pH in Biology
Dissociation of water molecules
leads to acidic and basic
conditions that affect living
organisms
Dissociation of Water
Sometimes, an H2O molecule loses a H+
ion that is in turn accepted by another
H2O
Ionization of Water
Occasionally, in water, a H+ is transferred
between H2O molecules
..
H :O : +
.. ..
H
..
..
..
:O:H
H:O:H +
+
H
..
..
:O:H -
H
water molecules hydronium hydroxide
ion (+)
ion (-)
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LecturePLUS Timberlake
• H+ leaves its electron behind and is
transferred as a single proton - a
hydrogen ion (H+)
• The water molecule that lost a proton is
now a hydroxide ion (OH-)
• The water molecule with the extra proton
is a hydronium ion (H3O+)
H2O <=> H+ + OH• This reaction is reversible.
• At equilibrium the concentration of water molecules greatly
exceeds that of H+ and OH• At equilibrium the concentration of H+ or OH- is 10-7M (25°C)
• Adding solutes (acids or bases) changes the concentration of
H+ and OHSolutions with more OH- than H+ are basic solutions.
Solutions with more H+ than OH- are acidic solutions.
Solutions in which concentrations of OH- and H+ are equal are
neutral solutions.
Pure Water is Neutral
Pure water contains small, but equal amounts of
ions: H3O+ and OH-
H2O + H2O
H3O+
+ OH-
hydronium hydroxide
ion
H3O+
OH-
1 x 10-7 M
ion
1 x 10-7 M
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LecturePLUS Timberlake
Acids & Bases
Acids are formed by
hydrogen cations
Bases are formed
by hydroxide anions
Acids
 Increase H+
 HCl (g) + H2O (l)
H3O+ (aq) + Cl- (aq)
 More [H3O+] than water > 1 x 10-7M
 As H3O+ increases, OH- decreases
[H3O+] > [OH-]
H3O+
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OH-
LecturePLUS Timberlake
Acids
• Donate protons (Hydrogen Ions) to water to
form hydronium ions
• pH 0-6.99
• Taste Sour
• Turn litmus paper red
• Strong acids completely dissociate to form ions
Bases
Increase the hydroxide ions (OH-)
H2O
NaOH (s) Na+(aq) + OH- (aq)
More [OH-] than water, [OH-] > 1 x 10-7M
 When OH- increases, H3O+ decreases
[OH] > [H3O+]
H3O+
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OH-
LecturePLUS Timberlake
Bases
• Donate hydroxide
• pH 7.01-14
• Accept protons
• Taste bitter
• Feel slimy
• Turn litmus paper blue
• Strong bases completely dissociate to form ions
Bases
• Base  increases [OH- ]
• Some bases reduce the H+ concentration directly by
accepting hydrogen ions
For example, NH3 + H+ <=> NH4+
• Other bases reduce H+ indirectly by dissociating to
OH-, which then combines with H+ to form water.
For example, NaOH -> Na+ + OHOH- + H+ ->
H 2O
NH3, A Bronsted-Lowry Base
When NH3 reacts with water, most of the reactants
remain dissolved as molecules, but a few NH3 reacts
with water to form NH4+ and hydroxide ion.
NH3
acceptor
+
+ H2 O
NH4+(aq) + OH- (aq)
donor
+
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LecturePLUS Timberlake
Strengths of Acids and Bases
Strong acids completely ionize (100%) in aqueous
solutions
HCl + H2O
H3O+ + Cl- (100 % ions)
Strong bases completely (100%) dissociate into ions
in aqueous solutions.
NaOH
Na+ (aq) + OH-(aq)
(100 % ions)
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LecturePLUS Timberlake
Strong and Weak
Acids and Bases
Strong acids
HCl, HNO3 , H2SO4
Most other acids are weak.
Strong bases
NaOH, KOH, and Ca(OH)2
Most other bases are weak.
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Weak Acids/Bases
–the binding and release of
hydrogen ions are reversible.
–At equilibrium, there will be a fixed
ratio of products to reactants
–Eg: NH3, carbonic acid
–Carbonic acid (H2CO3) is a weak
acid:
• H2CO3 <=> HCO3- + H+
Measuring Acidity and Alkalinity
• In any solution [H+] [OH-] = 10-14
– In a neutral solution, [H+] = 10-7 M and [OH-] = 10-7
M
• Adding acid to a solution shifts the balance
between H+ and OH- toward H+ and leads to a
decline in OH-.
– If [H+] = 10-5 M, then [OH-] = 10-9 M