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 |
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– → |
|
Redox |
Oxidizing or reducing agent |
Oxidizing or reducing agent |
Redox indicator |
MnO4– + 5Fe2+ |
|
Complexometric |
Metal ions, Cations |
Complexing agent like EDTA |
Metal indicator |
EDTA2- + Ca2+ → EDTA Ca.xH2O |
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 Ka and Kb 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