Non - Aqueous Acid - Base Titration
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East West University
Non-aqueous Acid-Base Titration PHRM-309
Tareq Hasan 10/6/2011
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
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
(WB) in H2O ...........................................................3
Aprotic Solvents ..............................................10
DIFFERENT ACID – BASE THEORY...................................3
Reasons for using Aprotic Solvents .................... 10
Arrhenius’s Acid – Base Theory .........................3
THEORY OF NON – AQUEOUS ACID – BASE TITRATION.....10
Limitations of Arrhenius’s Acid – Base Theory .....3
Titration of Weak Acid (WA) ............................11
Bronsted – Lowry Theory of Acid – Base ...........4
Theory ................................................................ 11
Advantages of Bronsted – Lowry Theory of Acid –
Apparatus for Controlled Environment ............. 12
Base ......................................................................4
Burette for protecting the Titrant ................. 12
Limitations of Bronsted – Lowry Theory of Acid –
Analytical / Titration Vessel for Analysis ....... 12
Base ......................................................................5
Practical Example ............................................... 12
Lewis’ Theory of Acid – Base .............................5
Titration of Weak Base (WB) ...........................14
Advantages of Lewis’ Concept of Acid – Base ......6
Theory ................................................................ 14
STRENGTH OF ACID & BASE .........................................6
Practical Example – 1 ......................................... 15
Strength of Acid ................................................6
Practical Example – 2 ......................................... 16
Explanation for a Strong Acid (SA) ........................6
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Non – Aqueous Acid – Base Titration
1. Interaction of the Titrant with H2O 2. Poor Solubility of Weakly Acidic (WA) or
In tro d u cti o n
Weakly Basic (WB) Analyte in H2O
A Non – Aqueous Acid – Base Titration
Interaction of H 2 O with the Titrant
involves the titration by neutralization of either acid or base by their opposite entities in a non – aqueous medium.
Non – aqueous Acid – Base Titration is to
eliminate
2
problems
encountered during the aqueous titration of weakly acidic or weakly basic analyte by a Strong
Acid
or
Strong
respectively. The 2 problems are –
Acidic / Weakly Basic Analyte by a Strongly Basic / Strongly Acidic Titrant
Reasons for Performing Non – Aqueous Acid – Base Titration
performed
In the aqueous titration of a Weakly
Base
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
Figure 1: Reaction of Water with titrants in Aqueous Titration of Weak Acid or Weak Acid
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Poor Solubility of Weak Acids (W A ) or Weak Bases (W B ) in H 2 O Most of the WA or WB Analytes are –
A Base is substance that can donate
Non – Polar
Hydroxyl ions (OH–) in water.
Organic
E.g. – NaOH in Water
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
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).
Diffe r en t Ac id – B ase T h eo r y
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
Limitations of Arrhenius’s Acid – Base Theory 1. Arrhenius’s Acid – Base Theory does not
Arrhenius’s Acid – Base Theory
explain the acidity or basicity of compounds in non – aqueous medium.
According to Arrhenius’s Acid – Base Theory –
E.g. Acidity of Acetic Acid is Liquid Ammonia.
An Acid is a substance that can donate
2. This theory cannot explain the acidity or
+
Protons or Hydrogen ions (H ) in water. E.g. – HCl in Water
basicity of ions. 3. Acidity or Basicity shown by compounds which do not donate H+ or OH-
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respectively cannot be explained by this theory. 4. This
theory
cannot
explain
Advantages of Bronsted – Lowry Theory of Acid – Base
the
1. This theory can explain the theory of
neutralization reaction between those
both neutral species and also ions. Here
acids and bases where water is not
–
produced.
An acid can be –
E.g. – Neutralization of HCl by Mercuric
Acetate
Neutral Species (E.g. – HCl)
produces
Cationic Species (E.g. – H3O+)
mercuric chloride and Acetic Acid
Anionic Species (E.g. – H2PO4-)
but not Water. Table 1: Different Acids in Bronsted – Lowry Theory
Acid
HCl
H+
+
Cl-
Bronsted – Lowry Theory of Acid – Base
H3O+
H+
+
H2O
According to Bronsted – Lowry Theory of
H2PO4-
H+
+
HPO42-
Proton +
Conjugate Bases
Acid and Base – A Base can be –
An Acid is a substance that dissociates into a proton (H+) and its conjugate
Neutral (E.g. – C5H5N)
base.
