Non - Aqueous Acid - Base Titration
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Transcript of Non - Aqueous Acid - Base Titration
East West University
Non-aqueous Acid-Base Titration
PHRM-309
Tareq Hasan10/6/2011
Table of Contents
NON – AQUEOUS ACID – BASE TITRATION..............2
INTRODUCTION.........................................................2
Reasons for Performing Non – Aqueous Acid – Base Titration 2
Interaction of H2O with the Titrant......................2
Poor Solubility of Weak Acids (WA) or Weak Bases (WB) in H2O 3
DIFFERENT ACID – BASE THEORY..................................3
Arrhenius’s Acid – Base Theory.........................3
Limitations of Arrhenius’s Acid – Base Theory.....3
Bronsted – Lowry Theory of Acid – Base...........4
Advantages of Bronsted – Lowry Theory of Acid – Base 4
Limitations of Bronsted – Lowry Theory of Acid – Base 5
Lewis’ Theory of Acid – Base.............................5
Advantages of Lewis’ Concept of Acid – Base......6
STRENGTH OF ACID & BASE........................................6
Strength of Acid................................................6
Explanation for a Strong Acid (SA).........................6
Explanation for a Weak Acid (WA)........................7
SOLVENTS USED IN NON – AQUEOUS ACID – BASE TITRATION 7
Protophillic Solvent...........................................7
Protogenic Solvent............................................8
Amphiprotic Solvents........................................9
Aprotic Solvents..............................................10
Reasons for using Aprotic Solvents....................10
THEORY OF NON – AQUEOUS ACID – BASE TITRATION. . .10
Titration of Weak Acid (WA)............................11
Theory................................................................11
Apparatus for Controlled Environment..............12
Burette for protecting the Titrant..................12
Analytical / Titration Vessel for Analysis........12
Practical Example...............................................12
Titration of Weak Base (WB)...........................14
Theory................................................................14
Practical Example – 1.........................................15
Practical Example – 2.........................................16
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2 | P a g e
N o n – A q u e o u s A c i d – B a s e T i t r a t i o n
In t roduct ion
A Non – Aqueous Acid – Base Titration
involves the titration by neutralization of
either acid or base by their opposite entities
in a non – aqueous medium.
Reasons for Performing Non – Aqueous Acid – Base Titration
Non – aqueous Acid – Base Titration is
performed to eliminate 2 problems
encountered during the aqueous titration of
weakly acidic or weakly basic analyte by a
Strong Acid or Strong Base Titrant
respectively.
The 2 problems are –
1. Interaction of the Titrant with H2O
2. Poor Solubility of Weakly Acidic (WA) or
Weakly Basic (WB) Analyte in H2O
Interaction of H2O with the Titrant
In the aqueous titration of a Weakly
Acidic / Weakly Basic Analyte by a
Strongly Basic / Strongly Acidic Titrant
respectively, the solvent also reacts
with the titrant, because of having both
Weakly Acidic and Weakly Basic
properties.
As a result –
More titrant will be required
Detected end – point will give
wrong result or no end – point will
occur
3 | P a g e
Figure 1: Reaction of Water with titrants in Aqueous Titration of Weak Acid or Weak Acid
Poor Solubility of Weak Acids (W A) or Weak Bases (WB) in H2O
Most of the WA or WB Analytes are –
Non – Polar
Organic
Less Soluble / Insoluble in polar H2O
/ Aqueous Solvent
This does not fulfill one of the basic
Criterias of the titration
The analyte must be soluble in a
solvent to form the analytical
solution
The Strong Acidic or Basic titrant
will rapidly react with the
compound / Analyte
Dif ferent Acid – Base Theory
Broadly, acid – base theories are classified
into –
1. Arrhenius’s Acid – Base Theory
2. Bronsted – Lowry Theory of Acid – Base
3. Lewis’s Theory of Acid – Base
Arrhenius’s Acid – Base Theory
According to Arrhenius’s Acid – Base Theory
–
An Acid is a substance that can donate
Protons or Hydrogen ions (H+) in
water.
E.g. – HCl in Water
4 | P a g e
A Base is substance that can donate
Hydroxyl ions (OH–) in water.
E.g. – NaOH in Water
Neutralization is an interaction
between an Acid and a Base to
produce Salt and Water.
