Page 17 - Pharmaceutical analytical chemistry |
P. 17
2. Normal concentration
The normal concentration of a solution (N) expresses the number of
equivalents of solute contained in one liter of solution or the number of
milliequivalents of solute per milliliter of solution.
Equivalent weighs in neutralization reactions
The equivalent of a substance participating in a neutralization reaction is
that amount of substance that either reacts with or supplies one mole of
H+ in that reaction.
EqW of substance = FW of substance/no. of moles of H+
The relationship between EqW and FW is straightforward for acids or
bases that contain a single reactive H+ or OH-, e.g. HCl and NaOH where
EqW = FW. In case of Ba(OH)2 which contains two identical OH-, it
reacts with two H+ ions, so its equivalent weight is one half of its formula
weight, i.e. EqW = ½ FW.
The situation becomes more complex for acids or bases that contain H+ or
OH¯ with different tendencies to dissociate, e.g. H3PO4.
H3PO4 + OH- → H2PO4- + H2O EqW = FW
H3PO4 + 2OH- → HPO4²- + 2H2O EqW = ½ FW
Normality calculations are treated like 1:1 reactions, i.e. one equivalent of
substance A always reacts with one equivalent of substance B.
3. Percent concentration
Concentrations are frequently expressed in terms of percent.
Three common methods are:
a) Percentage weight/weight,
16
