Water, Acids & Bases
and Buffers






2.1 WATER
Water is the solvent for life and is an
essential nutrient vital to existence of
both animals and plants
It is found inside and outside of cells
of the body. In the body, water is
found inside and around the cells and
within all blood vessels
The body is about 60% water; water
found inside cells comprises
the intracellular fluid and that found
outside cells comprises
the extracellular fluid.
The intracellular fluid volume
which accounts for two-thirds of the
body volume, gives form to the
tissues and organs and ultimately to
the body.
Water also helps in the regulation of
body temperature.
The average human body is
composed of 60% water, 17%
protein, 15% fat and 3% nitrogen.
Functions of Water





Principal fluid medium in which
nutrients, gases, and enzymes are
dissolved
Extracellular water bathes the cells,
serving as medium for transport of
nutrients and oxygen to the cells and
removing wastes from the cells
Intracellular water is the
physicochemical medium that allows
various metabolic processes to take
place
Intracellular fluid volume provides
form to tissues and organs and
ultimately to the body
Regulation of body temperature
Figure 1. Functions of water in the human body
Figure 2. Human body water composition
Chemistry of Water
boiling point
100 °C
freezing point &
0 °C,
melting point
maximum density
at 4 °C,
high specific heat
high heat of vaporization and fusion
high dipole moment
NOTE:
account for the ability
of water to form
Presence of dipole
hydrogen bonds
moment & its
making it a good solvent
polarity
for polar organic
molecules in the body
EXAMPLE
NO to fats or lipids which
are hydrophobic or waterfearing molecules and
cause hydrophobic
aggregation
Figure 3. Water molecule
YES to proteins and
carbohydrates which
are hydrophilic or
water-loving molecules
2.2 Acids, Bases and pH
Take Note:
pH = -log [H+ or H3O+]
(tetrahedral hydrogen bonding of water)





Water as Universal Solvent
Description
H-bonding – bond
between H and an
electronegative atom
such as O or N
Ion-dipole - bond
between an ion and a
molecule with an
electric dipole
moment such as H2O
Dipole-dipole - bond
between 2 molecules
with electric dipole
moments such as H2O
and an amide or
amine functional
group
Implication
A solvent is good at
dissolving a substance
(solute) in solution.
Water is polar, so it can
dissolve ions and other
polar organic molecules
such as proteins and
carbohydrates which are
hydrophilic (waterloving). Like dissolves like.
Water is a poor solvent
for non-polar molecules
which are hydrophobic
(water-fearing) such as
fats and oils resulting in
Hydrophobic aggregation.


ACID- strong acid, weak acid
Ka, pKa
BASE- strong base, weak base
Kb
Kw
Maintaining the pH of cells and
tissues is important for maintaining
body homeostasis or equilibrium.
Blood pH is maintained at
7.35 < pH < 7.45.
A slightly acidic pH < 6.95 can lead
to coma and death from acidosis,
A slightly basic pH > 7.70 can cause
convulsions and muscle spasms
from alkalosis.
Importance of pH in living cells





In animals, the maintenance of blood pH
→ (7.35 < pH < 7.45) is crucial for life.
Slightly acidic pH (6.95)
→ coma and death
Slightly basic pH (7.7)
→ convulsions and muscle spasms
Dramatic shifts in pH can play a role
in controlling cellular activities such
as egg division after fertilization and
enzymatic activities
Consequently, cells must work
constantly to maintain an acid-base
balance.
2.3 Buffer
1. How does the body maintain pH ?

Fat droplet in fat cells – example of
hydrophobic aggregation

Buffers present in physiologic fluids
maintain the body’s pH.
It is a solution that contains a weak
acid or base that is capable of
resisting or minimizing the change in



pH upon the addition of a strong acid
or base.
Can be prepared in the laboratory by
mixing a weak acid and its conjugate
base or a weak base and its conjugate
acid.
Used in all biochemical work to
control pH.
Buffer problems are solved using the
Henderson-Hasselbach equation.
5. Buffer Capacity
 It is the amount of H+ or OH- ions
that a buffer can absorb without a
significant change in its pH.
 It is affected by:
[salt/acid] ratio
Concentration of acid and
conjugate base component
2. In the body
 Blood contains the carbonic acidbicarbonate buffer system and
physiologic fluids in cells contain
the phosphate buffer system.
3. The Henderson-Hasselbach equation
 This is helpful in calculating the
proportions of weak acid and its
conjugate base or weak base and its
conjugate acid needed to prepare the
buffer.
We can also simply represent it as:
pH = pKa + log [salt/acid]
pOH = pKb + log [salt/base]
Figure 1. Tritration Curve showing the infliction
point, equivalence point and buffer region
6. Titration Curve
 Inflection point is where pH=pKa
7. Equivalence Point
 It is the point in the titration where
complete deprotonation occurs.
 Occurs when the added amount of
titrant (standard solution) is equal
to the total number of ionizable
protons of the titrand (analyte,
unknown solution).
 This is used to calculate the
concentration of the titrand.
Major Buffering Systems in the Body
4. Buffering Range for buffer sol’n
 Is defined as the pH + 1.
 This the range in which the capacity
of the buffer to resist drastic changes
in pH is maximal.
1. Carbonic Acid-Bicarbonate Buffer System
o
Extracellular
o
CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3
o
If there are too many H3O+ ions in the
blood, the H3O+ binds with the
HCO3 to form carbonic acid.
o
This carbonic acid will be
dissociated into water and
CO2, and will be released out
by the lungs
o
If there are too few H3O+ ions in the
blood, the carbonic acid will react to
form H3O+ and HCO3
o
During hyperventilation, CO2 is
rapidly excreted in the body, causing
the equilibrium to shift to the left,
therefore, pH increases
o
During hypoventilation, CO2 builds up
in the body causing the equilibrium to
the right, consequently decreasing
the pH.
o
Blood is an example of a buffered
system – pH maintained at pH 7.357.45
Bicarbonate buffering system of the blood
2. Phosphate Buffer System
o
Intracellular
o
H2PO4- + OH ⇌ HPO42- + H3O+
o
involves dihydrogen phosphate
H2PO4- (proton donor) and hydrogen
phoshphate HPO42- (proton acceptor)
o
When there is excess OH- in the cell,
it is neutralized by the dihydrogen
phosphate, shifting the equilibrium
to the right
When there is excess H3O+ in the
cell, it is neutralized by the hydrogen
phosphate, shifting the equilibrium
to the left
3. Protein Buffer
o
Nearly all proteins can act as buffer.
Proteins are made up of amino acids.
2.3 Buffer Preparation
a. Describe the preparation of 100 mL of 0.5M
acetic acid buffer, pH=5 from 1M HAc (pKa = 4.74)
and 1M NaAc.
b. Describe the preparation of 500 mL of 0.25M
acetic
acid
buffer,
pH=5.2
from
0.5M HAc (pKa=4.74) and 0.5M NaAc
c. Describe the preparation of 100mL of 0.5M
acetic acid buffer, pH=5 from 1M HAc (pKa=4.74)
and 1M NaOH.
d. Describe the preparation of 500mL of 0.25M
acetic acid buffer, pH=5.2 from 0.5M HAc
(pKa=4.74) and 0.5M NaOH