Volumetric
analysis
Often the
quantitative relationships introduced in concentration can be employed directly
in what are termed (volumetric analysis ), for example quantity
of base in some sample of an unknown can be determined by procedure called a (
titration ) in which a solution of an acid is dispensed into the solution
of the base from a burette until reaction is complete as signaled by some form
of indicator or( is a process based on addition of the standard solution to the
solution to be analyzed until the amount of reagent (standard) has been added
is equivalent to the amount of the substance being determined,( Until the
indicator just changes colures). This is called the (end – point )
of the titration, that means the procedure is stopped, and which the standard
solution is chemically equivalent to the unknown solution is called the
equivalent point.
equivalence point .
A common method
of end point detection in volumetric analysis involves the use of supplementary
chemical compound that exhibits a change in color as the total concentration of
the H+ changes occurring near the point such substance is called
(indicator)
(number of
equivalents of acid x volume of acid= number of equivalents of base x volume of
base)
Standard solution:- any solution
of accurately known concentration that is used in titration and prepared from
primary standard substance.
Properties of
primary standard substance
Material which is
used for preparation of primary standard
solution , so it have this characters :-
1- Very high purity.
2- Stable (absence
of hydrated water).
3- Must be available
and not too expensive.
4- It, s
reaction must be rapid and stoichiometric.
5- High equivalent
weight .
Titrant :- the solution
in the burette (known concentration).
Titrand (
analyte) :- the solution in the conical flask (to be
determined) .
Titration :- The process used
to determine the volume of standard required(known concentration) to react
completely with analyte.
End-point :- is the point
at which physical changes associated with the equivalence point can be observed.
Equivalent- point :- is the point
at which the amount of standard solution added is chemically equivalent to the
substance with which it reacts.
An Indicator:- is a chemical compound that exhibits a
change in its color as a result of H+ concentration changes
occurring near the equivalence point.
volumetric method :- a method in
which the analysis is completed by measuring the volume of solution accurately
known concentration (standard solution ) which is required to react quantitivly
with the volume of substance being determined (analyte).
For used in
volumetric (titrimetric )analysis a reaction must fulfill the following
conditions :-
1- 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 or
equivalent proportions.
2- The reaction
should be practically instantaneous or proceed with very great speed . ( most
ionic reaction satisfy this condition ). In some cases the addition of a
catalyst increases the speed of a reaction .
3- There must be a
marked change in some physical or chemical property of the solution at the
equivalence point .
4- 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.
The following are
required for volumetric analysis:-
1- Burette.
2- Flasks.
3- Pipette(bulb and
graduate).
4- Two beakers
5- Wash bottles
6- Small funnel.
7- Graduate
cylinder.
8- Note book, and
small towel.
9- Substances of know
purity for the preparation of standard solutions.
10- A visual
indicator or an instrumental method for detecting the completion of the
reaction.
Classification
methods of volumetric analysis
1- Neutralization:-
This comprises
volumetric determination based on reaction of acids with alkaline substants .
The neutralization method is used for determining the amount of acid ( alkalimetery)
or the amount of base (acidimetery).
2- Oxidation – reduction :-
The oxidation reaction
involves reaction associated with transfer of electron. In a reaction of this
type the oxidizing agent against electron and is reduced , the reducing agent
loses electron and is oxidized. The commonest of these are :-
1- Permanganatometry :- based on reaction of oxidation with potassium permanganate.
2- Iodometry :- based on oxidation by the action of free iodine or
reduction by ( I- ) ion.
3-precipitation:-
This volumetric
method is based on precipitation of various ion in the form of insoluble
compounds. The must important methods are those based on precipitation of insoluble
silver salt in accordance with the equation:
Where X- represent Cl- , Br- , I-
4-Complexation :-
This volumetric
reaction is based on the formation of various compound in the form of a weakly
dissociated complex. Normally used chelating agents such as EDTA in this type
of reactions.