6
chapter i
Water, Acids, Bases, and Buffers
TABLE 1-3
Percent Unprotonated Species and Ratio ofUnprotonated
Forms Relative to the Difference between the pH and pK'*
%A-
[A- ]/[HA]
Log [A- ]/[HA]
(pH-pKO
“100”
999/1
3 . 0 0
99
99/1
2.00
98
98/2
1.69
96
96/4
1.38
94
94/6
1.20
92
92/8
1.06
91
91/9
1.00
90
90/10
0.95
80
80/20
0.60
70
70/30
0.37
60
60/40
0.18
50
50/50
0.00
40
40/60
-0.18
30
30/70
-0.37
20
20/80
-0.60
10
10/90
-0.95
8
8/92
-1.06
6
6/94
-1.20
4
4/96
-1.38
2
2/98
-1.69
1
1/99
-2.00
“0”
1/999
-3.00
*Reproduced, with permission, from J. N. Aronson: The Henderson-
Hasselbalch equation revisited.
B io c h e m ic a l E d u c a tio n
11(2),
6 8
(1981).
seen from the Henderson-Hasselbalch equation, when the
pH of the solution equals the pK' of the buffer, [conju-
gate base] = [acid], and the buffer can therefore respond
equally to both added acid and added base. It also follows
from Equation (1.7) that when the pH of the solution is one
pH unit above or below the pK' value, the solution contains
approximately 9% unprotonated or protonated species, re-
spectively. Similarly, if the pH of the solution is two units
above or below the pK' value, the solution contains almost
entirely (99%) unprotonated or protonated species, respec-
tively. Table 1-3 provides percent unprotonated species
and the corresponding unprotonated/protonated ratios for
selected (pH-pK') values.
Buffer Systems of Blood and Exchange
of O2 and CO2
If the H+ concentration departs significantly from its nor-
mal value in blood, the health and survival of the human
TABLE 1-4
pH Values of Human Body Fluids and Secretions
Body Fluid or Secretion
pH
Blood
7.4
Milk
6.6-6.9
Hepatic bile
4
^
1
OOUl
Gall bladder bile
5.4-6.9
Urine (normal)
6.0
Gastric juice (parietal secretion)
0.87
Pancreatic juice
8.0
Intestinal juice
7.7
Cerebrospinal fluid
7.4
Saliva
7.2
Aqueous humor of eye
7.2
Tears
7.4
Urine (range in various disease
4.8-7.5
states)
Feces
7.0-7.5
Muscle cell, resting
6.94-7.06
(at 37°C; extracellular pH = 7.4)
(intracellular)
body are in jeopardy. H+ is the smallest ion, and it com-
bines with many negatively charged and neutral functional
groups. Changes of [H+], therefore, affect the charged
regions of many molecular structures, such as enzymes,
cell membranes, and nucleic acids, and dramatically al-
ter physiological activity. If the plasma pH reaches either
6 . 8
or 7.8, death may be unavoidable. Despite the fact that
large amounts of acidic and basic metabolites are produced
and eliminated from the body, buffer systems maintain a
fairly constant pH in body fluids (Table 1-4).
The major metabolic product from oxidation of ingested
carbon compounds is CO
2
. Hydration of CO
2
dissolved
in water yields the weak acid H
2
CO
3
(carbonic acid).
Depending on the type of food ingested and oxidized,
0.7-1.0 mol of CO
2
is produced per mole of O
2
consumed.
This results in the metabolic production of about 13 mol
of hydrated CO
2
each day in a normal person.
For efficient transport of relatively insoluble CO
2
from
the tissues where it is formed to the lungs where it must
be exhaled, the buffers of the blood convert CO
2
to the
very soluble anionic form HCOj" (bicarbonate ion). The
principal buffers in blood are bicarbonate-carbonic acid in
plasma, hemoglobin in red blood cells, and protein func-
tional groups in both. The normal balance between rates
of elimination and production of CO
2
yields a steady-state
concentration CO
2
in the body fluids and a relatively con-
stant pH.
