Chemistry Reference
In-Depth Information
0.4707
———
0.1864
2.53 tablets
Therefore, 1.0185 g lithium carbonate was found in 2.53 tablets, so the
weight of lithium carbonate in one tablet is 1.0185/2.53
0.4034 g.
The stated content of lithium carbonate is 400 mg per tablet, so the
percentage stated amount is given by (0.4034/0.4)
100
100.8%.
Non-aqueous titrations
Non-aqueous titrations are titrations carried out in the absence of water.
They are particularly useful for the assay of drugs that are very weakly acidic
or basic, so weak in fact that they will not ionise in aqueous conditions.
Water, being an amphoteric compound, acts to suppress the ionisation
of very weak acids and bases. All the apparatus and glassware for a non-
aqueous titration must be scrupulously dry, as even a drop of water will
ruin the whole assay. All glassware should be rinsed with distilled water,
rinsed again with a volatile solvent such as acetone, then dried thoroughly
in an oven or hot air dryer. It is also a good idea to remove all wash bottles
from the laboratory. There is no sadder sight than to watch a student con-
scientiously carry out a non-aqueous titration and then spoil all the hard
work by thoughtlessly adding water from a wash bottle.
Non-aqueous titrations are widely used in Volumes I and II of the
British Pharmacopoeia
for the assay of drug substances. A large number
of drugs are either weakly acidic (such as barbiturates, phenytoin or
sulfonamides), or weak bases (antihistamines, local anaesthetics, morphine,
etc.). The weak acids are usually titrated with tetrabutylammonium
hydroxide (N(Bu
n
)
4
OH) or potassium methoxide (CH
3
OK) in dimethyl-
formamide (DMF) as solvent. Weak bases are dissolved in glacial acetic
acid and titrated with perchloric acid (HClO
4
). When a strong acid, such as
perchloric acid, is dissolved in a weaker acid, such as acetic acid, the acetic
acid is forced to act as a base and accept a proton from the perchloric acid.
This generates an
onium
ion, which functions, in the absence of water, as a
super-strong acid, and it is this species that reacts with the basic drug.
The reactions occurring are as follows.
HClO
4
CH
3
COOH
CH
3
COOH
2
ClO
4
1
CH
3
COOH
2
base
CH
3
COOH
base H
1
