1. MD. MYNUL HASAN
SOUTHEAST UNIVERSITY Bangladesh
Acid-Base Titration

2. Content
 Acid base concept
 Role of this form of titration in pharmaceutical quality assurance
 Ionization
 Low of ionization
 Henderson hasselbarkh equation equation
 Neutralization curves
 Acid base indicators
 Mixed indicators used in polyprotic & amino acid systems during amino
acid titration

3. Introduction
Definition: An acid-base titration is a procedure which is used to determine the concentration of
an acid or base. A measured volume of an acid or base of known concentration is reacted with a
sample to the equivalence point.
A titration is a procedure used in analytical chemistry to determine the amount or concentration
of a substance. In a titration one reagent, the titrant, is added to another slowly. As it is added a
chemical stoichiometric reaction occurs until one of the reagents is exhausted, and some process
or device signals that this has occurred. The purpose of a titration is generally to determine the
quantity or concentration of one of the reagents, that of the other being known beforehand. In
any titration there must be a rapid quantitative reaction taking place as the titrant is added, and in
acid-base titrations this is a stoichiometric neutralization. The type of titration is simply the type
of chemical reaction taking place, and so in this section we consider acid-base titrations.
Acid-base titration can be dividing into:
1. Strong acid versus strong base
2. Weak acid versus strong base
3. Weak base versus strong acid
Acid-Base Titration Reactions
All acid-base titration reactions, as all acid-base reactions, are simply exchanges of protons. The
reaction could be strong acid + strong base (neutral) salt, as in the case of
HCl + NaOH NaCl + H2O,
although the reaction would be correctly written as
H3O+
+ OH-
H2O
since strong acids and strong bases are totally dissociated to protons and hydroxide ions in water.
For reactions which are
strong acid + weak base (acidic) salt,
such as the example
HCl + CH3NH2 CH3NH3
+
Cl-
,
or
strong base + weak acid (basic) salt,

4. such as the example
NaOH + CH3COOH Na+
CH3COO-
+ H2O,
the cations and anions could be omitted as they do not actually participate in the reaction. (Some
chemists call these bystander ions.)
Virtually all acid-base titrations are carried out using a strong acid or strong base. In most cases
the strong acid or strong base is used as the titrant. It is less common, but equally feasible, to
place the strong acid or strong base in the titration vessel and use the weak acid or weak base as
the titrant. A weak acid-weak base titration would have only a small pH change at the
equivalence point. This small change is difficult to detect, and for this reason weak acid-weak
base titrations are uncommon.
Standards in Acid-Base Titrations
One of the substances involved in a titration must be used as a standard for which the amount of
substance present is accurately known. The standard can be present either in the form of a pure
substance or as a standard solution, which is a solution whose composition is accurately known.
A standard can be prepared in only two ways: use a primary standard or standardize by titration
against some previously standardized solution. A primary standard is some substance such as
oxalic acid which can be precisely weighed out in pure form, so that the number of moles present
can be accurately determined from the measured weight and the known molar mass. For
example, we might prepare a 0.1000 molar solution of primary standard oxalic acid by weighing
out exactly 0.1 moles of oxalic acid and diluting to one liter in a volumetric flask.
The standard solutions used in an acid-base titration need not always be primary standards. A
standard solution which has been prepared by quantitative dilution of a primary standard is an
excellent secondary standard solution. Secondary standards can also be prepared by titration
against a primary standard solution.
Acid base concept
The terms ‘acids’ and ‘bases’ have been defined in many ways. According to Arrhenius,
probably the oldest, acids, and bases are the sources of H+ and OH- ions respectively. A
somewhat broader but closely related definition (Bronsted – Lowry) is that an acid is a substance
that supplies protons and a base is proton acceptor. Thus in water an acid increases the
concentration of hydrated proton (H3O+ ) and a base lowers it or increases the concentration of
OH- . In addition to Bronsted-Lowry concept there are others like solventsolvent definition and
the Lux and Flood definition but each one has its own limitations. One of the most general and
useful of all definitions in reference to complex formations was due to G. N. Lewis. Lewis
defined acid-base in terms of electron pair donor-acceptor capability. This definition includes

