Biopharmaceutics

Dialysis Membrane / Diffusion Calculator

The practical sheet's method, step by step: absorbance → C2 → C2/C1 → volume factor → B → ln(1 − B), then the graph against time, its slope and the rate constant.

Experiment

Drug / membrane (optional)

Printed on the lab record.

Calibration curve

Regression equation Y = a + bX (Y = absorbance, X = concentration)

Intercept (a)

Slope (b)

AU/(µg/mL)

Concentration unit (X)

Y = a + bX(Y = absorbance, X = conc. in µg/mL)

Open calibration tool

Slope b converts absorbance to concentration. B (volume-corrected ratio) is calculated later as (C2/C1) × [1 + V2/V1]. They are different quantities.

Concentration and volumes

Initial / donor concentration (C1)

µg/mL

C1 and C2 must be in the same unit — C2/C1 is unitless.

The practical sheet uses 5 mL.

The practical sheet uses 1500 mL.

Volume factor = 1 + V2/V1 = —

Time table

Time | Absorbance — add a row for every sample withdrawn

Sampling times and absorbances
#	Time(min)	Time(s)	Absorbance (Y)(AU)	Remove
Row 1		0
Row 2		300
Row 3		600
Row 4		900
Row 5		1,200
Row 6		1,500
Row 7		1,800

2–30 rows. Times must increase from row to row; rows are never re-ordered for you.

Load an example data set (you can edit every value)

Practical interpretation

How the slope of ln(1 − B) against time is reported

Laboratory calculation card

Enter the calibration line, C1, V1, V2 and every row's time and absorbance.

Time (s) = Time (min) × 60

C2 = (Y − a) / b (from Y = a + bX)

Volume factor = 1 + V2/V1

B = (C2/C1) × [1 + V2/V1]

ln value = ln(1 − B) — defined only when 1 − B > 0

ln(1 − B) = m·t + c, m = [nΣxy − (Σx)(Σy)] / [nΣx² − (Σx)²], c = [Σy − mΣx] / n

k = −slope (when the practical interprets the negative slope as the rate constant)

With V1 = 5 mL and V2 = 1500 mL the volume factor is 1 + 1500/5 = 301. Slope b belongs to the calibration line; B is the volume-corrected ratio. R² = 1 − SSE/SST for the ln(1 − B) line.

Calibration Curve CalculatorFits the calibration line that turns absorbance into C₂.

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

B has reached or passed 1, so 1 − B is zero or negative and has no logarithm. Check the absorbance, C1 and the volumes. That row is left out of the graph and the regression, and the calculator says so.

No. Slope b is the calibration line's slope (absorbance per unit concentration). B is the practical's volume-corrected ratio, (C2/C1) × [1 + V2/V1]. They only share a letter.

Either — the sheet records both. The slope and k change unit with the axis (k in s⁻¹ is k in min⁻¹ divided by 60); the intercept and R² do not change.

Choose “Report slope only”. The slope, intercept and R² are still calculated, and no rate constant is shown anywhere.

About the dialysis practical

A drug solution (volume V1, concentration C1) is placed inside a dialysis membrane and dipped into a receptor medium (volume V2). Samples of the receptor are read on a UV spectrophotometer at timed intervals and converted to concentration C2 with a calibration line.

The sheet corrects each ratio C2/C1 for the two volumes, B = (C2/C1) × [1 + V2/V1], and plots ln(1 − B) against time. A straight line with a negative slope describes diffusion across the membrane.

Before you start

Enter the calibration line of the same drug, in the same medium and wavelength

C1 must be in the same concentration unit as the calibration's X

Record time at each withdrawal — minutes or seconds, both work

Read the result carefully

If B reaches 1, ln(1 − B) is undefined — that reading is left out of the graph

Slope b of the calibration is not B of the practical

k is quoted in min⁻¹ or s⁻¹ — the unit follows the time axis you choose

Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy