This document discusses acids, bases, pH, and buffers. It defines acids as substances that donate hydrogen ions (H+) in water, raising the H+ concentration and making the solution acidic. Bases are defined as substances that accept H+ ions, lowering the H+ concentration and making the solution basic. The pH scale quantifies how acidic or basic a solution is based on the concentration of H+ ions, with lower pH indicating higher acidity. Buffers are discussed as mixtures that can absorb added H+ or OH- ions to minimize changes to pH.

1. ACID
BASE
BUFFER
pH
&
BY Dr. EKHLAQUE
ekhlaquebiotech@gmail.com

2. Substances dissolve in water, they release single, positively
charged hydrogen ions (H).
These charged matter can attach to other molecules, and in
doing so, change their properties.
For example, the protons in acid rain can damage plants, and
you are probably familiar with excess stomach acidity that
affects digestion.
Properties of substances that release H+ (called acids) and
substances that attach to H+ (called bases).

3. Acids donate H+
If hydrochloric acid (Hcl) is added to water, it
dissolves and ionizes, releasing the ions H+ and Cl–:
HCl → H+ + Cl–
Because its H+ concentration has increased, such a
solution is acidic.
Just like the combustion reaction of propane and
oxygen, the dissolution of HCl to form its ions is a
complete reaction.
HCl is therefore called a strong acid.

5. Cont…
Biological compounds that contain —COOH (the carboxyl
group) are also acids (such as acetic acid and pyruvic acid),
because
COOH → COO– + H+
Not all acids dissolve fully in water. For example, acetic
acid.
Because the reaction is not complete, acetic acid is a weak
acid.

6. Bases accept H+
If NaOH is added to water, it dissolves and ionizes, releasing OH–
and Na+ ions
NaOH → Na+ + OH–
Because the concentration of OH– increases and OH– absorbs H+ to
form water, such a solution is basic.
Because this reaction is complete, NaOH is a strong base.
Bicarbonate ion (HCO3
–), which can accept a H+ ion and become
carbonic acid (H2CO3), and ammonia (NH3), which can accept a H+
and become an ammonium ion (NH4
+).
Amino groups (—NH2) in biological molecules can also accept
protons, thus acting as bases:
 NH2 + H+ → NH3
+

7. The reactions between acids
and bases may be reversible
When acetic acid is dissolved in water, two reactions
happen. First, the acetic acid forms its ions:
CH3COOH → CH3COO– + H+
Then, once ions are formed, they re-form acetic acid:
CH3COO– + H+ →CH3COOH
This pair of reactions is reversible.
CH3COOH → CH3COO– + H+

8. Water is a weak acid
The water molecule has a slight but significant
to ionize into a hydroxide ion (OH–) and a
hydrogen ion (H+).
Actually, two water molecules participate in
this ionization.
One of the two molecules “captures” a
hydrogen ion from the other, forming a
hydroxide ion and a hydronium ion

9. pH is the measure of
hydrogen ion concentration
The terms “acidic” and “basic” refer only to solutions.
How acidic or basic a solution is depends on the relative
concentrations of H+ and OH– ions in it.
The terms “acid” and “base” refer to compounds and ions.
A compound or ion that is an acid can donate H+; one that
is a base can accept H+.
How do we specify how acidic or basic a solution is?
It is easier to work with the logarithm of the concentration,
because logarithms compress this range.

10. These H+ concentrations are expressed in terms of molarity, the
number of moles of a substance in a liter of solution.
In pure water, the concentration of H+ is 10–7 moles per liter (10–7
M).
In 1 M hydrochloric acid, the H+ concentration is 1 M; and in 1 M
sodium hydroxide, the H+ concentration is 10–14 M.
Because its values range so widely, the H+ concentration itself is an
inconvenient quantity to measure.
The pH value is defined as the negative logarithm of the hydrogen
ion concentration in moles per liter (molar concentration).
The equation for pH is
pH = –log10[H+]
Cont…

11. Cont…
Since the H+ concentration of pure water is 10–7
M, its pH is –log(10–7) = –(–7), or 7.
In practical terms, a lower pH means a higher H+
concentration, or greater acidity.
In 1 M HCl, the H+ concentration is 1 M, so the
pH is the negative logarithm of 1 (–log 100), or 0.
The pH of 1 M NaOH is the negative logarithm of
10–14, or 14.

13. Buffers minimize pH change
The earliest forms of life, live in and have adapted to
solutions with extremes of pH.
However, most organisms control the pH of the separate
compartments within their cells.
The normal pH of human red blood cells, for example, is 7.4,
and deviations of even a few tenths of a pH unit can be fatal.
The control of pH is made possible in part by buffers:
chemical mixtures that maintain a relatively constant pH even
when substantial amounts of an acid or base are added.

14. Cont…
A buffer is a mixture of a weak acid and its corresponding
base. for example, carbonic acid (H2CO3) and bicarbonate
ions (HCO3
–).
If an acid is added to a solution containing this buffer, not
all the H+ ions from that acid stay in solution.
Instead, many of them combine with the bicarbonate ions to
produce more carbonic acid.
This reaction uses up some of the H+ ions in the solution
and decreases the acidifying effect of the added acid:

15. If a base is added, the reaction essentially reverses.
Some of the carbonic acid ionizes to produce bicarbonate
ions and more H+, which counteracts some of the added
base.
In this way, the buffer minimizes the effects of an added
acid or base on pH.
This is what happens in the blood, where this buffering
system is important in preventing significant changes in
pH that could disrupt the ability of the blood to function in
carrying vital O2 to tissues.
A given amount of acid or base causes a smaller change in
pH in a buffered solution than in an unbuffered one
Cont…