5. Bronsted- Lowry definition as a special case. Some- what related to Lewis concept was an
approach developed by Pearson, who generalized complex formation in terms of hard-soft acids
and bases. All these concepts will be discussed in detail. Attempt will also be made to highlight
some of the structural and theoretical aspects regarding these concepts of few non-aqueous
solvent systems as they are relevant in this context.
Arrhenius Acid-Base Theory
The Arrhenius acid-base concept classifies a substance as an acid if it produces hydrogen ions
H(+) or hydronium ions in water. A substance is classified as a base if it produces hydroxide ions
OH(-) in water. This way of defining acids and bases works well for aqueous solutions, but acid
and base properties are observed in other settings. Other ways of classifying substances as acids
or bases are the Bronsted-Lowry concept and the Lewisconcept.
Acids are defined as a compound or element that releases hydrogen (H+
) ions into the solution.
In this reaction nitric acid (HNO3) disassociates into hydrogen (H+
) and nitrate (NO3
-
) ions when
dissolved in water.
Bases are defined as a compound or element that releases hydroxide (OH-
) ions into the solution.
In this reaction lithium hydroxide (LiOH) dissociates into lithium (Li+
) and hydroxide (OH-
) ions
when dissolved in water.
Bronsted-Lowry Acid-Base Concept
The Bronsted-Lowry theory classifies a substance as an acid if it acts as a proton donor, and as
a base if it acts as a proton acceptor. Other ways of classifying substances as acids or bases are
the Arrhenius concept and theLewis concept.

6. Lewis Acid-Base Concept
The Lewis theory classifies a substance as an acid if it acts as an electron-pair acceptor and as
a base if it acts as an electron-pair donor. Other ways of classifying substances as acids or bases
are the Arrhenius concept and theBronsted-Lowry concept.
Solvent –System Concept :
The Lowry-Bronsted concept of acid-base phenomenon is much broader than the one provided
by Arrhenius. In this concept the acid-base behaviour is neither restricted to nor dependent upon
any particular solvent. In fact the Bronsted concept applies to many of the solvents that contain
hydrogen. Liquid ammonia is another solvent most widely studied.
Ionization
Ionization is the process by which an atom or a molecule acquires a negative or positive charge
by gaining or losing electrons to form ions, often in conjunction with other chemical
changes.[1]
Ionization can result from the loss of an electron after collisions with subatomic
particles, collisions with other atoms, molecules and ions, or through the interaction with
light. Heterolytic bond cleavage and heterolytic substitution reactions can result in the formation
of ion pairs. Ionization can occur through radioactive decay by theinternal conversion process, in
which an excited nucleus transfers its energy to one of the inner-shell electrons causing it to be
ejected.
Ionization Process
To understand the process, we will consider the structure of sodium chloride. Sodium chloride is
the common salt, we use in our day to day life. The atomic number of Na and Cl are 11 and 17
respectively. That means sodium atom has 11 numbers of electrons and chlorine atom has 17
numbers of electrons in their orbits. The atomic structure or arrangement of electrons in their
orbits, in both the atoms are shown in the figure below.

7. It is clear from the figure beside, the Na atom has only one single electron in its outermost orbit.
Whereas chlorine contains seven electrons in its outermost orbit. But we know that for stability,
atoms generally require eight electrons in their outermost orbit. So, both of the shown atoms are
chemically active. When these atoms are brought together, Na atom loses its outermost electrons,
becomes positively charged and Cl atom gains one electron and becomes negatively charged. So
both atoms acquire eight electrons in their outermost orbit by exchanging these electrons. As the
Na atom is positively charged and Cl atom is negatively charged, electrostatic force acts between
them, due to which they will together and make one NaCl molecule.
Now according to Coulomb's law, the electrostatic force acting between two opposite charges is
expressed as,

8. Where, εr is the relative permittivity of the medium. So it is clear that electrostatic force between
two charges is inversely proportional to the relative permittivity of the medium in which the
charges are placed. The ionization process can easily be explained by relative permittivity of the
medium. The relative permittivity of air is 1.00058986 ± 0.00000050 or 1 and the relative
permittivity of water at 20°C is 80. So, in water the electrostatic force acting between Na and Cl
is 80 times smaller than that in the air. The electrostatic force between Na and Cl becomes so
small, that it becomes difficult to hold the Na and Cl together in water. That is why whenever
NaCl i.e. Sodium Chloride is dissolved in water, its molecules split into positive Na ion and
negative Cl ion even at room temperature and below. This is ionization of NaCl.
Ionization energy of atoms
The trend in the ionization energy of atoms is often used to demonstrate the
periodic behavior of atoms with respect to the atomic number, as summarized by
ordering atoms inMendeleev's table. This is a valuable tool for establishing and
understanding the ordering of electrons in atomic orbitals without going into the
details of wave functions or the ionization process. The periodic abrupt decrease in
ionization potential after rare gas atoms, for instance, indicates the emergence of a
new shell in alkali metals. In addition, the local maximums in the ionization energy
plot, moving from left to right in a row, are indicative of s, p, d, and f sub-shells.

