Biology H
Modified from http://www.visionlearning.com/library/module_viewer.php?mid=58
Acids, Bases and pH
Water is not always just H2O
You have learned that water is polar, with the O portion of the molecule attracting most of the
electrons and therefore acting negatively charged, while the H atoms are mostly left without
nearby electrons and so act positively charged. The opposite poles attract one another through
hydrogen bonds and create cohesion between water molecules. The
picture at left represents the situation in ice, where all the molecules are
locked into position. In liquid water, these hydrogen bonds are
constantly breaking and forming with new partners.
Sometimes, the O does such a good job of attracting H’s that it actually pulls one H+ away from
another water molecule like this:
This results in two ions. The hydronium
with the extra H+ is positively charged,
while the hydroxide that lost the H+ (but
still has the electron from it) is negatively
charged.
In a sample of 1 million water molecules, only a couple of molecules will do this at any one time
and they will always be perfectly balanced. In fact, what we really care about is not so much the
hydronium ions that can form, but the H+ ions themselves. The reaction can be shown as
H2O → H+ + OH− and the ions can reform into H2O.
and this is important because we define acids and bases by whether they release H+ (acid) or
OH− (base) when they are put in water.
Acids and Bases
For thousands of years people have known that vinegar, lemon juice and many other foods taste
sour. However, it was not until a few hundred years ago that it was discovered why these things
taste sour - because they are all acids. The term acid, in fact, comes from the Latin term acere,
which means "sour". While there are many slightly different definitions of acids and bases, in
this lesson we will introduce the fundamentals of acid/base chemistry.
Acids taste sour, are corrosive to metals, change litmus (a dye extracted from lichens) red, and
become less acidic when mixed with bases.
Bases feel slippery, change litmus blue, and become less basic when mixed with acids.
In the late 1800s, the Swedish scientist Svante Arrhenius proposed that water can dissolve many
compounds by separating them into their individual ions. Arrhenius suggested that acids are
compounds that contain hydrogen and can dissolve in water to release hydrogen ions into
solution. For example, hydrochloric acid (HCl) dissolves in water as follows:
H2O
H+(aq) +
HCl
Cl-(aq)
Note that (aq) means this is happening in water
Arrhenius defined bases as substances that dissolve in water to release hydroxide ions (OH-) into
solution. For example, sodium hydroxide (NaOH) is a typical base because it does this:
H2O
NaOH
Na+(aq) +
OH-(aq)
The Arrhenius definition of acids and bases explains a number of things. Arrhenius's theory
explains why all acids have similar properties to each other (and, conversely, why all bases are
similar): because all acids release H+ into solution (and all bases release OH-). The Arrhenius
definition also explains the observation that acids and bases counteract each other. This idea, that
a base can make an acid weaker, and vice versa, is called neutralization. This explanation didn’t
explain how substances like sodium bicarbonate NaHCO3 that don’t contain OH- could
neutralize acids, but they do it very well. Most stomach antacids contain sodium bicarbonate.
Neutralization
As you can see from the equations, acids release H+ into solution and bases release OH-. If we
were to mix an acid and base together, the H+ ion would combine with the OH- ion to make the
molecule H2O, or plain water:
H+(aq) +
OH-(aq)
H2O
But remember that an acid is a substance that releases the H+ ion and a base is the substance that
releases the OH- ion. This means that while we are happy that we have made a molecule of
water, we also have to think about what happens to the rest of the acid and base substances. The
answer is that those substances are also oppositely charged ions and will form an ionically
bonded compound called a salt if the water is removed.
The neutralization reaction of an acid with a base will always produce water and a salt, as shown
below:
Acid
Or
Base
Water
Salt
HCl
+
NaOH
H2O
+
NaCl
H+Cl-
+
Na+OH-
H+OH-
+
Na+ Cl-
HBr
+
KOH
H2O
+
KBr
So if you mix an acid and a base of equal strength, they will neutralize each other to form pure
water AND a salt.
Interesting to note that sodium chloride, aka NaCl, aka table salt is what you get if you mix
hydrochloric acid and sodium hydroxide (lye). Rearranging the ions turns two nasty substances
into two harmless, and even necessary ones!
In 1923, the Danish scientist Johannes Brønsted modified Arrhenius' theory of acids and
bases. The definition of acids remained any substance that can donate a hydrogen ion is an acid,
but the Brønsted definition of bases is, however, quite different. According to Brønsted, a base
is defined as any substance that can accept a hydrogen ion. In essence, a base is the opposite of
an acid. NaOH and KOH, as we saw above, would still be considered bases because they can
accept an H+ from an acid to form water. However, the Brønsted definition also explains why
substances that do not contain OH- can act like bases. Baking soda (NaHCO3), for example, acts
like a base by accepting a hydrogen ion from an acid as illustrated below:
Acid
HCl
Base
+ NaHCO3
Salt
H2CO3 +
NaCl
In this example, the carbonic acid formed (H2CO3) undergoes rapid decomposition to water and
gaseous carbon dioxide, and so the solution bubbles as CO2 gas is released. Bicarbonate is used
in many parts of the human body to accept hydrogen ions and keep acids in check.
pH
Great. So why should we care how the base is defined? Under the Brønsted definition, both
acids and bases are related to the concentration of hydrogen ions present. Acids increase the
concentration of hydrogen ions, while bases decrease the concentration of hydrogen ions (by
accepting them). The acidity or basicity of something, therefore, can be measured by its
hydrogen ion concentration. Now we have a way to label how strong and acid or base is.
