Non - Aqueous Acid - Base Titration

August 1, 2017 | Author: Apurba Sarker Apu | Category: Acid, Acid Dissociation Constant, Titration, Dissociation (Chemistry), Molecular Physics
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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

2|Page

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-

3|Page

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

5|Page

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

6|Page

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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