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