Page 19 - Phytochemistry 2 (06-PG 605)

Disadvantages of Gravimetric method:

• Unsuitable for volatile or thermolabile alk.
• The alk. residue must be entirely free from

any impurities, otherwise they are weighted as alkaloids.
(+ve ) or ( –ve) error ?
• The sample must be sufficiently large to
obtain a residue suitable for accurate weighing.

2- Volumetric (Titrimetric methods):
This method depends on titrimetric determination of total nitrogenous bases.

 Titration in aqueous medium
Titration in aqueous medium is carried out either by direct titration with a standard
acid solution; or indirectly by back titration of the unconsumed standard acid.

 Titration in non aqueous medium
Glacial acetic acid in perchloric acid and chloroform as solvent is usually used.

Non aqueous titration is suitable for:
 Determination of weakly basic alkaloids
 For alkaloids that are present in small quantities.

3- Spectrophotometric assays:
These assays are applied for:

 Pharmaceutical preparations, ampoules, eye drops, biological fluids (urine &
blood) where the alkaloids to be analyzed are in a very low concentration.

 Can be used as a means of making simultaneous analysis of multi-
component systems

4- Colourimetric assays:
 It can also be used for crude purified drug extracts when a specific colour
reaction is available.
 The colour reaction should be quantitative.

Examples Colourimetric assays
 The colour of Ergot alks with p-dimethyl- aminobenzaldehyd
 The colour of Morphine with Na nitrite / KOH→Rose colour.

5- Flourimetric Method:
 The flourescence of the alkaloid, when present, is measured at a certain
wavelength.
e.g. Quinine & quinidine in an oxygenated acid.

18