Biopharmaceutics
Partition / Distribution Coefficient Calculator
D and log D from aqueous and organic phase absorbances at several pH values — average log D, 1/log D, [H+], 1/[H+] and both graphs, with every step shown.
Experiment
Printed on the lab record.
Shared by every calibration equation on this page. D is a ratio, so the unit cancels.
Load an example data set (you can edit every value)
pH group 1
0–14
2–10; applies to every group at this pH.
Calibration equation (aqueous and organic phase)
Y = a + bX(Y = absorbance, X = conc. in µg/mL)
Experimental groups — absorbance readings (AU)
| # | Aqueous(AU) | Organic(AU) |
|---|---|---|
| B1 · R1 | ||
| B1 · R2 | ||
| B1 · R3 |
Laboratory calculation card
Enter each pH, its calibration equation and every absorbance reading to calculate D and log D.
Average absorbance = (B₁ + B₂ + … + Bₙ) / n
Y = a + bX → C = (Y − a) / b
D = C(organic phase) / C(aqueous phase) = CoP / Caq
log D = log10(D)
Average log D = Σ(log D) / number of groups
1/log D = 1 / average log D
[H+] = 10^(−pH) mol/L · 1/[H+] = 10^(pH) L/mol
Y is absorbance (AU), a the intercept and b the slope of the calibration line. The organic phase uses the aqueous equation unless you give it its own. Regression on the graphs is ordinary least squares on the group log D values.
Keep learning
References
- Sinko PJ, editor. Martin's Physical Pharmacy and Pharmaceutical Sciences. 8th ed. Philadelphia: Wolters Kluwer; 2023.
- Harris DC, Lucy CA. Quantitative Chemical Analysis. 10th ed. New York: W. H. Freeman (Macmillan Learning); 2019.
Common questions
The practical sheet tabulates the reciprocal of the average log D. 1/D would be the aqueous-to-organic ratio, a different quantity. This calculator shows 1/log D exactly as the sheet defines it and never substitutes 1/D.
log D needs D > 0, so both Caq and CoP must be positive. A zero or negative concentration means the mean absorbance is at or below the calibration intercept — check the blank, the dilution and the equation. That group is left out of the pH average and the calculator says so.
log P describes the un-ionised form only and does not depend on pH. log D is measured with whatever mixture of ionised and un-ionised drug exists at the chosen pH, which is why it changes along the pH series.
Only with at least three valid group log D values spread over at least two different x values. A line through two points always has R² = 1, so no statistic is shown for it.
Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy