32 Chapter 2 (^)
(^) Distilled water (^)
(^) Blood 7.4 (^)
(^) Milk 6.6 Egg white 8.0 (^)
(^) Black coffee 5.0 Baking soda 9.0 (^)
(^) Tomato 4.6 Milk of magnesia 10.5
Vinegar 3.0 Household ammonia 11.0^
Stomach gastric juice 2.0 (^)
Bleach 13.0 (^)
Oven cleaner 13.8 (^)
Hydrochloric acid 0.8 (^)
1.0 2.0^ 3.0^ 4.0^ 5.0 6.0
7.0 8.0 9.0 10.0 11.0 12.0 13.0
0.0 14.0^
Neutral (^)
Strong acid Strong base (^)
(^) ®
Learning
Cengage ©
Figure 2- 15 The pH of various solutions. A pH above 7 indicates a base; a value below 7 indicates an acid.
with H^1 ions, a base therefore lowers the H^1 ion concen-
tration in that solution. Basic, also called alkaline, solu-
tions have pH values above 7. Seawater with a pH of 8 is
10 times more basic than pure distilled water with a pH of
- In our bodies, saliva in our mouths has a pH value
slightly lower than 7 so it is just slightly acidic, whereas the
stomach with its gastric juice and hydrochloric acid is very
acidic with a pH value near 1. Our blood on the other hand
has a pH value of 7.4, making it just slightly basic. Urine
has a pH of 6, which, although acidic, is not as acidic as
tomato juice with a pH of 4.
The pH inside most cells and in the fluid surround-ing
cells is fairly close to 7. Because enzymes are ex-tremely
sensitive to pH, even a small change can render them
nonfunctional; thus, our bodies have buffers. A buffer is a
substance that acts as a reservoir for hydrogen
ions, donating them to a solution when their concentra-tion
falls, and taking the hydrogen ions from a solution when
their concentration rises. Buffers are necessary because the
chemical reactions in cells constantly are producing acids
and bases. Buffers help maintain ho-meostasis within cells
in regard to pH levels. Most buffers consist of pairs of
substances, one an acid and the other a base. For example,
the key buffer in human blood is the acid-base pair
bicarbonate (a base) and carbonic acid (an acid). Carbon
dioxide and water combine chemically to form carbonic
acid (H 2 CO 3 ). The carbonic acid then can dissociate in
water, freeing H^1 ions and -bicarbonate ions HCO 32. The
blood’s pH can be stabilized by the equilibrium between
these forward and reverse reac-tions that interconvert the
carbonic acid (H 2 CO 3 ) and the bicarbonate ion (HCO 32 )
(base).