Acid Base Titration – Titration Curve,

 Acid Base Titration

Titration is a method of analysis that
will
allow you
to det
ermine the precise endpoint
of a reaction and therefore the precise quantity of
reactant
in
the titration
fl
ask. 

The chemical reaction
invo
lved in acidbase titration
is known
as neutralisation reaction.

Indicator

An indicator is a substance which is used to determine the end point in a titration. 

In acid base
 titrations,  organic
 substances  (weak  acids  or  weak
 bases)  are
 generally  used  as
ind
icators. T

hey change their colour within a certain pH range. 

The colour change and the pH range of some
common indicators
are tabulated
below

Indicator

pH range

Colour change

Methyl orange

3.24.5

Pink to yellow

Methyl red

4.46.5

Red
to yellow

Litmus

5.57.5

Red
to blue

Phenol red

6.88.4

Yellow to red

Phenolphthalein

8.3-10.5

Colourless to pink

 

Theory
of Indicator

 

An acidbase indicator
is
a
weak acid or a weak
b
ase.
E
xamples
of indicto
rs used in acid base
reactions

Litmus

Phenolphthalein

Methyl orange

thymol blue, methyl yellow,
met
hyl orange,
b
romphenol blue,
b
romcresol green,
methyl red, bromthymol blue, phenol red, neutral red, phenolphthalein, thymolphthalein, alizarin yellow, tropeolin O, nitramine,
and trinitrobenzoic acid.

Indicators

pH range

Color for weeak acid

Color for conjugated base

Metyl orange

46

Orange

Yellow

Bromophenol
blue

67

Yellow

Blue

Thymol blue

89

Yellow

Blue

Phenolphthalein

910

Colourless

Pink

Alizarin yellow

1012

Yellow

Red

Theory of acidbase indicators:
Two theories have been proposed to explain the change of colour
of acidbase indicators with change in
pH.

 

Ostwald‘s
theory:


According to this theory, the colour change is due to ionisation of the acidbase indicator


The unionised form has different colour than the ionised form. 


The
ionisation of the indicator is
largely affected in acids and bases as it is either a weak acid or a weak base. 

If the indicator is a weak acid, its ionisation is very much low in acids due to common H+ ions while it is fairly ionised in alkalise. 


if the indicator is a weak base, its ionisation is large in acids and
low in alkalises due to
common OH- ions.

 

Considering two important indicators
phenolphthalein (a weak acid) and methyl orange (a
weak base), Ostwald
theory can be
illustrated as
follows:

 

Phenolphthalein: 

It
can
be
re
presented as HPh. 

It
ionises in solution to
a small extent as:

HPh H+ + Ph

 

Colourless Pink Applying law of mass
action,

K = [H+][Ph ]/[HpH]



The undissociated molecules of phenolphthalein are
colourless while Ph
ions are pink in
colour.  

Let us derive Handerson equation
for an indicator

 

HIn + H2O ↔ H3O+ + In         Acid formBase
formMethyl
orange:

It is a very weak base and can be represented as MeOH. It is ionized in solution to give Me+

 

and OH ions.

 

MeOH ↔ Me+ + OH

 

Yellow
Red Applying law of mass
action,
K = [Me+ ][OH- ]/[MeOH]

 

In presence of an acid, OH ions are removed in the form of water molecules and the above equilibrium
shifts to right hand side. 


Thus, sufficient Me+ ions are produced which impart red colour to the solution. 


On addition of alkali, the concentration of OH
ions increases in
the solution and the equilibrium shifts to left hand side, i.e., the ionisation of MeOH is practically negligible. 


Thus, the solution acquires the colour of unionised methyl orange molecules, i.e., yellow.

 



























Indictors       
 pKind      
 pH



 

Methylorange

3.7

3.14.4

Phenophthaline

9.3

8.310.0

 

Titration curve

 

 

1) Titration of a strong acid with a
strong base

 

In a strong acidstrong base titration, the acid and base will react to form a neutral solution. At the equivalence point of the reaction, hydronium (H+) and hydroxide (OH-) ions will react to form water, leading to a pH of 7

2) Titration of a weak acid with a strong base


In the titration of a weak acid with a strong base, the conjugate base of the weak acid will make the pH at the equivalence point greater than 7. Therefore, you would want an indicator to change in that pH range.

