equilibrium
concentration in following time,Such as [H+] instead
hydrogen ion equilibrium concentration.
Step 1,Calculate the pure water‘s pH change value
The pure water‘s hydrogen ion exponent (pH)
value:
pH(pure water) = 7.00
When add the hydrochloric acid:
[H+] = 0.01mol/1 liter =0.01mol.L-1
pH = -log 0.01 = 2.00
pH = 7.00 – 2.00 = 5.00 (pH unit)
Step 2,Calculate contain acetic acid and its
conjugate base solution‘s pH value
In the solution acetic acid‘s concentration and
acetate ion‘s concentration
[HAc] = 0.10 mol.L-1
[Ac-] = 0.10 mol.L-1
Acetic acid equilibrium-constant
Ka = 1.70?10-5
In this solution has a acid-base equilibrium
HAc H+ + Ac-
Starting 0.10 0.10
Substitute the concentrations into the equilibrium-
constant equation
Ka = [H+][Ac-]/[HAc]
[H+] = Ka? [HAc]/[Ac-]
= 1.70? 10-5? 0.10/ 0.10
= 1.70? 10-5 (mol.L-1)
pH = 4.77
Step 3,Calculate solution‘s pH value when add
hydrochloric acid
HAc H+ + Ac-
Starting 0.10 0.10
Change +0.01 -0.01
Equilibrium 0.10+0.01 0.10-0.01
Step 4,Substitute the equilibrium concentration
into the equilibrium-constant equation
[H+] = Ka? [HAc]/[Ac-]
= 1.70? 10-5? 0.11/ 0.09
= 2.07? 10-5
pH = 4.68
pH = 4.77–4.68= 0.09 (pH unit)
Definition of buffer solution,
A buffer solution is a solution
characterized by the ability to
resist changes in pH when limited
amounts of strong acid or strong
base are added to it.
2,Composition of Buffer Solution
Buffer solution consist of weak acid
and its conjugate base or weak base and its
conjugate acid,The conjugate acid-base pair
are called buffer system or buffer pair.
Important buffer pair such as:
HAc ---- Ac-
NH4+ ---- NH3
H2CO3 ---- HCO3-
H3PO4 ---- H2PO4-
H2PO4- ---- HPO42-
HPO42- ---- PO43-
3,Calculate the pH value of Buffer
Solution
Henderson-Hasselbalch Equation:
A buffer solution made up of a weak acid HA and
its conjugate base A-,The acid-ionization equilib-
rium is
HA(aq) H+(aq) + A-(aq)
and the acid-ionization constant is
Ka = [H+][A-]/[HA]
According to the constant equation,we can get an
equation for the H+ concentration.
[H+] = Ka? [HA]/[A-]
Take the negative logarithm of both sides of the
equation,That is,
-log[H+] = -log{Ka? [HA]/[A-]}
= -log Ka – log{ [HA]/[A-]}
The left side equals the pH,The pKa of a weak
acid is defined in a manner similar to pH and pOH.
pKa = -log Ka
The previous equation can be written
pH = pKa - log{ [HA]/[A-]}
= pKa + log{ [A-]/[HA]}
More generally,we can write
[base]
pH = pKa + log
[acid]
This is known as the
Henderson-Hasselbalch equation.
Buffer ratio,[Base/acid]
Henderson—Hasselbalch equation change form:
pH＝ pKa+lg ＝ pKa+lg
]HB[
]B[?
c(conjugate base)
c(conjugate acid)
pH＝ pKa+lg ＝ pKa+lg
Vn
Vn
/)HB(
/)B(?
)HB(
)B(
n
n?
pH＝ pKa+lg ＝ pKa+lg
)HB()HB(
)B()B(
Vc
Vc
)HB(
)B(
V
V?
Buffer Capacity and Buffer Effective Range
1,Buffer capacity(?)
is definited as the amount of substance of
strong acid or strong base when the pH
value change one unit of pH unit in 1 liter
of buffer solution.