E.g. – Hydrochloric Acid (HCl) is
neutralized by Sodium Hydroxide
(NaOH) to produce Sodium
chloride (NaCl) salt and water
(H2O).
Limitations of Arrhenius’s Acid – Base Theory
1. Arrhenius’s Acid – Base Theory does not
explain the acidity or basicity of
compounds in non – aqueous medium.
E.g. Acidity of Acetic Acid is Liquid
Ammonia.
2. This theory cannot explain the acidity or
basicity of ions.
3. Acidity or Basicity shown by compounds
which do not donate H+ or OH-
respectively cannot be explained by this
theory.
4. This theory cannot explain the
neutralization reaction between those
acids and bases where water is not
produced.
E.g. – Neutralization of HCl by
Mercuric Acetate produces
mercuric chloride and Acetic Acid
but not Water.
Bronsted – Lowry Theory of Acid – Base
According to Bronsted – Lowry Theory of
Acid and Base –
An Acid is a substance that dissociates
into a proton (H+) and its conjugate
base.
A Base is a substance that accepts the
proton (H+) and forms its conjugate
acid.
The general reactions can be represented as
–
Advantages of Bronsted – Lowry Theory of Acid – Base
5 | P a g e
1. This theory can explain the theory of
both neutral species and also ions. Here
–
An acid can be –
Neutral Species (E.g. – HCl)
Cationic Species (E.g. – H3O+)
Anionic Species (E.g. – H2PO4-)
Table 1: Different Acids in Bronsted – Lowry Theory
Acid Proton + Conjugate Bases
HCl H+ + Cl-
H3O+ H+ + H2O
H2PO4- H+ + HPO4
2-
A Base can be –
Neutral (E.g. – C5H5N)
Anionic Species (HPO42-)
Table 2: Different Bases in Bronsted – Lowry Theory
Base + Proton Conjugate Acid
C5H5N + H+ C5H5NH+
HPO42- + H+ H2PO4
-
2. This theory can explain the acidity and
basicity shown by same compounds.
E.g. –
Water
Acetic Acid
3. This theory is capable of explaining the
neutralization reaction in non –
aqueous medium. E.g. Neutralization of
HCl by Mercuric acetate in Acetic Acid.
Figure 2: Neutralization of HCl by Mercuric acetate in Acetic Acid
Limitations of Bronsted – Lowry Theory of Acid – Base
1. According to Bronsted – Lowry theory –
An acid shows its acidity in the
presence of a base
A Base shows its basicity in the
presence of an acid
2. It does not explain the acidity shown by
Non – Protic compounds. E.g. – BF3,
BCl3, Ag+ etc.
Lewis’ Theory of Acid – Base
In Lewis’ Theory of Acid – Base –
An Acid is a Compound / Atom / Ion
capable of accepting one / one pair /
pairs of electron from a base; Thus
referring as Lewis Acid.
E.g. – H+ is a Lewis acid, since it can
accept an electron to fulfill its outer
shell.
6 | P a g e
H+ + e- H
A Base is a Compound / Atom / Ion
capable of donating one / one pair /
pairs of electron to an acid; Thus
referring as Lewis Base.
E.g. – AlCl4- is a Lewis Base, since it
can donate a pair of electrons
AlCl4- AlCl3 + 2e-
So, according to this theory, a neutralization
reaction is a formation of Coordinate
Covalent Bond between the donors and
acceptors of electron pair atoms. E.g. –
Reaction between Boron trichloride and
Triethylamine.
Figure 3: Reaction between Boron trichloride and Triethylamine
Advantages of Lewis’ Concept of Acid – Base
Lewis’ Concept of Acid and Base can
explain the acidity / basicity of any
molecule / atom / ion in any type of
solvent.
Strength of Acid & Base
Strength of Acid
Strength of Acid depends on –
1. Dissociation of an Acid into Proton (H+)
2. Environment / Solvent in which the acid
dissociates
This can be explained for both Strong and
Weak Acid.
Explanation for a Strong Acid (S A)
A Strong Acid (SA) such as HCl will
slowly dissociate into proton (H+) and
Chloride ion (Cl-) in Acetic Acid
(CH3COOH) than in H2O; and thus acts
as a Weak Acid in Acetic Acid.
