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CONTENT
Introduction
Types Of Solvents
1. Aprotic solvents
2. Protophilic solvents
3. Protogenic solvents
4. Amphiprotic solvents
Non Aqueous Titration Of Weak Acids
Non Aqueous Titration Of Weak Bases
INTRODUCTION
Definition
Non aqueous titration are those in which titration of weakly acidic or basic substances are carried out using non aqueous solvents so as to get sharp end point.
Non aqueous solvents the disadvantages of poor solubility and weak reactivity.
Moisture and corbondioxide should be avoided using non aqueous procedures.
Moisture should be held to less than 0.05%.
Standardisation &titration should be carried out as far as possible at the same temperature.
In non aqueous solvents the acidity is decreases in the
following order
HCIO4> HBr>H2SO4>HCI>HNO3
o HCl in water…….strongly acidic
o HCl in acetic acid……weakly acidic
o Acetic acid in water……weakly acidic
o Acetic acid in ammonia….strongly acidic
WHY NONAQUEOUS TITRATION??
let’s consider the titration of a 50 mL solution of 10 –4 M
strong acid with equimolar strong base. Before the
equivalence point, the pH is determined by the untitrated
strong acid, whereas after the equivalence point the
concentration of excess strong base determines the pH. In an
aqueous solution the concentration of H3O+ when the
titration is 90% complete is
The pH is 5.3. when the titration is 110% complete, the
concentration of OH- is :
Or a pOH of 5.3. the pH, therefore is
pH = pKW-pOH = 14.0-5.3 = 8.7
The change in pH when the titration passes from 90% to 110%
completion is
If the same titration is carried out in a nonaqueous solvents
with a Ks of 1 x 10 -20, the pH when the titration is 90%
complete is still 5,3. however, when the titration is 110%
complete is now :
Types of solvents
The solvents are divided into 4 groups
1. Aprotic solvents
2. Protophilic solvents
3. Protogenic solvents
4. Amphiprotic solvents
Aprotic solvents(inert solvents)
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 picric acid gives a colorless solution in benzene or
toluene which becomes yellow on adding aniline shows that
picric acid is not dissociated in benzene or toluene solution.
This type of solvents neither accept or donate protons
Ex:-Benzene , dioxan , chlorobenene,chloroform,ethyl acetate,
carbon tetra chloride.
Protophilic solvent
Basic in character and react with acids to form solvated proton
HB + Sol. ⇌ Sol.H+ + B-
Acid + Basic solvent ⇌ Solvated proton + Conjugate base of acid
A weakly basic solvent has less tendency than a strongly basic one
to accept a proton.
Similarly a weak acid has less tendency to donate protons than a
strong acid.
As a result a strong acid such as perchloric acid exhibits more
strongly acidic properties than a weak acid such as acetic acid
when dissolved in a weakly basic solvent.
Ex:-acetone , ether, pyridine ,liquid ammonia
Protogenic solvents
Acidic in nature and donate protons
Ex:- sulphuric acid , formic acid, propionoic acid ,acetic
anhydride etc.
They have high dielectric constant and ionised
Because of their strength and ability to donate protons.
Amphiprotic solvents
Have both protophilic and protogenic properties.
This can accept or donate protons.
Ex:-water , alcohols, acetic acid
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.
When a weak base, such as pyridine is dissolved in acetic acid,
equivalent amount of acetate ions are produced which have more
tendency to accept protons.
Therefore, to titrate a solution of a weak base in acetic acid with
perchloric acid in acetic acid, and obtain a sharp endpoint.
HClO4 + CH3COOH ⇌ CH3COOH2+ + ClO4
-
onium ion
C5H5N + CH3COOH ⇌ C5H5NH+ + CH3COO-
Acetate ion
CH3COOH2+ + CH3COO- ⇌ 2CH3COOH
Burette conical flask
Adding HClO4 + C5H5N ⇌ C5H5NH+ + ClO4-
DETERMINING END POINT
A. Potentiometric titration
B. Indicator method
A. Potentiometric titration :
potentiometric method for the detection of the equivalence point
The end point is determined by using indictor electrode(glass electrode) and
reference electrode(saturated calomel electrode)
Indicator method:
Crystal violet
(0.5 per cent in glacial acetic acid) violet blue-green yellowish-
green
α-Naphtholbenzein
(0.2 per cent in glacial acetic acid)
blue or blue-
greenorange dark-green
Oracet Blue B
(0.5 per cent in glacial acetic acid)
blue purple pink
Quinaldine Red
(0.1 per cent in methanol)magenta
-------
almost
colour less
Indicator Basic Neutral Acidic
Color changes
Thymol Blue yellow blue
Acidimetry in Nonaqueous Solvents
Solvents used in the titration of weak bases;
Neutral solvents :
Ex: - alcohol,chloroform,benzene,chlorobenzene
Acidic solvents:
Ex:-formic acid,glacial acetic acid,propionic acids
Titrant used in the titration of weak basess:
Ex:-Perchloric acid
Indicatores used in the titration of weak bases
Ex:-oracat blue ,crystal violet,1-naphtholbenzein(weak bases)
A solvent such as acetic acid reacts with base, B, to yield the conjugate acid of the
base, BH+, and the conjugate base anion, CH3COO-
B + CH3COOH BH+ + CH3COO-
The conjugate base anion can then react with perchloric acid in the following
manner :
The titrant reacts with the solvent, and the solvent is protonated :
HClO4 + CH3COOH CH3COOH2+ + ClO4
-
The titration reaction, with the conjugate base anion, is as follows :
CH3COO- + CH3COOH2+ 2CH3COOH
So that the overall reaction can be represented as :
B + HClO4 BH+ClO4-
Many weakly acidic substances (alcohol or aprotic solvent)can be titrated in an appropiate non aqueous solvents with a sharp end point .
Ex:- acidic halides, acids,amino acids, enols(barbiturates,xanthines), phenols, pyrrolessulphonamides etc,.
1) Solvents used in the titration of weak acids:
Ex:-Ethylenediamine,n-butylamine,morpholine
2) Titrant used in the titration of weak acids:
Ex:-sodium methoxide, lithium methoxide, potasium methoxide ,tetrabutyl ammonium hydroxide etc.
3) Indicators used in the titration of weak acids
Ex:-azo violet.thymol blue,thymolphthalein.,O-Nitro aniline
Alkalimetry in Nonaqueous Solvents