In 1909, the Danish biochemist Sören Sörensen invented the pH scale for measuring acidity.
The pH scale is described by a terrible formula that you don’t need to worry about, but it does
make some sense if we consider the concentration of H+ ions in a solution. For example, a
solution with an H+ concentration = 1 x 10-7 moles/liter has a pH equal to 7 (a simpler way to
think about pH is that it equals the exponent on the H+ concentration, ignoring the minus sign). A
solution with a pH of 6 would have an H+ concentration of 1 x 10-6 moles/liter, making it 10
times more concentrated than the solution with a pH of 7. The pH scale ranges from 0 to 14.



Substances with a pH between 0 and less than 7 are acids; pH and H+ concentration are
inversely related - lower pH means higher H+.
Substances with a pH greater than 7 and up to 14 are bases; higher pH means lower H+
concentration.
Right in the middle, at pH = 7, are neutral substances, for example, pure water.
Remember that dissociation of H2O into H+ and OH- that doesn’t happen very often? 1 x 10-7
moles/liter is exactly the concentration of H+ in a sample of pure water.
The relationship between [H+] and pH is shown in the table below alongside some common
examples of acids and bases in everyday life.
[H+]
pH Example
1 X 100
0
HCl
1 x 10-1
1
Stomach acid
1 x 10-2
2
Lemon juice
1 x 10-3
3
Vinegar
1 x 10-4
4
Soda
1 x 10-5
5
Rainwater
1 x 10-6
6
Milk
Neutral 1 x 10-7
7
Pure water
1 x 10-8
8
Egg whites
1 x 10-9
9
Baking soda
Acids
Bases
1 x 10-10 10
Tums® antacid
1 x 10-11 11
Ammonia
1 x 10-12 12
Mineral lime - Ca(OH)2
1 x 10-13 13
Drano®
1 x 10-14 14
NaOH
For an interactive pH scale and more explanation of the basis for the pH scale, go to
http://www.johnkyrk.com/pH.html
Biology?
You should recall that living things must be able to maintain homeostasis, and the pH of various
bodily fluids and tissues is critical. Most body pH is maintained between about 6.5 and 7.5. The
blood must be maintained at a pH of between 7.35 and 7.45, but stomach acid must be at a low
pH (generally between 1 and 3). This acidic environment allows digestive enzymes from the
stomach to work properly. After leaving the stomach, partially digested food must be brought to
a pH of 8 or higher in order for the digestive enzymes of the small intestines to work. Saliva
typically has a pH of between 6.35 and 7.4.
Buffers
In all of these areas, we maintain the H+ concentration between these various desired ranges by
using buffers. A buffer is a substance like a weak acid or base that can either absorb or release
H+ as needed to maintain a steady pH, and bicarbonates play a big role here. Blood uses a
bicarbonate to interact with carbonic acid to maintain pH. Sodium bicarbonate is the active
ingredient in many antacids. Also note that a buffer does not necessarily neutralize a solution,
instead it keeps the pH stable at whatever level is needed.
Name
Biology H
What is pH ?
: Hydroxide Ion (OH-)
A
: Hydrogen Ion (H+)
B
C
D
1. Count the Hydrogen and Hydroxide ions for each beaker. Rank them in order of their
approximate pH from most acidic to the most basic.
2. Which beaker contains only pure water? Explain how you know.
3. What is the basis for the pH scale devised by Sörensen?
4. Consider a solution with a pH of 4 and a solution with a pH of 5. Explain what the numbers
tell us about the relative numbers of hydrogen ions in these solutions.
5. a. What is the pH range of most areas of your body?
b. Name two places in your body where the pH is NOT in this range. Include what the pH is
in each place and why is it necessary for the pH to be different in order for each part to
function properly.
6. What is a buffer?
7. Explain why it is incorrect to say “buffers exist in the blood to make sure the pH of blood
remains neutral.”
8. What happens to the concentration of hydrogen ions when an acid is added to a beaker
containing water? What happens to the pH?
9. What happens to the concentration of hydroxide ions when a base is added to a beaker
containing water? What happens to the pH?
10. Solution A (pH 4) and Solution B (pH 9) are mixed.
a. What type of reaction happens?
b. What are the two products that will form?
c. Approximately where on the pH scale will the resulting mixture fall?