pH Scale
We measure acidity and alkalinity using the pH scale, pH being short
for “potential of hydrogen”. The pH scale ranges from 0 to 14. It is
logarithmic, so that a difference of one pH unit represents a tenfold
change in hydrogen (or rather, hydronium) ion concentration. While we
are not concerned with how to calculate pH, it is important to know how
pH changes with hydronium ion concentration: it is a reciprocal scale.
This means that as the pH values decrease, the concentration of
hydronium ion (H3O+) increases. For instance, a substance with a pH of
2 has 10 times the concentration of hydronium ion concentration as a
substance with a pH of 3. As the pH values increase, concentration of
hydroxide ions (OH-) increases.
A substance with a pH of 10 has 100 times the hydroxide ion
concentration as a substance with a pH of 8. Although it may be
confusing, remember that if you hear about a solution having a low pH it
actually means it is quite acidic.
Water is a neutral compound, except for that very small amount-one
half a billion molecules-that is dissociated. When this is translated to
the pH scale, water has a pH of 7. This is considered the neutral point.
Acids have pHs lower than 7, and bases have pHs higher than 7. One
way to think of this is that for every pH point lower than 7, the solution
has 10 times more h+ floating around than is present in regular water.
Likewise, every pH point above 7 means that 10 times more OH- is
present than is in water.
The pH of a substance can be determined by a pH meter or by an
indicator, such as litmus. Indicators turn different colors, depending on
the pH of a solution. You have probably heard of a litmus test. This
involves placing a drop of solution onto litmus paper, which contains
the indicator, and observing the color change, if any. For instance, acids
will turn blue litmus paper red, and bases will turn red litmus paper
blue. However, the range over which indicators change colors is small.
Litmus is only an effective indicator for solutions in the pH range form
5.5 -8.0. Therefore, chemists use many different indicators to test the
pH of solutions across the entire pH scale.
Again you ask, what does this mean to living things? The pH of solutions
has far-reaching consequences for living organisms. For larger
multicellular organisms, highly acidic solutions are often used to digest
foods. One common example is stomach acid (gastric acid). It is made
up mostly of HCl, a highly acidic compound. However, the pH of the
stomach can range from between 1.5 to 7.0 depending on the stage of
digestion. The cells lining the stomach are the only cells in the human
body that can withstand strong acids. So, when foods enter the lower
part of the stomach, sodium bicarbonate is added to the food slurry.
The sodium bicarbonate is produced by the pancreas and released into
the duodenum (lower part of the stomach). The result is a
neutralization reaction where salt and water are produced.
But let’s think small. What effect does pH have on the cells of
organisms? Or for that matter, how does pH affect unicellular
organisms? Remember pH means “potential of hydrogen”. This is just a
scientific way of saying the ability of the solution to give away its
protons (or hydrogen atoms, H+) a really strong acid (one with a low
pH) really, really wants to give away hydrogen atoms. Conversely, a
really strong base (one with a high pH) really, really wants to accept
proton (or forcibly take them from other unsuspecting molecules).
Adding or taking away hydrogen atoms changes the physical structure
of the molecule In biological systems, the function of a molecule is
highly dependent on its three-dimensional structure. Changing its
structure can severely alter its function. Solutions with an extreme pH
value can break apart proteins inside cells, tear down cell membranes
or cell walls and, most importantly, inhibit the function of enzymes.
The final result: death or destruction of the cell. Inhibiting the function
of enzymes can prevent metabolic reactions form occurring. Most
biological systems function best within a neutral pH range. We’ll talk
more about enzymes and their importance to biological systems later in
this chapter.
Section Review
1. Define the following terms:
a. pH scale
2.
3.
4.
5.
6.
7.
b. indicator
Which value represents a strong acid?
a. 2
b. 7
c. 8
d. 14
Which of the following substances is basic?
a. Milk
b. Vinegar
c. Bleach
d. rainwater
Orange juice has a pH of about 2 and tomato juice has a pH of
about 4. Orange juice has a lower pH because
a. It is more basic than tomato juice
b. It is a stronger base than tomato juice
c. It is a stronger acid than tomato juice
d. It is a weaker acid than tomato juice
Which pH value is best to use when culturing an amoeba?
a. 3
b. 10
c. 14
d. 7
A solution has a pH of 12. It is
a. A strong acid
b. A strong base
c. A neutral solution
d. A weak acid
How many atoms of hydrogen are in 1 molecule of glucose?
(C6H12O6)?
a. 3
b. 6
c. 12
d. 18