pKa
Definition
The negative logarithm of the acid dissociation constant. For a basic drug it is the pH at which half the molecules are protonated and half are free base. For an acidic drug it is the pH at which half are protonated (HA) and half are deprotonated (A minus). Partition into organic solvent is favoured when the drug is in its un-ionised form.
- Definition
- Negative log of the acid dissociation constant
- Basic drug meaning
- pH where half the molecules are protonated
- Acidic drug meaning
- pH where half are HA and half are A-minus
- Extraction relevance
- Un-ionised form partitions into organic solvent
Common questions
How is pKa used practically in forensic toxicology extractions?+
Analysts adjust the sample pH away from the drug's pKa so most molecules sit in their un-ionised form, which favours partition into an organic solvent during liquid-liquid extraction and improves recovery ahead of instrumental analysis.
Why do basic and acidic drugs need opposite pH adjustments for extraction?+
A basic drug is un-ionised and extractable at high pH, while an acidic drug is un-ionised and extractable at low pH, so the same extraction protocol cannot be applied without first knowing which class the target compound belongs to.
Related terms
- Acid Fraction
- The organic phase recovered when the biological aqueous is acidified to pH 2 to 3 and shaken with diethyl ether or chloroform....
- 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...
- 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...