Anionic Species (HPO42-)
A Base is a substance that accepts the +
proton (H ) and forms its conjugate acid.
Table 2: Different Bases in Bronsted – Lowry Theory
Base
+
Proton
Conjugate Acid
C5H5N
+
H+
C5H5NH+
HPO42- +
H+
H2PO4-
The general reactions can be represented as –
2. This theory can explain the acidity and basicity shown by same compounds. E.g. – Water 4|Page
E.g. – H+ is a Lewis acid, since it can
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.
accept an electron to fulfill its outer shell.
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 + 2eFigure 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
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.
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 Ac id – 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.
Figure 3: Reaction between Boron trichloride and Triethylamine
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Advantages of Lewis’ Concept of Acid – Base
Explanation for a Strong Acid (S A ) A Strong Acid (SA) such as HCl will
Lewis’ Concept of Acid and Base can
slowly dissociate into proton (H+) and
explain the acidity / basicity of any
Chloride ion (Cl-) in Acetic
molecule / atom / ion in any type of
Acid
(CH3COOH) than in H2O; and thus acts
solvent.
as a Weak Acid in Acetic Acid. It is because, H2O is more basic than
St ren g t h o f Ac id & Ba se
that of Acetic Acid (CH3COOH) and thus
Strength of Acid
can rapidly accept H+ than that of Acetic Acid (CH3COOH).
Strength of Acid depends on –
This is because, the higher the pkb value
1. Dissociation of an Acid into Proton (H+)
of a compound the lower is the basicity
2. Environment / Solvent in which the acid
–
dissociates
pkb of H2O = 7
This can be explained for both Strong and
pkb of CH3COOH = 9.25
Weak Acid. Table 3: Strength of Hydrochloric Acid in different Solvents
HCl in H2O Acts as a Strong Acid
HCl Hydrochloric Acid
H2O
+
Water
H+
H+
+
Proton
ClChloride ion
H3O+
Proton
[Rapid Dissociation]
[Rapid Acceptance]
Hydronium Ion HCl in CH3COOH acts a Weak Acid
HCl Hydrochloric Acid
CH3COOH Acetic Acid
+
H+ Proton
H+ Proton
+
Cl-
[Slow Dissociation]
Chloride ion
CH3COOH2+
[Slow Acceptance]
Onium ion
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Explanation for a Weak Acid (W A )
H2O and thus can rapidly accept H+ released by Acetic Acid.
A Weak Acid (WA) such as Acetic Acid
This is because; the higher the pkb value
(CH3COOH) will rapidly dissociate into
of a compound the lower is the basicity
Proton (H+) and Acetate ion (CH3COO-)
–
in Liquid Ammonia than in H2O; thus
pkb of Ammonia = 4.75
act as a Strong Acid in NH3.
pkb of H2O = 7
It is because, Ammonia (NH3) is a stronger base (pkb = 4.75) than that of Table 4: Strength of Acetic Acid in different Solvents
Acetic Acid in H2O acts as Weak Acid
CH3COOH Acetic Acid H2O
+
Water
H+
+
Proton H+
CH3COOAcetate Ion
H3O+
Proton
[Slow Dissociation]
[Slow Acceptance]
Hydronium Ion
Acetic Acid in Liquid Ammonia acts as Strong Acid
CH3COOH Acetic Acid NH3
+
Liquid Ammonia
H+ Proton
H+
Proton
Sol vent s u s ed in N o n – A q u eo u s Ac id – Bas e T i tr atio n
Solvents used in Non – Aqueous Acid – Base Titration are – 1. Protophillic Solvent 2. Protogenic Solvent 3. Amphiprotic Solvent
+
CH3COO-
[Rapid Dissociation]
Acetate ion NH4+
[Rapid Acceptance]
Ammonium ion
Protophillic Solvent Protophillic Solvents are – Proton – loving Compounds for having high affinity to accept proton (H+) Basic in Nature; Also called Basic Solvents
4. Aprotic Solvents 7|Page
Used to provide rapid dissolution of
React with the employed strong
weak acids analytes where it can rapidly donate proton (H+) and behave like a
base titrant Conjugate Base of Weak Acid by
strong acid
reacting with Weak Acid Analyte
Protophillic Solvents accept H+ released by the weak acid and form –
Effect produced by these solvents is called Leveling Effect.
