40
UNIT 1
Organization of the Body
2
Bicarbonate ion (HCO
3
2
)
, an important base in the body,
is particularly abundant in blood.
Ammonia (NH
3
)
, a common
waste product of protein breakdown in the body, is also a base. It
has one pair of unshared electrons that strongly attracts protons.
By accepting a proton, ammonia becomes an ammonium ion:
NH
3
1
H
1
S
NH
4
1
ammonium
ion
pH: Acid-Base Concentration
Te more hydrogen ions in a solution, the more acidic the solu-
tion is. Conversely, the greater the concentration of hydroxyl
ions (the lower the concentration of H
1
), the more basic, or
alkaline
(al
9
kuh-līn), the solution becomes. Te relative concen-
tration of hydrogen ions in various body fluids is measured in
concentration units called
pH units
(pe-āch
9
).
Te idea for a pH scale was devised by a Danish biochemist
and part-time beer brewer named Sören Sörensen in 1909. He
was searching for a convenient means of checking the acidity of
his alcoholic product to prevent its spoilage by bacterial action.
(Acidic conditions inhibit many bacteria.) Te pH scale that
resulted is based on the concentration of hydrogen ions in a so-
lution, expressed in terms of moles per liter, or molarity. Te pH
scale runs from 0 to 14 and is
logarithmic
. In other words, each
successive change of one pH unit represents a tenfold change in
hydrogen ion concentration
(Figure 2.13)
. Te pH of a solu-
tion is thus defined as the negative logarithm of the hydrogen
ion concentration [H
1
] in moles per liter, or
2
log[H
1
]. (Note
that brackets [ ] indicate concentration of a substance.)
At a pH of 7 (at which [H
1
] is 10
2
7
M
), the solution is
neutral
—neither acidic nor basic. Te number of hydrogen
ions exactly equals the number of hydroxyl ions (pH
5
pOH).
Absolutely pure (distilled) water has a pH of 7.
Solutions with a pH below 7 are acidic—the hydrogen ions
outnumber the hydroxyl ions. Te lower the pH, the more acidic
the solution. A solution with a pH of 6 has ten times as many
hydrogen ions as a solution with a pH of 7.
Solutions with a pH higher than 7 are alkaline, and the rela-
tive concentration of hydrogen ions decreases by a factor of 10
with each higher pH unit. Tus, solutions with pH values of 8
and 12 have, respectively, 1/10 and 1/100,000 (1/10
3
1/10
3
1/10
3
1/10
3
1/10) as many hydrogen ions as a solution of
pH 7.
Te approximate pH of several body fluids and of a number
of common substances appears in Figure 2.13. Notice that as the
hydrogen ion concentration decreases, the hydroxyl ion con-
centration rises, and vice versa.
Neutralization
What happens when acids and bases are mixed? Tey react with
each other in displacement reactions to form water and a salt.
For example, when hydrochloric acid and sodium hydroxide
interact, sodium chloride (a salt) and water are formed.
HCl
1
NaOH
S
NaCl
1
H
2
O
acid
base
salt
water
Concentration
(moles/liter)
[OH
-
]
10
0
10
-
14
10
-
1
10
-
13
10
-
2
10
-
12
10
-
3
10
-
11
10
-
4
10
-
10
10
-
5
10
-
9
10
-
6
10
-
8
10
-
7
10
-
7
10
-
8
10
-
6
10
-
9
10
-
5
10
-
10
10
-
4
10
-
11
10
-
3
10
-
12
10
-
2
10
-
13
10
-
1
[H
+
]
pH
Examples
1
M
Sodium
hydroxide (pH=14)
Oven cleaner, lye
(pH=13.5)
Household ammonia
(pH=10.5–11.5)
Neutral
Increasingly basic
Increasingly acidic
Household bleach
(pH=9.5)
Egg white (pH=8)
Blood (pH=7.4)
Milk (pH=6.3–6.6)
Black coffee (pH=5)
Wine (pH=2.5–3.5)
Lemon juice; gastric
juice (pH=2)
1
M
Hydrochloric
acid (pH=0)
10
-
14
10
0
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Figure 2.13
The pH scale and pH values of representative
substances.
The pH scale is based on the number of hydrogen ions
in solution. The actual concentrations of hydrogen ions, [H
1
], and
hydroxyl ions, [OH
2
], in moles per liter are indicated for each pH
value noted. At a pH of 7, [H
1
]
5
[OH
2
] and the solution is neutral.
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