 

3) Titration of a strong acid with a weak base

 

 

In a weak base-strong acid titration, the acid and base will react to form an acidic solution. 

A conjugate acid will be produced during the titration, which then reacts with water to

 form hydronium ions

This results in a solution with a pH lower than 7

4) Titration of a weak base with a weak acid


 

When a weak acid reacts with a weak base, the equivalence point solution will be basic if

 the base is stronger and acidic if the acid is stronger; if both are of equal strength, then the

 equivalence pH will be neutral.

Non aqueous titration

 

Non aqueous titration is the titration of substances dissolved in solvent other than
water. 

It is the most common titrimetric procedure used in pharmacopoeial assays and serves a double
purpose: it is suitable for the titration
of very weak acid and
 very weak base, and it provides

a solvent in which oirganic compound are soluble. 

The  most  commonly
 used
 procedure  is  the  titration
 of organic base with perchloric acid 
in anhydrous acetic acid.

These assays sometimes take some perfecting
in terms of bein
able to judge the endpoint  precisely.

 

The Karl Fischer Titration for water
content is another
nonaqueous titration, usually done in methanol 
or sometimes in ethanol.  

Since water is the analyte in this method, it cannot also
be used as the solvent.

 

Need of Non aqueous
titrations

 

Often
t
imes we need to perform an acidbase titration in
no
n
aqueous solvent due to:

 The
a
nalyte is too weak
acid
or a base to be 
titrated in H
2O

 Reactants
or products are insoluble in H
2O

 Reactants
or products react
with H
2O

 Titration in H2O doesn’t allow a sharp end point
 but
in a
nonaqueous solvent with
a stronger base than OH
it
is possible to get
an sharp end point

 

 

Bronsted Lowry;
a general
definition applicable  to both
aqueous
and
nonaqueousS systems



 

Lewis theory: Acids: electron pair
acceptors

 

Bases: electron pair donors

 

Strong
acids
in water:

 

HCl       +        H2O       →                H3O+                      +         Cl

(Acid)                 (Base)               (Conjugated Acid)      
(Conjugated base)

Weak acids in water:

 

HCOOH         +       
 H
2O    <-> H3O+                   +          HCOO

(Acid)                         (Base)                    (Conjugated Acid)      
 (Conjugated
base)

 

Weak acids in nonaqueous solvents:

HCOOH         +       
 CH
3NH2              <->   CH3NH4+             
+         HCOO (Acid)                (Base)                    (Conjugated Acid) (Conjugated
base)

 

It follows from these
definitions that an acid may be either:

*    an
electrically neutral
molecule, e.g.
HCl, or

 

*    a positively charged cation, e.g. C6H5NH3+,
or

 

*    a negatively charged anion,
e.g. HSO4-.  A base may be either:

 

*    an
electricially neutral molecule, e.g.
C6H5NH2, or an anion,
e.g.  Cl.

 

*    Substances which are potentially acidic can function as acids only in the presence of a
base to which they can donate a proton. Conversely
basic properties do not become apparent  unless an acid also
is present.

*    The apparent strength of an acid or base is determined by  the
extent of its reaction
with a solvent.

*    In aqueous solution all strong acids appear equally strong
 because they react with the solvent to undergo almost   complete conversion
to hydronium
ion (H3O+) and the acid
anion.

*    In a weakly protophilic solvent such as acetic acid, the   extent of formation of the
acetonium ion (CH3COOH2+)  
due to the addition of a proton provides a more sensitive 
 differentiation
of the strength of acids and
shows that the   order
of decreasing
strength for acids is perchloric,  hydrobromic, sulfuric, hydrochloric, and nitric.

*    Acetic acid reacts incompletely
with water to form
 hydronium ion and is, therefore, a
weak acid.

*    In contrast, it dissolves in a base such as ethylenediamine,  and reacts so completely with the solvent
that
it behaves as  a strong acid.This so-called
levelling effect.

 

Levelling effect
or solvent levelling

 

*    Levelling effect or solvent: leveling refers to the effect of solvent on the properties of acids
and bases.

