Molarity:

“Molarity” is defined as the number of moles of solute per litre of solution.

Moles of solute

Molarity (M) =

Volume of solution in litre

Standard solution:

A standard solution is one, which contains a known weight of the reagent in a definite volume of solution, and for many years concentrations were expressed in terms of molarity (i.e., number of moles per litre) and normality (i.e., number of equivalents per litre).

So, the standard solutions are now commonly expressed in terms of molar concentration or molarity (M).

Classification of the reactions in titrimetric analysis:

The reactions employed in titrimetric analysis fall into two main classes…

a. Those in which no change in oxidation state occurs; these are dependent upon the combination of ions.

b. Oxidation-reduction reaction; these involve a change of oxidation state or a transfer of electrons.

For purpose of convenience, these two types of reaction are divided to four main classes. These are ..

ü Neutralization (acid-base) reactions,

ü Complex-forming reactions

ü Precipitation reactions

ü Oxidation-reduction reactions.

1.Neutralization reaction( or Acidimetry and alkalimetry):

These reactions involve the combination of hydrogen and hydroxide ions to form water. These reactions are two types ….

v Acidimetry

v Alkalimetry

Acidimetry:

These include the titration of free bases or those formed from salts of weak acids by hydrolysis, with a standard acid.

Alkalimetry:

The titration of free acids or those formed from by hydrolysis of salts of weak bases, with a standard base.

2.Complex formation reactions:

These depend upon the combination of

ü Ions,

ü Other than hydrogen or hydroxide ions, to form a

o Soluble,

o Slightly dissociated ion or compound.

For example:

In the titration of a solution of a cyanide with silver nitrate

2CN^- + Ag⁺ [Ag (CN) ₂] ^-

or, in the titration of chloride ion with mercury(ІІ) nitrate solution.

2Cl¯ + Hg²⁺ HgCl₂

3.Precipitation reactions:

These depend upon the combination of ions to form a simple precipitate.

For example:

Ag⁺ + Cl¯ AgCl

4.Oxidation-Reduction reactions:

These include all the reactions involving change of oxidation number or transfer of electrons among the reacting substances.

The standard solutions are either oxidizing or reducing agent.

The principle oxidizing agents are…

ü KMnO₄,

ü K₂Cr₂O₇,

ü I₂,

ü KI,

ü KBr.

The principle reducing agents are…

ü Fe(ΙΙ),

ü Sn(ΙΙ),

ü As(ΙΙΙ) oxide,

ü Sodium thiosulphate,

ü Mercury(Ι) nitrate,

ü Chromium(ΙΙ) chloride or Sulphate,

Some terms involved in Titrimetric Analysis:

Standard Solution:

The solution of accurately known strength is called the standard solution. Standard solutions are expressed by normality (N), molarity (M) etc.

Titrand & Titrant:

In titrimetric analysis the regent of known strength is called ‘Titrant’ and the substance being titrated is called ‘Titrand’. ‘Titrant’ taken in Burette and ‘Titrand’ taken in conical flask.

Example: If a known volume of NaOH solution is titrated with 0.1N HCl, then NaOH solution called ‘Titrand’ and HCl solution called ‘Titrant’.

Titration:

The process of adding standard solution to an unknown solution until the reaction is complete i.e. until the end point is achieved is termed as titration.

Equivalence/End point:

The point at which all the reagent of unknown solution (Titrand) react with known solution (Titrant) is called ‘Equivalence’ or ‘End point’.

At the end point, titration is detected by some physical change (usually color change) produced by indicator.

Indicator:

Indicator is a standard solution, which changes its color at the end point of a titration, or indicator is an organic dye that signals the end point by visual change in color.

Actually indicator formed a complex with excess Titrant, which added after the end point and change their color. For example-

Phenolphthalein in acidic solution

No color change

Addition of base excess than end point

Phenolphthalein in basic solution

Color change due to Phenolphthalein formed complex with base

Titration error:

100% accurate measure is impossible. Practical value and theoretical value is never same.

So, the difference between the theoretical value and practical value of a titration is known as Titration error.

i.e. Titration error = Theoretical value – Practical value.

Example: If the theoretical value of a titration is 5 but the examination value or practical value is 4.91.

Then titration error = 5 – 4.91

= 0.09

Condition of titrimetric analysis:

For use in titrimetric analysis a reaction must fulfill some conditions. They are as follows –

1.Chemical equation:

There must be a simple reaction, which can be expressed by a chemical equation. The substance to be determined should react completely with the reagent in stoichiometric (molar basis) or equivalent proportions.

NaOH + HCl NaCl + H₂O

2.Fast:

The reaction should be relatively fast. In some cases the addition of a catalyst may be necessary to increase the speed of a reaction.

3.Alteration of physical condition:

At the end point there must be a change in some physical condition.

4.Indicator:

An indicator should be available which, by a change in physical properties (color or formation of a precipitate), should sharply define the end point of the reaction.

Introduction:

The term ‘Titrimetric Analysis’ refers to quantitative chemical analysis carried out by determining the volume of a solution of accurately known concentration, which is required to react quantitatively with a measured volume of a solution of the substance to be determined.
In Titrimetric Analysis, the substance to be determined is allowed to react with an appropriate reagent added as a standard solution, and the volume of solution needed for complete reaction is determined. The common types of reaction which find use in titrimetry are..
 Neutralization (acid-base) reactions,
 Complex-forming reactions
 Precipitation reactions
 Oxidation-reduction reactions.