PREPARATION AND STANDARDIZATION OF REAGENT SOLUTIONS

PREPARATION AND STANDARDIZATION OF REAGENT SOLUTIONS
Chemical reagents are manufactured and marketed in different grades of purity.
In general, the purest reagents are marketed as “analytical reagent” or AR-grade.
Other labels are “LR”, meaning laboratory reagent, and “CP”, meaning chemically
pure . The strength of chemicals is expressed as normality or
molarity. Therefore, it is useful to have some information about the strength of the
acids and alkalis most commonly used in chemical laboratories
Some important terms that are often used in a laboratory for chemical analysis
are defined/explained below.
Molarity
A one-molar (M) solution contains one mole or one molecular weight in grams
of a substance in each litre of the solution. The molar method of expressing
concentration is useful because equal volumes of equimolar solutions contain
equal number of molecules.
Normality
The normality of a solution is the number of gram equivalents of the solute per
litre of the solution. It is usually designated by the letter N. Semi-normal, pentinormal,
desi-normal, centi-normal and milli-normal solutions are often required;
these are written as 0.5N, 0.2N, 0.1N, 0.01N and 0.001N, respectively. However,
molar expression is preferred because “odd” normalities such as 0.121N are
clumsily represented in fractional form.
The definition of normal solution uses the term “equivalent weight”. This
quantity varies with the type of reaction. Hence, it is difficult to give a clear
definition of equivalent weight that covers all reactions. It often happens that
the same compound possesses different equivalent weights in different chemical
reactions. A situation may arise where a solution has a normal concentration
when employed for one purpose and a different normality when used in another
chemical reaction. Hence, the system of molarity is preferred.