تحميل برنامج لمشاهدة الافلام

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Tests for detection of alkaloids

Precipitation reactions :

- Precipitating reagent + neutral or slightly acidic aqueous solution of alkaloidal salts = crystalline precipitates of different colors.
- Reagents that contain heavy metals (Hg, Pt, Bi, Cd) = double salts with most alkaloids.
- Reactions are used for extraction & purification.

Precautions:
– Certain alkaloids do not react e.g. caffeine + Mayer’s reagent = no ppt.
– False positive response may be obtained [most reagents also precipitate proteins, tannins, coumarins & certain flavonoids].

Color reactions :

- Color reagents + solid free bases [not the salts] - characteristic colored solutions.
- Reagents generally contain conc. H2SO4 + oxidizing agent.
- Reagents - colors with most alkaloids, or may be specific for one alkaloid or a group of related alkaloids.
 
Examples :
–Van-Urk's test: ergot alkaloids + para-dimethyl amino-benzaldehyde (PDAB) in conc. H2SO4 = blue color.
–Vitalis' test: solanaceous alkaloids + conc. HNO3 + alcoholic KOH = violet color.

Isolation of volatile & sublimable alkaloids :

Isolation of volatile alkaloids e.g. nicotine & coniine : By steam distillation
 
1.Immerse plant material in water.
2.Add caustic soda (NaOH) or Na2CO3 to liberate free bases (NH3 not used due to its volatility).
3.Steam distill the liberated alkaloid.
 
- Isolation of sublimable alkaloids e.g. caffeine : By direct sublimation from the dried powdered plant.

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