*    The strength of a strong acid is limited (leveled)
by the basicity of the   solvent.

 

Similarly the
strength
of a
strong base is leveled
by
the acidity o
the solvent.

 

*    When a strong acid is dissolved in water, it reacts with it to form hydronium   ion (H3O+).[2] An example of
this
would be the following
reaction, where
 “HA” is the
strong acid:

*    HA + H2O A− + H3O+

 

*    Any acid that is stronger than H3O+ reacts with H2O to form H3O+.
 Therefore, no
acid stronger
than H3O+ exists
in H2O.

*    Similarly, when ammonia is the solvent, the strongest acid is ammonium 
 (NH4+), thus HCl
and a super acid exert the same
ac
idifying effect.

*    The same argument applies to bases. In water, OH is the strongest base.  Thus,
even though sodium amide (NaNH2)
is an exceptional base
(pKa of  NH3 ~ 33), in water it is only as good
as sodium hydroxide.

*    On the other hand, NaNH2 is a
fa
r more basic reagent in
ammonia
than is  NaOH.

Solvents used in non aqueous titration

*    Solvent
which are used in
non aqueous titration  are called
non aqueous solvent.

 

*    They are following types:

1.   Aprotic Solvent

 

2.   Protogenic
Solvent

 

3.   Protophillic Solvent

 

4.   Amphiprotic
Solvent

 

*    Aprotic  solvents
 
are  neutral,
 chemically  inert
 substances  
 such  as
 benzene
 and chloroform. They have a low dielectric  
constant, do not react with either acids or bases and  therefore do not favor
ionization.The fact that picric acid gives a colorless solution in benzene  which becomes yellow on adding aniline shows that picric  acid
is not dissociated in benzene solution and also that in
 the presence of the base aniline it functions as an acid, the
 development of yellow color being due to formation of the picrate ion.Carbon tetrachloride
and toluene come in this group; they   possess low
dielectric constants, do not cause
ionization in  solutes and do not undergo
reactions with acids and bases. Aprotic solvents are frequently used to dilute reaction 
mixture

*    Protogenic solvents
are acidic substances, e.g.
 sulfuric acid. They exert a leveling effect on bases. Anhydrous acids such as hydrogen fluoride and
 sulphuric acid fall in this category, because of their  strength and ability to donate protons, they enhance
the strength of weak bases.Ex: sulphuric acid , formic acid, propanoic acid, acetic anhydride etc.They have high
dielectric constant
and ionised  because of
their
strength and ability to donate protons.

*    Protophilic solvents are the substances that  possess a high affinity for protons. The over
all  reaction can be represented as:

 

HB+S
SH+
+ B

The
e
quilibrium in this reversible reaction will
be  generally influenced by the nature
of the acid and  the solvent.Weak acids
are
normally used in the presence of
 strongly
protophilic solvents
as their acidic strengths
are
then enhanced and
then
become
comparable to these of strong acids;
this is  known
as the levelling effect.

 

*    Amphiprotic solvents have both
protophilic and protogenic properties.
Ex
amples are acetic acid and the alcohols. They are
 dissociated to a slight extent. The dissociation of acetic acid, which is frequently used as a solvent for titration of basic
 substances, is shown
in the equation below:

CH3COOH H+ + CH3COO−

Here the acetic acid
is functioning as
an acid. If a very strong acid such as perchloric acid is
dissolved in acetic acid, the latter
can function as a base and combine with protons  donated
by
the perchloric acid
to form protonated
acetic acid,  an onium
ion:

HClO4 H+
+ ClO4−

CH3COOH + H+ CH3COOH2+ (onium ion)

Since the CH3COOH2+ ion readily donates its
proton to a base,
a solution of perchloric acid

 

in glacial acetic acid 
functions as a strongly acidic solution.

 

 

Titrants
used in non aqueus titration

 

Acidic
titrants:

 

o Perchloric acid

 

o p- Toluenesulfonic
acid,

 

o 2,4Dinitrobenzenesulfonic acid

Basic
titrants

 

o Tetrabutylammonium hydroxide

 

o Sodium acetate

 

o Potassium methoxide

 

o Sodium aminoethoxide

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