9. Law of Ionization in Acid-Base Titration
Acid Ionization Constant (Ka)
The acid ionization constant (Ka) is the measure of the strength of an acid in solution.
The acid ionization constant, Kb
The acid dissociation constant, Ka, comes from the equilibrium constant for the breakdown
of an acid in aqueous solution:
HA + H2O A-
+ H3O+
Where H3O+
is the hydrogen ion is solution, it may also be written H+
(aq). The equilibrium
law for this dissociation is:
As the concentration of water is effectively constant, a new constant, the
acid dissociation constant, Ka, is defined as:
The acid dissociation constant, Ka gives a measure of the extent of the
dissociation. If Ka is a large value then the acid is strong and dissociates
into ions easily. These constants are only useful for weak acids.
Acid Ka
Methanoic acid 1.78 x 10-4
Ethanoic acid 1.74 x 10-5

10. Propanoic acid 1.35 x 10-5
Butanoic acid 1.51 x 10-5
Hydrocyanic acid 3.98 x 10-10
Hydrofluoric acid 5.62 x 10-4
Carbonic acid 4.26 x 10-7
From the list it may be seen that methanoic acid is a stronger acid than
ethanoic acid, i.e. it dissociates further releasing more hydrogen ions in
solution.

11. Example: Hydrogen ion concentration from Ka.
Calculate the [H+(aq)] of 0.2 M ethanoic acid (Ka = 1.78 x 10-5
)
Ethanoic acid is a weak acid. It only partially dissociates according to the equation:
CH3COOH CH3COO-
+ H+
Therefore the acid dissociation constant:
We can assume that as the acid only slightly dissociates then the concentration of the
acid at equilibrium is the same (to a close approximation) as the concentration of the
original acid (in this case = 0,2 M)
Therefore:
And as the hydrogen ion concentration equals the ethanoate ion concentration, then:
0.2 x 1.78 x 10-5
= [H+
]2
[H+
] = √ (3.56 x 10-6
)
[H+
] =1.89 x 10-3

12. The base ionization constant, Kb
When bases interact with water they do so by removing a hydrogen ion creating hydroxide ions
in solution.
NH3 (aq) + H2O NH4
+
(aq) + OH-
(aq)
Once again, the water concentration is effectively constant and allows us to define a new
constant, Kb:
Kb is referred to as the base equilibrium constant and gives a measure of the extent of the
equilibrium. Large values for Kb means strong base. However, as for acids, the values are usually
very small and the -log(10) form of the information is used to generate convenient sized
numbers.
pKb = - log(10) Kb
The lower the pKb value the stronger the base.
Acid base indicators
Acid base indicators are substances that respond to a change in the hydrogen ion concentration of
a solution. You will be able to recognize this change most often through a color change. Keep in
mind that acid base indicators are typically weak acids. A weak acid is a compound that partially
dissociates (breaks apart) in solution. As we will see shortly, the role of a weak acid in the acid
base indicator process is very important. Some common examples of acid-base indicators include
blue grapes, which can change color from deep red in an acid to violet in a base; beets, which
change from red to purplish when in a very basic substance; and blueberries, which turn red in
strong acids. Acid base indicators refers as a pH indicator. These terms are interchangeable.pH
refers to a numerical scale that tells you whether a solution is acidic or basic. A pH value less
than 7 is considered acidic, while pH values greater than 7 are basic. A pH equal to 7 is neutral.

13. Example of Correlation Between Acid Base Indicator, Color Change, and pH
Acid-base indicators are similar to dyes. When exposed to chemicals that have different pH
values, the color of the indicator changes accordingly. Acid-base indicators often come in the
form of litmus papers, which change color depending on whether they are submerged in an
acidic or basic substance .
Acid-base indicators are important because they help chemists get an estimate of the pH value of
a given substance. These indicators can be used to classify substances as acids or bases, which
are two important classifications in the world of chemistry. Acid-base indicators are usually
weak acids or bases and are able to respond to changes in the hydrogen ion concentration of an
unknown substance.The pH scale measures how acidic or basic a given chemical is. Chemicals
that are extremely acidic or basic are considered reactive, which means they can be highly
volatile. People who are exposed to highly acidic or basic chemicals can be burned, injured or
even killed. For this reason, the classifying of a substance as either acidic or basic, by getting an
estimate of its place on the pH scale, is important