With differential coefficient:
β =
d p H
d )b(a
V
n
β＝ ＝ 2.303× [HB][B-]/ctotal
dp H
d )b(a
V
n
(1)
(2) 0.1mol.L-1 HAc+ NaOH
(3) 0.2mol.L-1 HAc+NaOH
(4)
pH
βmax＝ 2.303× (ct /2)(ct /2) /ct
＝ 0.576ct
∵ ct＝ [HB]+[B-]
β＝ 2.303× [HB][B-]/ct
＝ 2.303× ([HB] /ct )([B-]/ct )× ct
＝ 2.303× ([HB] /ct )(1-[HB-]/ct )× ct
2,Buffer effective range
When the buffer ratio is 1︰ 10 or 10︰ 1,
we cognize this buffer solution without
the buffer ability,So,we can definite the
buffer effective range as:
pH = pka± 1
Prepare the Buffer solution
Principle and Step
1,Choose correspondence of buffer system
2,The total concentration of buffer solution
3,Calculate the amount of buffer solution
4,Calibrate the buffer solution
Tris,trihydrixymethylaminomethane
(HOCH2)3CNH2
Standard of Buffer Solution
Potassium hydrotartrate (KHC4H4O6)
Potassium acid phthalate (KHC8H4O4)
Borax (Na2B4O7·10H2O)
Phosphate (KH2PO4-Na2HPO4)
Buffer System in Blood
In plasma:
H2CO3-HCO3-,H2PO4--HPO42-、
HnP-Hn-1P-(HnP instead of protein)
In red cell,
H2b-Hb- (H2b instead of haemoglobin)、
H2bO2-HbO2- (H2bO2 instead of
oxyhemoglobin),H2CO3-HCO3-、
H2PO4--HPO42-
CO2(solute)+H2O H2CO3 H++ HCO3-
pH＝ pKa,+lg [HCO3
-]
[CO2]solute
[HCO3-]
[CO2]solute＝ 6.10 + lg
In plasma:
[HCO3-] = 0.024mol·L-1
[CO2]solute= 0.001 2mol·L-1
pH＝ 6.10 + lg
＝ 6.10 + lg
＝ 7.40
1
1
Lm o l0 0 1 2.0
Lm o l0 2 4.0
1
20
SUMMA
1,Buffer solution is composed of a pair of conjugate acid-base
pair with enough concentration.The component of anti-acid is
conjugate base,The component of anti-base is conjugate acid,
When a few strong acid or base is added in to the buffer
solution.The component of anti-acid and anti -base will move
the equilibrium of dissociation to stabilize pH value.
2,We can use the Henderson-Hasselbalch‘s equation to calculate
the pH value of buffer solution:
pH＝ pKa+lg ＝ pKa+lg ＝ pKa+lg
]HB[
]B[? )HB( )B(cc
)HB(
)B(
n
n?
3,Buffer capacity is decided by pan-concentration
and buffer ratio,When the pan-concentration of the
buffer pair is settled,the buffer ratio is the more
close with to 1,the buffer capacity will be the more
large,When the buffer ratio is equal to 1,the buffer
capacity will attain the maximum?β max‘,When the
buffer ratio is equalization,the pan-concentration of
the buffer pair is the more large,the buffer
capacity will be the more large.
4,pH＝ pKa土 1 is stipulated the buffer effective
range,Over this range,the buffer solution will be
to lose the buffer ability too low of the buffer
capacity,Dispense the buffer solution,It may be
according to the table of this book,The methods
have three kinds,The first kind is ―weak acid add
its conjugate base‖,The second kind is ―weak acid
(overdose) add strong base‖,The third kind is
―strong acid add weak base (overdose)‖.
5,The buffer solution apply to medical science
about the buffer system and buffer action
principle in blood is important,In the human
blood,the major buffer pair have H2CO3-HCO3-、
H2PO4--HPO42-,HnP-Hn-1P-,In these buffer pair,
the carbonic acid buffer pair have highest
concentration and buffer ability is highest too
in blood,It have important affection to maintain
the pH value in 7.35~7.45 range of blood.
