Volumetric analysis – Pharmaceutical Analysis 1 B. Pharma 1st semester

Volumetric analysis

Content

•   Types of
volumetric analysis

Learning
Objectives

At the end of this
lecture, the student will be able to

• List the types of volumetric analysis

• Explain the direct method with examples

• Explain the indirect method with examples

• Explain the back titration method with examples

Volumetric Titrations

Method of quantitative chemical analysis in which the amount of a substance is determined by measuring the volume that it occupies

Types of Volumetric Titrations

Type of titration	Substance that can be analysed	Reagent	Indicator	Chemical reactions
Acid Base	Acid or Base	Alkali or acid	pH indicator		OH– + HA → A– + H2O
Precipitation	Ions that form insoluble salt	Silver nitrate, ammonium thio cyanate	conductivity		Ag+ + Cl– →  AgCl (s)
Redox	Oxidizing or reducing agent	Oxidizing or reducing agent	Redox indicator		MnO4– +  5Fe2+ + 8H+ → 5Fe3+ + Mn2++ 4H2O
Complexometric	Metal ions, Cations	Complexing agent like EDTA	Metal indicator		EDTA2- + Ca2+ → EDTA Ca.xH2O

Different
Methods of Volumetric Titration

Methods

1. Direct             

2. Indirect                

3. Back

Direct
titration:

• Substance is directly titrated with suitable titrant by
using suitable indicator

• Direct titration is useful for:-

A- Strong acid

B- Strong base

C- Weak acid or base if K_a and K_b not
less than 10^-7

 

Examples:

•Acid estimated by
base

Potassium hydrogen phthalate by sodium hydroxide

KHC8H4O4    +
NaOH          —–>   KNaC8H4O4       +     
H2O

•Base estimated by
acid

Sodium hydroxide by hydrochloric acid

NaOH + Hcl →   Nacl +
H2O

Indirect
titration

• Substance is not directly titrate but product formed has
to be titrated

• Sometime not feasible due to:

I. Reaction kinetic or the reaction rate is slow

II. No suitable indicator in the direct titration

III. The color change is slow or delay

IV. The end point is far from the equivalent point

Determination of
ammonium salt (Formol titration)

4NH4Cl + 6HCHO  à (CH2)6N4 + 4HCl + 6H2O

Hcl + NaOH à  Nacl + H2O

N.B. Formaldehyde
must be neutralized from any formic acid due to aerial oxidation

Indicator: Phenolphthalein

Colour change: Colorless to pink

Back
titration:

• In this titration a known but excess amount of reagent is
added to the sample solution and the excess/unreacted reagent is back titrated
with a standard solution

When do we use back titration?

1- When sample is volatile .eg. NH3, formic acid

2- When sample is insoluble eg. ZnO, CaO, CaCO3, BaCO3

3- When reaction require heat of standard solution

4- When reaction proceed only in presence of excess reagent

eg. With lactic
acid

Back titration is generally a two-stage analytical
technique:

• Reactant A of unknown concentration is reacted with excess
reactant B of known concentration

A  +  B(excess)  
——>   C + Unreacted B

• Titration is performed to determine the amount of
unreactant B in excess

B(Unreacted) +  D
——->   E

Note: A blank
titration is carried out similarly, without sample to know exactly how much of
B has reacted with A to form C

Example:

Zno+H2SO4 → ZnSO4   + H2O
+ H2SO4 (unreacted)

H2SO4 + 2NaOH → Na2SO4+H2O

Note:  A calculated amount of ammonium chloride is
added in order to prevent interaction between zinc sulphate and sodium
hydroxide, so that a sharp end point is obtained

Indicator: Methyl
orange

Color Change:
Yellow to pinkish red

Summary:

• Methods of volumetric titration: Direct, Indirect, and back
titration

• Direct method:  Substance is directly titrated with suitable
titrant by using suitable indicator

• Indirect titration:
Substance is not directly titrate but product formed has to be titrated

• Back titration:
In this titration a known but excess amount of reagent is added to the sample
solution and the excess/unreacted reagent is back titrated with a standard
solution