Acids and alkalis
Acids
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They are substances that give hydrogen ions in solution i.e. they are
+
potential proton (H ) donors.
The strength of an acid depends on the degree of dissociation
(ionization) in solution. Hydrochloric acid (HCl) is almost completely
dissociated so, it is a strong acid while acetic acid (CH3COOH) is a
weak acid as it slightly dissociates.
Types of acidity
There are 2 types of acidity; true and titratable acidity
1- True acidity
It is the concentration of hydrogen ions in solution..
It depends on the degree of ionization.
It is measured by pH meter.
2- Titratable (total) acidity
It is the total concentration of hydrogen available for ionization
whether ionized or not.
It does not depend on the degree of ionization.
It is measured by titration against an alkali of certain normality such
as 0.1N sodium hydroxide.
Alkalis
They are substances, which give hydroxyl ions in solution. Also, they
are potential proton acceptors.
The strength of an alkali depends on the degree of dissociation
(ionization) in solution. Sodium hydroxide (NaOH) and potassium
hydroxide (KOH) are completely dissociated so, they are strong
alkalis while ammonium hydroxide (NH4OH) is a weak alkali as it
slightly
Bases
+
They are substances, which can accept protons (H ).
All alkalis are bases but not all bases are alkalis.
Thymine, uracil, adenine and guanine are examples of bases.
Amphoteric substances
They are substances that can act as acids in certain media and as
alkalies in other media. Proteins and amino acids are examples for
amphoteric substances.
Buffers
They are solutions that resist appreciable changes in pH when an acid
or alkali is added to it. Biological systems use buffers to control pH.
Buffers may be formed of one of the following mixtures:
1- Weak acid and its salt with strong base e.g. carbonic acid and
sodium bicarbonate.
2- Weak base and its salt with strong acid e.g. ammonium
hydroxide and ammonium chloride.
Mechanism of buffer action
In the case of bicarbonate buffer (NaHCO3 / H2CO3)
If strong alkali as sodium hydroxide (NaOH) is added it will react
with the carbonic acid forming sodium bicarbonate, which is a weak
alkali. So there is no appreciable change in the pH.
NaOH + H2CO3
NaHCO3 + H2O
If strong acid as hydrochloric acid (HCl) is added it will react with the
sodium bicarbonate forming carbonic acid, which is a weak acid. So
there is no appreciable change in the pH.
HCl + NaHCO3
H2CO3+ NaCl.
Buffer systems of the blood
1- Plasma buffers
They include:
a)- Bicarbonate buffer (NaHCO3 / H2CO3)
b)- Phosphate buffer (Na2HPO4 / NaH2PO4)
c)- Plasma proteins (Na proteinate / H protein)
2- RBCs buffers
They include:
a)- Bicarbonate buffer
(KHCO3 / H2CO3)
b)- Haemoglobin buffer
(KHb /HHb)
c)- Oxyhaemoglobin buffer (KHbO2 /HHbO2)
Clinical significance of blood buffers
The pH of blood is maintained in a narrow range around 7.4 (7.37 7.43) by the buffer systems in blood plasma and red blood cells.
A decrease in blood pH, which can be compensated, is called acidosis
while uncompensated decrease in blood pH is called acidaemia.
An increase in blood pH, which can be compensated, is called
alkalosis while uncompensated increase in blood pH is called
alkalaemia.
2
Even relatively small changes in this value of blood pH can lead to
severe metabolic consequences. Therefore, blood buffers are
extremely important in order to maintain homeostasis.
Solutions
A solution is formed when a substance is dissolved in a solvent.
Any solution is formed of 2 phases; the solute which is also called
dispersed phase and solvent which is also called dispersion phase
Types of solutions
Solutions can be classified according to the concentration or the size
of solute particles
A- According to the concentration of solute particles,
solutions may be:
1- Normal solution
It is a solution, which contains the equivalent weight of the
solute in grams dissolved in one liter of the solution.
2-Molar solution
It is a solution, which contains the molecular weight of the
solute in grams dissolved in one liter of the solution.
3-Molal solution
It is a solution, which contains the molecular weight of the
solute in grams dissolved in one kilogram of the solvent.
B- According to the size of solute particles, solutions may be:
1- Crystalloids (True solutions)
They are solutions in which the size of solute particles is less
than 1 mµ (millimicron) e.g. sodium chloride solution.
2- Colloids
These are solutions in which the size of solute particles ranges
from 1 to 100 mµ. e.g. plasma proteins
3- Suspensions
They are solutions in which the size of solute particles is more
than 100 mµ e.g. suspension of sand in water.
Colloids
These are solutions in which the size of solute particles ranges from 1
to 100 mµ. e.g. plasma proteins.
Types of colloids
According to their ability to take up the solvent, colloids are classified
into emulsoids and suspensoids
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1- Emulsoids
They are lyophilic (solvent loving) colloids. If the solvent is water
they are called hydrophilic colloids.
4
They are more viscid.
They are more stable and not easily precipitated as the solute particles
are surrounded by 2 stability factors:
i- Negative or positive charge.
ii- A shell (layer) of solvent.
They can be precipitated by dehydration
followed by neutralization of the charge.
charge
Examples for emulsoids include protein, starch and egg white a layer of solvent
solutions.
2- Suspensoids
They are lyophobic (solvent hating) colloids. If the solvent is water
they are called hydrophobic colloids.
They are less viscid than emulsoids.
They are less stable and easily precipitated as the
solute particles are surrounded by one stability
factor that is negative or positive charge.
They can be precipitated by neutralization of the charge.
Examples for emulsoids include colloidal gold and colloidal iron
solutions.
The charge on the colloidal particles may be due to
1- Ionization of some groups on the surface of colloidal particles.
2- Ions from the solvent may be adsorbed on the surface of the
colloidal particles.
Stability factors of colloids
1- Presence of negative or positive charge on the surface of the
colloidal particles.
2- Presence of a shell (layer) of solvent surrounding the
particles of emulsions.
3- The size of colloidal particles; the smaller the size, the more
stable the colloid.
Separation of colloids
1- Ultracentrifugation, which means centrifugation of the
solution at a very high speed.
2- Ultrafiltration, which means forcing the solution, under
pressure, through membranes of varying porosity.
3- Electrophoresis, which means migration of colloidal particles
in electric field.
Precipitation of colloids
Colloids can be precipitated by:
1- Dehydrating agents.
2- Strong agitation, freezing and heating.
3- Colloids of opposite charge.