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Water, pH and Buffers

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The structure and function of water. Dissociation of weak acids and ... Hydrophobicity. From Lehninger, 2nd ed., Ch 4. College of Medicine & Medical Sciences ... – PowerPoint PPT presentation

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Title: Water, pH and Buffers


1
Water, pH and Buffers
College of Medicine Medical Sciences
  • Lectures 1-3, MBC226-222-241-224
  • Dr Ayyub Patel

2
Outline
  • Homeostasis
  • The structure and function of water
  • Dissociation of weak acids and weak bases
  • pH and the Henderson-Hasselbalch equation
  • Buffers, biological/physiological examples

College of Medicine Medical Sciences
3
HOMEOSTASIS
  • The dynamic that defines the distribution of
    water and the maintenance of pH and electrolyte
    concentrations

College of Medicine Medical Sciences
4
HOMEOSTASIS
  • Water distribution maintained by the kidneys,
    antidiuretic hormone, hypothalamic thirst
    response, respiration and perspiration

College of Medicine Medical Sciences
5
HOMEOSTASIS
  • Clinically, need to be aware of water depletion
    caused by decreased intake (coma, wandering the
    desert) or increased loss (diarrhea, renal
    malfunction, over-exercise), and excess body
    water due to increased intake (too much I.V.) or
    decreased excretion (renal failure)

College of Medicine Medical Sciences
6
WATER
  • Comprises approx 70 of human mass (45-60
    intracellular, 25 extracellular/blood plasma)
  • dipolar partial negative charge on oxygen,
    partial positive charge on hydrogens
  • dipolar nature leads to formation of many low
    energy hydrogen bonds

College of Medicine Medical Sciences
7
Structure of H20
College of Medicine Medical Sciences
From Lehninger, 2nd ed., Ch 4
8
Properties of water
  • Very polar
  • Oxygen is highly electronegative
  • H-bond donor and acceptor
  • High b.p., m.p., heat of vaporization, surface
    tension

9
Water dissolves polar compounds
solvation shell or hydration shell
10
Non-polar substances are insoluble in water
Many lipids are amphipathic
11
How detergents work?
12
Hydrogen Bonding of Water
One H2O molecule can associate with 4 other H20
molecules
  • Ice 4 H-bonds per water molecule
  • Water 2.3 H-bonds per water molecule

Crystal lattice of ice
13
Biological Hydrogen Bonds
14
(No Transcript)
15
non-covalent interactions
16
Relative Bond Strengths
Bond type kJ/mole H3C-CH3 88 H-H 104 I
onic 40 to 200 H-bond 2 - 20 Hydrophobic
interaction 3 -10 van der Waals 0.4 - 4
17
Water Solubility / Hydrophilic
College of Medicine Medical Sciences
From Lehninger, 2nd ed., Ch 4
18
Hydrophilic/Hydrophobic
College of Medicine Medical Sciences
From Lehninger, 2nd ed., Ch 4
19
Hydrophobicity
College of Medicine Medical Sciences
From Lehninger, 2nd ed., Ch 4
20
Hydrophobicity/Micelles
College of Medicine Medical Sciences
From Lehninger, 2nd ed., Ch 4
21
Water and pH relationship
  • In solution water shows a very low dissociation
    (probability of H in water is 1.8 x 10-9)
  • H2O H OH-
  • H is actually associated with a cluster of water
    molecules and exists in solution as H3O or H5O2
    or H7O3

College of Medicine Medical Sciences
22
Ionization of Water
23
Ionization of Water
H20 H OH-
Keq1.8 X 10-16M
Keq H OH- H2O
H2O 55.5 M
H2O Keq H OH-
(1.8 X 10-16M)(55.5 M ) H OH-
1.0 X 10-14 M2 H OH- Kw
If HOH- then H 1.0 X 10-7
24
pH Scale
  • Devised by Sorenson (1902)
  • H can range from 1M and 1 X 10-14M
  • using a log scale simplifies notation
  • pH -log H
  • Neutral pH 7.0

25
Water and pH relationship
  • For Dissociation of water,

College of Medicine Medical Sciences
  • Where denotes concentration and K is the
    dissociation constant

26
Water and pH relationship
  • 1 mole of water 18g
  • 1 L of water contains
  • 1000 18 55.56 mol (ie pure water 55.56M)
  • Molar concentration of H (or OH-) ions can be
    calculated
  • H 1.8x10-9 x 55.56 1.0x10-7
  • In order to avoide using ve numbers, the H is
    expressed as pH which is ve log (base 10) of
    H
  • pH - log10 H pH of pure water
    is 7
  • Acidic solutions have pH lt 7 while basic
    solutions have pH gt 7

College of Medicine Medical Sciences
27
Water and pH relationship
  • H1x10-6 what is the pH?
  • pH - log10 H
  • H0.24x10-4 what is the pH?
  • H3.4x10-3 what is the pH?

