1. Acid Base Titration
1. The Arrhenius theory of acids and bases states
that “an acid generates H+ ions in a solution
whereas a base produces an OH– ion in its solution”.
2. The Bronsted-Lowry theory defines “an acid as a
proton donor and a base as a proton acceptor”.
Acid Base+ H+
3. Finally, the Lewis definition of acids and bases
describes “acids as electron-pair acceptors and
bases as electron-pair donors”.
2. An acid-base titration is a quantitative analysis of acids and bases;
through this process, an acid or base of known concentration
neutralizes an acid or base of unknown concentration.
The titration progress can be monitored by visual indicators, pH
electrodes, or both.
The reaction’s equivalence point is the point at which the titrant
has exactly neutralized the acid or base in the unknown analyte; if
you know the volume and concentration of the titrant at the
equivalence point, you can calculate the concentration of a base
or acid in the unknown solution.
The pH range of phenolphthalein is 8.0 – 9.8.
In solutions with pH less than 8.0, it is colorless and
in solutions with pH of more than 9.8, its color is
pink. Phenolphthalein is colorless in weak alkaline
solution and pink in strong alkaline solution
“Acid base indicators are those substances which
have one color in acidic solution and another color
in alkaline solution i.e. their color changes with
proper change in pH value.”
4. It is clear that it is not necessary that the color of
an acid base indicator is the same in all alkaline
solutions. Likewise, it is also not necessary that
the color of an acid base indicator be the same in
all acidic solution
Theory of Indicators: (explain the behavior of
acid base indicators.)
1. Ostwarld Theory
2. Quinonoid Theory
5. Ostwarld Theory
Theory of indicator action was suggested by W.ostwald.
Most of acid base indicator are weak organic acids or bases &
colour changes due to ionization of indicator
The unionized indicator has different colour than ionized indicator
Example: phenolphthalein is a weak acid. It is colorless. On
dissolving it in water, it is ionized into colorless H ions and pink
Consider HPh is weak organic acid indicator & in aqueous solution it
is well dissociate
HPh(colorless) ⇋ H(colorless) + Ph(Pink)
In the above reaction, dissociated indicator have different colour
than undissociated indicator
6. In acid base titration , if the indicator placed in
acidic solution of acid HA, following equilibrium
HPh ⇋ H+ + Ph-
HA ⇋ H+ + A-
Hph is a weak acid ,so it poorly dissociate & due to
common ion effect the dissociation further
decreased. The equilibrium is shifts towards left
Therefore in presence of acid the dissociation of
Hph is almost negligible & indicator is mostly in
undissociated form & show undissociated indicator
colour ( colourless)
7. But if the same indicator added in alkali solution
of base BOH, the dissociation of Hph will be
HPh ⇋ H+ + Ph- (dissociation increased)
BOH ⇋ OH- + B+
The hydrogen ion removed by OH- by formation
of water molecules & equilibrium is shifted
towards right side & concentration of Ph- will
more than Hph which give dissociated indicator
colour. i.e phenolphthalein give pink colour
8. Similarly if the indicator is weak organic base , it poorly
dissociated in alkali solution but dissociation
increased in acidic solution.
Thus the color change of methyl orange can also be
explained. Methyl orange is a weak base and thus dissociates
MeOH(Yello) ⇋ Me+(Red) + OH-(colorless)
In basic medium:
Due to the common ion effect in the presence of OH ions in
alkaline medium, ionization of MeOH is very low, i.e the
concentration of Me is very low and the color of the solution
9. MeOH ⇋ Me ++ OH-
HCl ⇋ H ++ Cl-
H ++ OH- ⇋ H2O
Me+ + Cl- ⇋ MeCl
10. Quinonoid theory
Quinonoid theory suggest that the colour change
of indicator is due to structural changes of Indicator.
In acidic solution indicator exist in different
structural form & in basic solution it exist in
different structural form
These structural form have different colours,
therefore in acidic solution indicator give different
colour and in basic solution different colour.
11. According to this theory, indicator are aromatic carbonic
compounds used in acid base titrations.
They are a mixture of at least two movable forms
Among these, one form is in acidic medium and the
other form in alkaline medium in greater proportion.
These two forms are called benzenoid and quinonoid
Both these forms have different colors. The color of the
quinonoid form is darker than the color of the benzenoid
12. Therefore, when the pH value of a solution changes, there is a
change in the benzenoid form to the quinonoid form or the
quinonoid form to the benzenoid form causing a change in color.
The acidic solution of phenolphthalein, being in its benzenoid form, is
When this solution is added to such a base that the solution becomes
alkaline, the entire benzenoid form turns into a quinonoid form.
The color of phenolphthalein’s alkaline solution also becomes red as
the color of the anions obtained from the quinonoid form is red.
14. Similarly methyl orange is also found in two tautomeric
In acidic solution, this compound remains in quinonoid
form and in alkaline solution it remains in benzenoid form.
The color of quinonoid form is red and the color of
benzenoid form is yellow orange.
Therefore, the color of acidic solution of methyl orange is
red and the color of alkaline solution is yellow orange.
15. Classification of acid base Titration
1. Titration of strong acid with strong base
2. Titration of weak acid with strong base
3. Titration of weak base with strong acid
4. Titration of weak base with weak acid
5. Titration of polybasic acid with strong base
27. Neutralization curve
Neutralization curve represents the neutralization reaction
between acid & base in neutralization titration.
It is plotted between the pH and volume of acid or base
The following information derived from neutralization curve
1. The curve is useful in studying the neutralization process
by studying the change in hydrogen ion concentration
2. It represent the progress of acid base titration
3. It indicate the end point of titration
4. It indicate the sensitivity of titration & chances of error
5. On the basis of pH conditions near inclination point the
selection of indicator for particular titration is made.
28. Non Aqueous Titration
Non aqueous titration are the titration which carried out using non
aqueous solvent (other than water)
Why choose non aqueous titration:
1. If the reactants or productants are insoluble in water.
2. React with water
3. Too weak base or acid that cannot be titrated in water due to their
poor infliction in pH at end point in water
Example of non-aqueous solvent:
- Glacial acetic acid
- Formic acid
- Ethylene diamine
29. Non aqueous solvent
1. Protophilic solvents: These are the solvent which have high affinity for proton &
are basic in nature.
HB + S SH+ + B-
Acid solvent solvated proton conjugate base
Example: Liquid ammonia, ketones and amines
2. Protogenic solvents: which readily donate protons & are acidic in nature
Example: sulphuric acid, hydrogen fluoride
3. Amphiprotic solvents: Solvents which are able to donate as well as accept proton(
Protogenic & protophilic solvent)
Example: Acetic acid
CH3COOH CH3COO- + H+
While in presence of strong acid like perchloric acid, it act as base & accept proton
CH3COOH + HClO4 CH3COOH2 + ClO4
4. Aprotic solvent: solvents which are chemically inert & does not donate or accept
proton , therefore they don't have any basic or acidic nature.
Example: Toluene, chloroform, benzene etc.
30. ALKALIMETRY & ACIDIMETRY
Alkalimetry refers to the determination of
strength of basic substances by titrating them
with standard acid solution.
Acidimetry refers to the determination of
strength of acidic substances by titrating them
with standard base solution.
In non-aqueous titration weak base & weak
acid are determined.
31. Titration of weak base:
Titration of weak base done by standard acid
mainly perchloric acid.
Acetic acid used as solvent for dissolving weak
Reaction involved can be explained as follows.