It is because, H2O is more basic than
that of Acetic Acid (CH3COOH) and thus
can rapidly accept H+ than that of Acetic
Acid (CH3COOH).
This is because, the higher the pkb value
of a compound the lower is the basicity
–
pkb of H2O = 7
pkb of CH3COOH = 9.25
7 | P a g e
Table 3: Strength of Hydrochloric Acid in different Solvents
HCl in H2O Acts as a Strong Acid
HCl H+ + Cl- [Rapid Dissociation]
Hydrochloric Acid Proton Chloride ion
H2O + H+ H3O+ [Rapid Acceptance]
Water Proton Hydronium Ion
HCl in CH3COOH acts a Weak Acid
HCl H+ + Cl- [Slow Dissociation]
Hydrochloric Acid Proton Chloride ion
CH3COOH + H+ CH3COOH2+ [Slow Acceptance]
Acetic Acid Proton Onium ion
Explanation for a Weak Acid (W A)
A Weak Acid (WA) such as Acetic Acid
(CH3COOH) will rapidly dissociate into
Proton (H+) and Acetate ion (CH3COO-)
in Liquid Ammonia than in H2O; thus
act as a Strong Acid in NH3.
It is because, Ammonia (NH3) is a
stronger base (pkb = 4.75) than that of
H2O and thus can rapidly accept H+
released by Acetic Acid.
This is because; the higher the pkb value
of a compound the lower is the basicity
–
pkb of Ammonia = 4.75
pkb of H2O = 7
Table 4: Strength of Acetic Acid in different Solvents
Acetic Acid in H2O acts as Weak Acid
CH3COOH H+ + CH3COO- [Slow Dissociation]
Acetic Acid Proton Acetate Ion
H2O + H+ H3O+ [Slow Acceptance]
8 | P a g e
Water Proton Hydronium Ion
Acetic Acid in Liquid Ammonia acts as Strong Acid
CH3COOH H+ + CH3COO- [Rapid Dissociation]
Acetic Acid Proton Acetate ion
NH3 + H+ NH4+ [Rapid Acceptance]
Liquid Ammonia Proton Ammonium ion
Solvents used in Non – Aqueous Acid – Base T i t ra t ion
Solvents used in Non – Aqueous Acid – Base
Titration are –
1. Protophillic Solvent
2. Protogenic Solvent
3. Amphiprotic Solvent
4. Aprotic Solvents
Protophillic Solvent
Protophillic Solvents are –
Proton – loving Compounds for having
high affinity to accept proton (H+)
Basic in Nature; Also called Basic
Solvents
Used to provide rapid dissolution of
weak acids analytes where it can rapidly
donate proton (H+) and behave like a
strong acid
Protophillic Solvents accept H+ released by
the weak acid and form –
Solvated Proton
Conjugate Acid of the Basic Solvent
React with the employed strong
base titrant
Conjugate Base of Weak Acid by
reacting with Weak Acid Analyte
Effect produced by these solvents is called
Leveling Effect.
E.g. – Acetone, Ether such Dioxane, Liquid
Ammonia
Table 5: Leveling Effect of Protophillic Solvent on Weak Acid Analyte
HWA H+ + WA-
Weak Acid Analyte Proton Conjugate Base of WA
S + H+ SH+
9 | P a g e
Protophillic Solvent
Proton Solvated Proton
SB + SH+ SBH+ + S
Strong Base Titrant
Solvated Proton
Conjugated Acid of Strong Base
Protophillic Solvent
HWA + SB WA- + SBH+
Weak Acid Analyte
Strong Base Titrant
Conjugate Base of WAConjugated Acid of
Strong Base
Protogenic Solvent
Protogenic Solvents are –
Proton – generating Solvents for rapid
release of proton (H+)
Acidic in Nature; also called Acidic
Solvents
Used to provide rapid dissolution of
weak acid analyte where it can rapidly
accept H+ and behave like a strong base.
Protogenic Solvents generate / donate H+
and forms Conjugate Base of Acidic Solvent
that rapidly accepts the H+ released by the
Strong Acid titrant
Effect produced by these solvents is called
Leveling Effect.
