Non - Aqueous Acid - Base Titration

East West University

Non-aqueous Acid-Base Titration

PHRM-309

Tareq Hasan 10/6/2011

1 | P a g e 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

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 Introduction

 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 H

O 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

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Poor Solubility of Weak Acids (W

)

or Weak Bases (W

) in H

O

 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

Different 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

 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

-4 | P a g e 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

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. –

5 | P a g e  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.

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

AlCl



AlCl

+ 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

6 | P a g e

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 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

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

H

O

+

H



H

O

[Rapid Acceptance]

Water Proton Hydronium Ion

HCl in CH3COOH acts a Weak Acid

HCl



H

₊

_Cl

_{[Slow Dissociation]}

Hydrochloric Acid Proton Chloride ion

CH

COOH +

H



CH

COOH

[Slow Acceptance]

7 | P a g e

Explanation for a Weak Acid (W

)

 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]

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 Titration

 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

8 | P a g e  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

HW



H

+

W

A-Weak Acid Analyte Proton Conjugate Base of WA

S

+

H

_

_SH

Protophillic

Solvent Proton Solvated Proton

S

+

SH



S

H

+

S

Strong Base Protophillic Solvent

HW

+

S



W

+

S

H

Weak Acid Analyte

Strong Base

Titrant Conjugate Base of WA

Conjugated 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.

9 | P a g e  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

HS



H

+

S

A-Strong Acid Titrant Proton Conjugate Base Strong

Acid Titrant

S

₊

_H



_HS

Conjugate Base of

Acidic Solvent Proton Acidic Solvent

B

+

HS



BH

+

S

A-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

CH

COOH



H

+

CH

COO

-Acetic Acid Proton Acetate Ion (Conj. Base)

B

+ H



_BH

10 | P a g e

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

CH

COOH + H



CH

COOH

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

 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 Titration

 Acidity of Compounds falls from strong to weak with increasing value from 1 to 14 on the pk_ascale.

 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

11 | P a g e

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

)

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 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

HW



H

+

W

A-Weak Acid Analyte Proton Conjugate Base of WA

S

+

H



_SH

Protophillic

Solvent Proton Solvated Proton

S

+

SH



S

H

+

S

Strong Base Protophillic Solvent

HW

+

S



W

+

S

H

Weak Acid Analyte

Strong Base

Titrant Conjugate Base of WA

Conjugated Acid of Strong Base

12 | 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.

13 | 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 –

14 | P a g e

Titration of Weak Base (W

)

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

HS

+

HA



H

A

+

S

A-SA Titrant Acidic Solvent Solvated Proton

Species Conj. Base of SA

W

+

HA



HW

+

A

-WB Analyte Acidic Solvent Conj. Acid of WB Conj. Base of Acidic

Solvent

H

A

+

A



2HA

Solvated Proton Species

Conj. Base of Acidic

Solvent Acidic Solvent

W

+

HS



HW

+

S

A-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

HS

+

HA



H

A

+

S

A-SA Titrant Acidic Solvent Solvated Proton Species Conj. Base of SA

W

+

H

A



HW

+

HA

WB Analyte Solvated Proton Species Conj. Acid of WB Acidic Solvent

W

+

HS



HW

+

S

15 | 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 soluble in H2O.

 Ephedrine in Glacial Acetic Acid is titrated by Perchloric Acid in Acetic Acid.

 The Reactions are illustrated as –

16 | P a g e

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 –