Page 20 - phytochemistry II -pharmD general

3. There is no need for consideration of chemical equivalence, contributions
due to alkalinity of glass, choice of indicator,… etc; as in the case with
volumetric procedures.

4. It is useful for weakly basic alkaloids.
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.

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