E.g. –
Formic Acid
Glacial Acetic Acid
Sulfuric Acid
Liquid HCl
Liquid HF
Table 6: Leveling Effect of Protophillic Solvent on Weak Base Analyte
HS H+ + S-
Acidic Solvent Proton Conjugate Base of Acidic Solvent
B + H+ BH+
Weak Base Analyte Proton Conjugate Acid of Weak Base Analyte
HSA H+ + SA-
Strong Acid Titrant Proton Conjugate Base Strong
10 | P a g e
Acid Titrant
S- + H+ HS
Conjugate Base of Acidic Solvent
Proton Acidic Solvent
B + HSA BH+ + SA-
Weak Base Analyte Strong Acid Titrant
Conjugate Acid of Weak Base Analyte
Conjugate Base Strong Acid Titrant
Amphiprotic Solvents
Amphiprotic Solvents are capable of acting
as both H+ acceptor and donor.
Solvents of this category produce Leveling
Effect on both Weak Acid and Base Analyte
E.g. – Glacial Acetic Acid, Alcohols
Table 7: Acetic Acid Acting as a Protogenic Solvent
Acetic Acid Acting as a Protogenic Solvent
CH3COOH H+ + CH3COO-
Acetic Acid Proton Acetate Ion (Conj. Base)
B + H+ BH+
Weak Base Analyte Conjugate Acid of Weak Base Analyte
Table 8: Acetic Acid Acting as a Protophillic Solvent
Acetic Acid Acting as a Protophillic Solvent
HA H+ + A-
Weak Acid Analyte Proton Conjugate Base of Weak Acid
CH3COOH + H+ CH3COOH2+
Acetic Acid Proton Onium ion (Conj. Acid)
Aprotic Solvents
Aprotic Solvents are chemically inert for
neither accepting nor donating protons
They are also called Neutral Solvents
E.g. –
Hydrocarbons
11 | P a g e
Carbon Tetrachloride (CCl4)
Chloroform
Benzene
Reasons for using Aprotic Solvents
1. Aprotic Solvents are used to increase
the volume of the analytical solution for
easy and accurate detection of End –
point
2. They are used as additives in various
titration methods
3. They are useful to study the reaction
free of solvents effects.
Theory of Non – aqueous Acid – Base T i t ra t ion
Acidity of Compounds falls from strong to
weak with increasing value from 1 to 14 on
the pka scale.
And, Basicity of compounds falls from
strong to weak with decreasing value from
14 to 1 on the pka scale.
So, compounds which have pka value close
to –
1 (or 1 – 3) is a Strong Acid
1 (or 1 – 9) is Weak Base
Table 9: pka values of some Strong Acid and Weak Bases
Strong Acid pka Weak Base pka
Benzylpenicillin 2.76 Benzocaine 2.78
Aspirin 3.49 Aniline 4.58
Picric Acid 0.38 Sulfadiazine 2.00
Saccharine 1.6 Apomorphine 7.00
And, compounds which have pka values
close to –
14 (or 4 – 14) are Weak Acids
14 (or 9 – 14) are Strong Base
Table 10: pka values of some Weak Acid and Strong Bases
Weak Acid pka Strong Base pka
Caffeine 14.00 Ammonia 9.25
Phenol 9.99 Amphetamine 9.37
Sulfanilamide
10.43 Ephedrine 9.6
Acetic Acid 4.75 Trimethylamine 9.74
So, Non – aqueous acid – base titration is
performed for those compounds which are
–
Partially soluble or insoluble in H2O
Acidic compounds with pka 4 – 14
Basic Compounds with pka 1 – 4
Titration of Weak Acid (W A)
Theory
Here, a WA analyte is titrated by a
Strong Base (SB) titrant in a Non –
Aqueous Protophillic / Basic Solvent.
In this case, the WA analyte reacts with
the Protophillic / Basic solvent and
forms the Solvated Proton Species (A
Conjugated Acid of the Protophillic
12 | P a g e
Solvent) which ultimately reacts with
the SB Titrant.
So, the general reaction can be written
as –
Table 11: Titration of Weak Acid by Strong Base Titrant in a Basic Solvent
HWA H+ + WA-
Weak Acid Analyte Proton Conjugate Base of WA
S + H+ SH+
Protophillic Solvent
Proton Solvated Proton
SB + SH+ SBH+ + S
Strong Base Titrant
Solvated Proton
Conjugated Acid of Strong Base
Protophillic Solvent
HWA + SB WA- + SBH+
Weak Acid Analyte
Strong Base Titrant
Conjugate Base of WAConjugated Acid of
Strong Base
13 | P a g e
Figure 4: Titration Vessel and Burette for Non – aqueous Titration of Weak Acid
Apparatus for Controlled Environment
Burette for protecting the Titrant
Titrants used in Non – Aqueous
Titration of Weak Acid are very
reactive and can react with
atmospheric compounds like
Oxygen.