Solvated Proton
E.g. – Acetone, Ether such Dioxane, Liquid
Conjugate Acid of the Basic Solvent
Ammonia
Table 5: Leveling Effect of Protophillic Solvent on Weak Acid Analyte
HWA Weak Acid Analyte
S
+
Protophillic Solvent
SB Strong Base Titrant
Weak Acid Analyte
H+
SH+
SB
WAConjugate Base of WA
SH+ Solvated Proton
Solvated Proton
+
+
Proton
Proton
+
HWA
H+
SBH+
+
Conjugated Acid of Strong Base
Strong Base Titrant
Protogenic Solvent
WA-
S Protophillic Solvent
+
Conjugate Base of WA
SBH+ Conjugated Acid of Strong Base
Used to provide rapid dissolution of weak acid analyte where it can rapidly
Protogenic Solvents are – Proton – generating Solvents for rapid release of proton (H+) Acidic in Nature; also called Acidic Solvents
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. 8|Page
E.g. –
Liquid HCl
Formic Acid
Liquid HF
Glacial Acetic Acid Sulfuric Acid Table 6: Leveling Effect of Protophillic Solvent on Weak Base Analyte
HS
H+
Acidic Solvent
B
H+
Weak Base Analyte
Proton
BH+ Conjugate Acid of Weak Base Analyte
HSA
H+
Strong Acid Titrant
+
Conjugate Base of Acidic Solvent
B Weak Base Analyte
+
H+
SAConjugate Base Strong Acid Titrant
Proton
HS
Proton
+
SConjugate Base of Acidic Solvent
Proton
+
S-
+
Acidic Solvent
HSA Strong Acid Titrant
BH+ Conjugate Acid of Weak Base Analyte
Amphiprotic Solvents
+
SAConjugate Base Strong Acid Titrant
Solvents of this category produce Leveling Effect on both Weak Acid and Base Analyte
Amphiprotic Solvents are capable of acting as both H+ acceptor and donor.
E.g. – Glacial Acetic Acid, Alcohols
Table 7: Acetic Acid Acting as a Protogenic Solvent
Acetic Acid Acting as a Protogenic Solvent
CH3COOH
Acetic Acid
B Weak Base Analyte
H+ Proton
+ H+
+
CH3COOAcetate Ion (Conj. Base)
BH+ Conjugate Acid of Weak Base Analyte
9|Page
Table 8: Acetic Acid Acting as a Protophillic Solvent
Acetic Acid Acting as a Protophillic Solvent
HA Weak Acid Analyte
CH3COOH + Acetic Acid
H+
H+
+
Proton
Conjugate Base of Weak Acid
Proton
ACH3COOH2+
Onium ion (Conj. Acid)
Aprotic Solvents
Acidity of Compounds falls from strong to weak with increasing value from 1 to 14 on
Aprotic Solvents are chemically inert for neither accepting nor donating protons They are also called Neutral Solvents E.g. – Hydrocarbons Carbon Tetrachloride (CCl4)
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
Chloroform
to –
Benzene
1 (or 1 – 3) is a Strong Acid 1 (or 1 – 9) is Weak Base
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
Table 9: pk a values of some Strong Acid and Weak Bases
Strong Acid
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
pka
free of solvents effects. And, compounds which have pka values Theo ry o f No n – aq u eo u s Ac id – B ase T itr atio n
close to – 14 (or 4 – 14) are Weak Acids 14 (or 9 – 14) are Strong Base
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Table 10: pk a values of some Weak Acid and Strong Bases
Titration of Weak Acid (W A ) Theory
Weak Acid
pka
Strong Base
pka
Caffeine
14.00
Ammonia
9.25
Here, a WA analyte is titrated by a
Phenol
9.99
Amphetamine
9.37
Strong Base (SB) titrant in a Non –
Sulfanilamide 10.43
Ephedrine
9.6
Acetic Acid
Trimethylamine 9.74
4.75
Aqueous Protophillic / Basic Solvent. In this case, the WA analyte reacts with the Protophillic / Basic solvent and
So, Non – aqueous acid – base titration is
forms the Solvated Proton Species (A
performed for those compounds which are
Conjugated Acid of the Protophillic
–
Solvent) which ultimately reacts with
Partially soluble or insoluble in H2O
the SB Titrant. So, the general reaction can be written
Acidic compounds with pka 4 – 14
as –
Basic Compounds with pka 1 – 4
Table 11: Titration of Weak Acid by Strong Base Titrant in a Basic Solvent
HWA Weak Acid Analyte
S
+
Protophillic Solvent
SB
+
Weak Acid Analyte
H+
SH+
SB Strong Base Titrant
WAConjugate Base of WA
SH+ Solvated Proton
Solvated Proton
+
+
Proton
Proton
Strong Base Titrant
HWA
H+
SBH+
+
Conjugated Acid of Strong Base
WAConjugate Base of WA
S Protophillic Solvent
+
SBH+ Conjugated Acid of Strong Base
11 | P a g e
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 Figure 4: Titration Vessel and Burette for Non – aqueous Titration of Weak Acid
Apparatus for Controlled Environment
Protect the Analytical Solution from the atmosphere
Titrants used in Non – Aqueous Titration of Weak Acid are very and
atmospheric
can
react
with
compounds
like
Obtain Accurate Result Perform
the
titration
in
Controlled Environment The Middle neck of the flask provides the entry of burette tip.
