Acid Fraction
Definition
The organic phase recovered when the biological aqueous is acidified to pH 2 to 3 and shaken with diethyl ether or chloroform. Contains drugs with pKa below 7, mostly acidic compounds: barbiturates, salicylates, NSAIDs, paracetamol, glutethimide and phenols.
- Extraction pH
- 2 to 3
- Solvents
- Diethyl ether or chloroform
- Compound class
- Acidic drugs, pKa below 7
- Example drugs
- Barbiturates, salicylates, NSAIDs, paracetamol, glutethimide
Common questions
Why is the sample acidified before extracting the acid fraction?+
Lowering the pH suppresses ionisation of acidic drugs, converting them to a neutral, lipid-soluble form so they partition into the organic solvent instead of staying dissolved in the aqueous phase.
How does the acid fraction differ from the alkaline fraction in the same scheme?+
The alkaline fraction is collected by raising the pH to free basic drugs such as amphetamines and opioids for the same solvent extraction, so the two fractions target opposite ends of a drug's acid-base behaviour.
Related terms
- Alkaline Fraction
- The organic phase recovered when the aqueous is basified to pH 9 to 10 with ammonium hydroxide or sodium hydroxide and shaken...
- Cut-Off
- The minimum analyte concentration that an SFSL reports as positive. SAMHSA-style serum cut-offs commonly used at Indian labs include morphine 2 ng/mL,...
- DFC
- Drug-facilitated crime, the umbrella term for sexual assault, robbery or kidnapping in which a victim has been incapacitated by a clandestinely administered...
- Neutral Fraction
- The organic phase recovered when the aqueous is buffered to pH 7 to 9 and extracted with a less polar solvent like...
- pKa
- The negative logarithm of the acid dissociation constant. For a basic drug it is the pH at which half the molecules are...
- Post-Mortem Redistribution
- The movement of drugs from solid tissues into central blood after death, due to diffusion along concentration gradients, autolytic enzyme release, and...