So for obtaining accurate result, the
titrant is protected from the
atmosphere by a special burette
with reservoir which is flushed out
with N2 (or other inert gases) and a
layer of N2 (or other inert gases) is
laid over the titrant.
The entire device is sealed with
Teflon stopcocks.
Analytical / Titration Vessel for Analysis
Figure 5: Titration Vessel
The analyte reacts with the basic
solvent to form the solvated
proton, a very reactive species and
can also react with the atmospheric
compounds
So, A Three – necked Flask is used
to –
Protect the Analytical Solution
from the atmosphere
Obtain Accurate Result
Perform the titration in
Controlled Environment
The Middle neck of the flask
provides the entry of burette tip.
The Left and Right necks provide
the entry and exit of the N2 gas (or
other inert gas) respectively.
Practical Example
14 | P a g e
A Practical Example of Non – aqueous
Titration of Weak Acid is the titration of
Benzoic Acid in n – butylamine by
Sodium methoxide
Here,
Benzoic Acid (Weak Acid Analyte,
pka = 4.2)
Sodium methoxide (CH3ONa, Strong
Base Titrant)
n – butylamine (Basic / Protophillic
Solvent)
So, the reaction can be –
Figure 6: Titration of Weak Acid by Strong Base
15 | P a g e
Titration of Weak Base (W B)
Theory
In the titration of Weak Base (WB) or a
H2O – insoluble / Poorly H2O – soluble
Strong Base (SB), the analyte is titrated
by a Strong Acid (SA) titrant in
Protogenic / Acidic Solvent.
SA Titrant is dissolved in the Protogenic
Solvent to make the Solvated Proton
Species and Conj. Base of SA Titrant.
Here, the Protogenic Solvent acts as a
base in the stronger acidic SA Titrant.
When, the Analyte is dissolved in the
Protogenic Solvent, they will react with
each other to form –
1. Conjugate (Conj.) Acid of WB
2. Conj. Base of SA
Then, the Solvated Proton Species and
Conjugate (Conj.) Base of Acidic
Solvent to form the Acidic Solvent
HSA + HA H2A+ + SA-
SA Titrant Acidic Solvent Solvated Proton Species
Conj. Base of SA
WB + HA HWB+ + A-
WB Analyte Acidic Solvent Conj. Acid of WBConj. Base of Acidic
SolventH2A+ + A- 2HA
Solvated Proton Species
Conj. Base of Acidic Solvent
Acidic Solvent
WB + HSA HWB+ + SA
-
WB Analyte SA Titrant Conj. Acid of WB Conj. Base of SA
When, the analyte is dissolved in the
Aprotic Solvent, Solvated Proton
Species and WB Analyte will react to
form –
Conj. Acid of WB
Acidic Solvent
HSA + HA H2A+ + SA-
SA Titrant Acidic Solvent Solvated Proton Species Conj. Base of SA
WB + H2A+ HWB+ + HA
WB Analyte Solvated Proton Species Conj. Acid of WB Acidic Solvent
16 | P a g e
WB + HSA HWB+ + SA
-
WB Analyte SA Titrant Conj. Acid of WB Conj. Base of SA
Practical Example – 1
Although Ephedrine is a Strong Base
(pka = 9.6), it is titrated in non –
aqueous solvent for being poorly
soluble in H2O.
Ephedrine in Glacial Acetic Acid is
titrated by Perchloric Acid in Acetic
Acid.
The Reactions are illustrated as –
17 | P a g e
Figure 7: Titration of Ephedrine in Acetic Acid by Acetous Perchloric Acid
Practical Example – 2 Ephedrine in Aprotic Solvents such as
CCl4, Benzene, and CHCl3 etc. is titrated
by Perchloric Acid in Dioxane.
The Reactions are illustrated as –
18 | P a g e
Figure 8: Titration of Ephedrine in Aprotic Acid by Perchloric Acid in Dioxane
19 | P a g e