6
College of Medicine Medical Sciences
4.6
2.5
28
College of Medicine Medical Sciences
From Devlin, 3rd ed., Ch 1
29
Henderson-Hasselbach Equation
Consider the dissociation of a general acid HA
HA H A-
We can define a dissociation constant (K) where
College of Medicine Medical Sciences
Rearranging gives
Taking logarithms on both sides and multiplying
by -1 gives -logH -logK
log HA/A- or pH
pK log A-/HA
30
Dissociation Constant and pH
College of Medicine Medical Sciences
From Marks, Marks, Smith, Ch 4
31
Henderson-Hasselbalch Equation
College of Medicine Medical Sciences
32
Henderson-Hasselbalch Equation
  • This equation can be used to determine the pH if
    the pK and ratio of the ionised and unionised
    forms is known.
  • The pKa (a for acid) is the ve log of the
    dissociation constant of the acid. It is the pH
    at which the ratio of the ionised and unionised
    species is equal to 1. ie the molar
    concentration of the ionised and unionsed species
    is the same.
  • Similarly pKb is ve log of the dissociation
    constant of the base

College of Medicine Medical Sciences
33
College of Medicine Medical Sciences
From Devlin, 3rd ed., Ch 1
34
College of Medicine Medical Sciences
From Devlin, 3rd ed., Ch 1
35
Weak Acids and Bases Equilibria
  • Strong acids / bases disassociate completely
  • Weak acids / bases disassociate only partially
  • Enzyme activity sensitive to pH
  • weak acid/bases play important role in
  • protein structure/function

36
Acid/conjugate base pairs
HA H2O A- H3O HA A- H HA
acid ( donates H)(Bronstad Acid) A- Conjugate
base (accepts H)(Bronstad Base)
Ka pKa value describe tendency to loose
H large Ka stronger acid small Ka weaker
acid
Ka HA- HA
pKa - log Ka
37
pKa values determined by titration
38
Phosphate has three ionizable H and three pKas
39
Buffers
  • Buffers are aqueous systems that resist changes
    in pH when small amounts of a strong acid or base
    are added.
  • A buffered system consist of a weak acid and its
    conjugate base.
  • The most effective buffering occurs at the region
    of minimum slope on a titration curve
  • (i.e. around the pKa).
  • Buffers are effective at pHs that are within /-1
    pH unit of the pKa

40
Henderson-Hasselbach Equation
HA weak acid A- Conjugate base
1) Ka HA- HA
2) H Ka HA A-
3) -logH -log Ka -log HA A-
H-H equation describes the relationship
between pH, pKa and buffer concentration
4) -logH -log Ka log A- HA
5) pH pKa log A- HA
41
Case where 10 acetate ion 90 acetic acid
  • pH pKa log10 0.1


  • 0.9
  • pH 4.76 (-0.95)
  • pH 3.81

42
Case where 50 acetate ion 50 acetic acid
  • pH pKa log10 0.5


  • 0.5
  • pH 4.76 0
  • pH 4.76 pKa

43
Case where 90 acetate ion 10 acetic acid
  • pH pKa log10 0.9


  • 0.1
  • pH 4.76 0.95
  • pH 5.71

44
Cases when buffering fails
  • pH pKa log10 0.99


  • 0.01
  • pH 4.76 2.00
  • pH 6.76
  • pH pKa log10 0.01


  • 0.99
  • pH 4.76 - 2.00
  • pH 2.76

45
Sample pH problems
College of Medicine Medical Sciences
From Devlin, 3rd ed., Ch 1
46
Sample pH Problem (cont)
College of Medicine Medical Sciences
From Devlin, 3rd ed., Ch 1
47
Buffers
  • Definition A weak acid plus its conjugate base
    that cause a solution to resist changes in pH
    when an acid or base are added
  • Effectiveness of a buffer is determined by 1)
    the pH of the solution, buffers work best within
    1 pH unit of their pKa 2)
    the concentration of the buffer the more
    present, the greater the buffering capacity

College of Medicine Medical Sciences
48
Physiological Buffers
  • Carbon Dioxide-Bicarbonate System a major
    regulator of blood pH
  • Phosphate System major regulator of cytosolic pH
  • Proteins as buffers
  • CO2 and HCO3 are much higher than PO4 in
    blood the reverse is true in the cytosol, PO4
    gtgtgt HCO3

College of Medicine Medical Sciences
49
Examples - Physiological Buffers
College of Medicine Medical Sciences
From Marks, Marks, Smith, Ch 4
50
College of Medicine Medical Sciences
From Marks, Marks, Smith, Ch 4
51
pH Titration Curves
College of Medicine Medical Sciences
From Lehninger, 2nd ed., Ch 4
52
Blood Bicarbonate and Metabolic Acidosis
The bicarbonate blood buffer in a normal
adult maintains the blood pH at about 7.40. If
the blood pH drops below 7.35, the condition is
referred to as an ACIDOSIS. A prolonged blood pH
below 7.0 can lead to death. Clinically for an
acidosis, the acid-base parameters (pH, HCO3-
, CO2 ) of the patients blood should be
monitored. The normal values for these are pH
7.40 HCO3- 24 mM CO2 1.2 mM.
College of Medicine Medical Sciences
53
Sample Problem Metabolic Acidosis
  • The blood values of a patient were pH 7.03 and
    CO2 1.1 mM. What is the patients blood
    HCO3- and how much of the normal HCO3- has
    been used in buffering the acid causing the
    condition?
  • The pK for HCO3-/CO2 6.10

College of Medicine Medical Sciences
54
Solution
  • Substitute into Henderson-Hasselbalch equation
  • 7.03 6.10 log HCO3-/1.1 mM, or
  • 0.93 log HCO3-/1.1 mM
  • The anti-log of 0.93 8.5, thus
  • 8.5 HCO3-/1.1 mM, or HCO3- 9.4 mM
  • Since normal HCO3- equals 24 mM, there was a
    decrease of 14.6 mmol of HCO3- per liter of
    blood in this patient. This would be approaching
    the point where, if left untreated, the HCO3-
    buffering capacity would be no longer effective
    in this patient.

College of Medicine Medical Sciences
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