THE pH SCALE
There must be a balance between acids and bases in order to maintain life. An
imbalance disrupts homeostasis. The acid-base balance is measured using the
pH scale. The pH scale(Fig. 2-7) measures the acidity or alkalinity (base) of a
substance using a pH value from 0 to 14. Values on the pH scale are logarithmic
values. A pH value of 7 is neutral, which is the pH of pure water. A pH value
greater than 7 is a base or alkaline. A pH value less than 7 is an acid. A change
in one pH value is a large change because it is a logarithmic scale. For example
a pH of 1 has 10 times more hydrogen ions than a pH of 2 and 100 times more
hydrogen ions than a pH of 3 (pH = −log 10 [H+]).
Adding a substance that will increase or decrease the concentration of hydro-
gen ions can change the pH value of a substance. Increasing the concentration
of hydrogen ions makes the substance more acidic and decreasing the concen-
tration makes the substance more alkaline.(^38) CHAPTER 2 Chemical Elements of Microorganisms
Fig. 2-7. The pH scale is a logarithmic scale that measures
the acidity or alkalinity of a substance.
0 1 2 3 4 5 6 7 8 9
10
11
12
13
14
Distilled water
Oven cleaner
Stomach acid
Acidity
Alkalinity
(Base)
Lemon juice
Vinegar
Tomatoes
Black coffee
Urine
Bile
Baking soda
Milk of Magnesia
Household Bleach
Ammonia
Lime water