OBJECTIVES
1. Understand the polar nature of water and the varieties of non-covalent bonds formed in it.
2. Understand pH and how to calculate it.
3. Know the definition of weak and strong acids and bases, buffers, and how a buffer functions.
4. Know how to solve pH and buffer problems.
5. Understand titration curves and pKa values.
I. Importance of pH
A. pH homeostasis is critical to life
1. blood pH
2. biochemical reactions
3. Clinical correlations: Di Beatty and Teresa Livermore (p. 37+M)
B. pH is fundamental to understanding of biological processes
1. organ function
2. subcellular compartmentation
a. cytoplasm
b. mitochondria
c. lysosomes
d. endocytosis/cell "traffic"
3. molecular interactions
C. pH regulation
1. gas exchanges/excretion [lung and kidney]
2. proton pumps
3. buffers
a. HCO3 -/CO2
b. proteins
c. H2PO4-1/HPO4-2
II. Properties of water
A. Water is polar (Figure 1-2 D)
B. Effect on non-covalent bonds
1. electrostatic (p. 66D) A+ ......... B- 2. hydrogen bonds (Figure 1-3&1-4D) -A-H.......B- (A,B=N,O,S)
3. hydrophobic bonds (Figure 2.47D) (exclusion by water)
4. van der Waals forces (Figure 2.51D) optimal distance
C. Water ionization
1. H2O <========> H+ + OH-
[H+] [OH- ] [H+] [OH- ]
2. Keq = -------------- = --------------
[H2O] [55.5]
Kw = Keq x 55.5 = [H+] [OH- ] = 1 x 10-14 @ 25øC
3. Pure water: [H+] = [OH- ] = 1 x 10-7
III. Definition of acids and bases (Lowry-Bronsted)
A. General: p"X" = log (1/"X") = -log"X"
B. pH = -log[H+]
C. pOH = -log[OH- ]
D. pKw = -log Kw = pH + pOH = 14
E. pKa = -log Ka
F. A ten-fold change in [H+] alters the pH by 1.0 unit.
IV. Titration curves and buffers
A. Titration curves (Figures 1.7D)
(Note: sometimes x and y axes are reversed; e.g. x axis is equiv. of base)
B. Buffers
1. A buffer is a mixture of a weak acid and the salt of that acid.
2. The practical buffer zone is pKa ñ 1 pH.
3. The Henderson-Hassalbalch equation is used for calculations involving
buffers.
[A- ]
****** pH = pKa + log _____
[HA]
(See p. 9D for derivation.)
4. pH of a buffer is theoretically independent of dilution (depends only on
the ration of A-/HA and pKa).
V. Solving pH problems
A. Strong acids (completely dissociated)
[H+] = normality of acid
pH = -log[H+]
B. Strong bases (completely dissociated)
[OH- ] = normality of base
pOH = -log[OH- ]
pH + pOH = 14
C. Weak acids: two kinds of problems
1. % ionization x molarity = [H+]
[H+ ] [A- ] x2 x2
2. Ka = ________ = ________ ~ ________
[HA] [HA] - x [HA]
1/2
x = [H+] = {Ka x [HA]}
D. Buffers: use Henderson-Hasselbalch equation
SUMMARY OF EQUATIONS
EQUATION USE
pH = -log[H+] pH calculations
Kw = [H+] [OH- ] = 1 x 10-14 conversion
pKw = pH + pOH = 14 conversion
[H+] [A- ]
Ka = __________ weak acids
[HA]
[A- ]
pH = pK + log _______ buffers
[HA]