Transcript pH , Buffers and acid balance balance

pH, Buffers and Acid Base
Balance
Objectives
• Define the terms: pH, acid, base, acidity of a solution &
anion gap.
• Define the buffers
• Explain how buffers act.
• List the different biological buffers in our bodies.
• Outline the importance of biological buffers.
• Define the disturbance of acid – base balance.
• Mention two causes for each type of acid – base
balance disturbances
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
pH Scale
• Devised by Sorenson (1902)
• Definition : pH is mathematically defined as
negative logarithm of hydrogen ion
concentration(-log10[H+]). It provides a
convenient way of stating H+ and OHconcentrations.
• pH range is from 1.0 to 14.0.
• Neutral pH is 7.0.
• Acidic solutions have pH < 7.0 while
basic solutions have pH > 7.0
• pOH = 14- pH
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
Relationship of pH and Ionization of Water
H+ + OH-
H 20
H20 + H 20
H3O +
+
• Keq= [H+] [OH-]
[H2O]
•
[H2O] Keq = [H+] [OH-]
•
55.5 M )(1.8 X 10-16M)= [H+] [OH-]
•
1.0 X 10-14 M = [H+] [OH-] = Kw
OH -
[Keq = 1.8 X 10-16M]
[ Kw is ionic product of water]
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If [H+]=[OH-] then [H+] = 1.0 X 10-7
Thus Neutral pH is -log 1.0 X 10-7 or 7.0
Concentration of water
[H2O] = 55.5 M
• Acids: Are defined as substances which
give hydrogen ions or protons when
dissolved in water.
•
Examples : Hydrochloric acid ( strong), Acetic acid (week).
• Bases: Are defined as substances which
give hydroxyl ions or accept protons when
dissolved in water.
• Examples : Sodium hydroxide ( strong), Sodium bicarbonate(week).
• Question : why are some acids and bases strong
and some are week????
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
• Ampholytes :
– Substances which can functions both as acids and bases are
called as Ampholytes. e.g Water,
• Anion Gap :
– Defined as the difference between total concentration of
measured ions (Na+ and K+) and that of measured anions (Cl –
and HCO3).
A – = (Na+ + K+ ) - (Cl – + HCO3).
• Anion Gap, in fact, represents the unmeasured anions
in plasma.
• Normal range 10-16 mEq/L
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
Buffers
• Definition:
A solution of weak acid and its conjugate base
which resists a change in pH when small
quantities of an acid or base are added.
• Mechanism of Buffer action :
– The bicarbonate buffer system contains carbonic acid (H2CO3) and sodium
bicarbonate (NaHCO3).
H2CO3 ↔ H+ + HCO3– The salt produces sodium and bicarbonate ions.
NaHCO3 ↔ HCO3- + Na+
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
Mechanism of Buffer action-con’td

The function of the Bicarbonate (conjugate base) is
to neutralize H+ from acids (e.g HCl). The Carbonic
acid in the product adds to the weak acid available.
NaHCO3- + HCl
•
H2CO3 + NaCl
The function of the weak acid is to neutralize a
base (e.g NaOH) . The Bicarbonate ion in the
product adds to the available week base.
H2CO3 + NaOH
NaHCO3- + H2O
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
• Buffering Capacity (BC):
The efficiency of a buffer in maintaining a constant
pH on the addition of small amounts of acid
or base is called as its buffering capacity.
• Buffering capacity is determined by:
– the pH of the solution; Buffers work best within 1 pH
unit of their pKa.
– the concentration of the buffer, the stronger the
buffer , the greater its buffering capacity.
– Ratio of acid to conjugate base.
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
Physiological Buffers
• The Important buffer systems:
– Bicarbonate buffer
– Phosphate buffer
– Protein buffer
– Hemoglobin buffer
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
Physiological Buffer-con’td
• Bicarbonate buffer system
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Predominant buffer system in ECF particularly plasma ( Sodium salts).
Also operates intracellularly e.g in RBCs ( Potassium salts)
Carbonic acid /bicarbonate (Na or K)
Bicarbonate to Carbonic acid ratio of 20 : 1 at pH 7.4 which is referred to
as alkali reserve.
• Phosphate buffer system
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Made up of sodium dihydrogen phosphate (Na H2PO4 -acid)and disodium
hydrogen phosphate (Na2HPO4 –base)
Ratio of base to acid is 4 :1.
More important in intracellular fluids than in plasma becoz of low concentration.
Other very effective becoz of its pK of 6.8 which is close to physiological pH.
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
Physiological Buffer-con’td
• Protein Buffer system
– Proteins contain – COO- groups, which, like bicarbonate ions can act as proton
acceptors.
– Proteins also contain – NH3+ groups, which, like carbonic acid can donate
protons.
– If acid comes into blood, H ions can be neutralized by the – COO- groups
--- COO- + H+ → - COOH + H2O
– If base is added, it can be neutralized by the – NH3+ groups
……..NH3+ + OH- → …. NH2 + H2O
• Hemoglobin buffer system
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About 60% of protein buffering capacity because of Hb.
Due to high content of imidazole groups of histidines.
Responsible for buffering most of carbon dioxide added to blood by
tissues.
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
Importance of Physiological Buffers
• Most of the biochemical reactions in the body
need strict conditions of pH which are
facilitated by buffers. These processes
include:
– Enzyme catalyzed reactions
– Binding and transport of hormones and other
important molecules
– Biding and transport of small molecules like
oxygen.
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
Disturbances of Acid base balance
•
The body has efficient mechanism of pH regulation
with the result the blood pH is maintained within 7.35
ad 7.45.
• Under certain conditions the human body encounters
disturbances in Acid base balance.
• The acid base disorders are mainly classified as :
– Acidosis: a decline in blood pH
• Metabolic acidosis
• Respiratory acidosis
– Alkalosis- arise in blood pH
• Metabolic acidosis
• Respiratory alkalosis
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
Disturbances of Acid base balance--con’td
• Metabolic acidosis…Primary defect is reduction of
bicarbonate ( HCO3- ).
• Abnormal increase in anions (organic acidosis, ingestion of
acidifying salts, renal insufficiency)
• Abnormal loss of bicarbonate(severe diarrhea, fistulas )
• Respiratory acidosis… Primarily retention of carbon
dioxide ( H2CO3 )
• Depression or suppression of respiration (brain damage, drug
poisoning, loss of ventillatory functions, Effects of pain
• Impairment of CO2 diffusion, Obstacle to the escape of CO2
• Insufficient pulmonary blood flow.
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
Disturbances of Acid base balance--con’td
• Metabolic alkalosis…Primary defect is increase in bicarbonate
( HCO3- ).
– Excessive loss of HCl from stomach
– Alkali administration
– Potassium deficiency
• Respiratory alkalosis… Primary abnormality is decreased
H2CO3 concentration.
– Stimulation of respiratory centre. ( CNS disease like
meningitis, salicylate poisoning, hyperpyrexia)
– Other causes like hysteria, high altitude ascending,
injudicious use of respirator.
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY
DR KHALID MAJID FAZILI,
QASEEM UNIVERSITY