Oxygen. So for obtaining accurate result, the titrant
So, A Three – necked Flask is used to –
Burette for protecting the Titrant
reactive
compounds
is
protected
from
the
atmosphere by a special burette with reservoir which is flushed out
The Left and Right necks provide the entry and exit of the N2 gas (or other inert gas) respectively.
Practical Example
with N2 (or other inert gases) and a 12 | 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,
Sodium methoxide (CH3ONa, Strong Base Titrant) n – butylamine (Basic / Protophillic Solvent) So, the reaction can be –
Benzoic Acid (Weak Acid Analyte, pka = 4.2)
Figure 6: Titration of Weak Acid by Strong Base
13 | P a g e
Titration of Weak Base (W B )
Here, the Protogenic Solvent acts as a base in the stronger acidic SA Titrant.
Theory
When, the Analyte is dissolved in the
In the titration of Weak Base (WB) or a
Protogenic Solvent, they will react with
H2O – insoluble / Poorly H2O – soluble
each other to form –
Strong Base (SB), the analyte is titrated
1. Conjugate (Conj.) Acid of WB
by a Strong Acid (SA) titrant in
2. Conj. Base of SA Then, the Solvated Proton Species and
Protogenic / Acidic Solvent. SA Titrant is dissolved in the Protogenic
Conjugate
(Conj.)
Base
of
Acidic
Solvent to form the Acidic Solvent
Solvent to make the Solvated Proton Species and Conj. Base of SA Titrant.
HSA
+
SA Titrant
WB
Acidic Solvent
+
Conj. Base of Acidic Solvent
+
SA Titrant
Solvent,
Solvated
Conj. Base of SA
HWB+
+
AConj. Base of Acidic Solvent
2HA HWB+
+
SA-
Conj. Acid of WB
When, the analyte is dissolved in the Aprotic
SA-
Acidic Solvent
HSA
WB Analyte
+
Conj. Acid of WB
A-
Solvated Proton Species
WB
HA
WB Analyte
H2A+ Solvated Proton Species
Acidic Solvent
+
H2A+
HA
Conj. Base of SA
Acidic Solvent
Proton
Species and WB Analyte will react to form – Conj. Acid of WB
HSA
+
SA Titrant
WB
WB Analyte
Acidic Solvent
+
WB Analyte
WB
HA H2A+ HSA SA Titrant
+
Solvated Proton Species
Solvated Proton Species
+
H2A+ HWB+
Conj. Base of SA
+
Conj. Acid of WB
HWB+ Conj. Acid of WB
SAHA Acidic Solvent
+
SAConj. Base of SA
14 | P a g e
Practical Example – 1 Although Ephedrine is a Strong Base (pka = 9.6), it is titrated in non – aqueous solvent for being poorly
Ephedrine in Glacial Acetic Acid is titrated by Perchloric Acid in Acetic Acid. The Reactions are illustrated as –
soluble in H2O.
Figure 7: Titration of Ephedrine in Acetic Acid by Acetous Perchloric Acid
15 | P a g e
Practical Example – 2
The Reactions are illustrated as –
Ephedrine in Aprotic Solvents such as CCl4, Benzene, and CHCl3 etc. is titrated by Perchloric Acid in Dioxane.
Figure 8: Titration of Ephedrine in Aprotic Acid by Perchloric Acid in Dioxane
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