Pharmaceutics
Dissolution Rate Constant Calculator
Builds the dissolution practical's table from your corrected readings — midpoint, dC/dt, Cs − C and both k columns — then averages the midpoint-based k.
Experiment inputs
Printed on the lab record.
The solubility of the drug in the dissolution medium.
Used for Cs and every reading.
dC/dt and k are reported per this unit.
Experimental data
Time | Corrected Reading — one row per sampling point.
| # | Time(min) | Corrected Reading(mg/mL) | Remove |
|---|---|---|---|
| T1 | |||
| T2 | |||
| T3 | |||
| T4 | |||
| T5 |
Load an example data set (you can edit every value)
Laboratory calculation card
Enter the saturation concentration and every time point's corrected reading to build the table.
1. Midpoint
Midpoint = (C₁ + C₂) / 2
The average of a reading and the one after it — the concentration roughly in the middle of that interval. The last observation has no reading after it, so the supplied practical table averages it with the previous one instead, which makes the final midpoint a repeat of the row above.
2. Rate of concentration change
dC/dt = (C₂ − C₁) / (t₂ − t₁)
How fast the concentration moved between two samples. If the second reading is lower, dC/dt is negative — that is experimental variability, and the calculator keeps the sign rather than hiding it.
3. Saturation difference
Cs − C
How far the solution still is from saturation, using the row's own corrected reading. This is the driving force for dissolution: it is largest at the start and shrinks as drug goes into solution.
4. First k
k = x / y where x = dC/dt and y = Cs − C
The measured rate divided by the driving force. Its unit is per unit time (min⁻¹). It has no value on the final row, because dC/dt does not exist there.
5. Midpoint-based k
k = x / y where x = midpoint and y = Cs − C
The same division with the interval's average concentration in place of the rate. This is the column the practical sheet averages and reports. Written this way it divides a concentration by a concentration, so as printed it carries no unit — worth knowing when you write the result up.
6. Average
Average k = (k₁ + k₂ + … + kₙ) / n
The mean of the midpoint-based column. Which rows are included is set by the “Average over” control above — the supplied practical table uses the interior rows only.
Keep learning
References
- Sinko PJ, editor. Martin's Physical Pharmacy and Pharmaceutical Sciences. 8th ed. Philadelphia: Wolters Kluwer; 2023.
- Aulton ME, Taylor KMG, editors. Aulton's Pharmaceutics: The Design and Manufacture of Medicines. 6th ed. Edinburgh: Elsevier; 2022.
Common questions
They are different calculations and the practical sheet records both. The first divides the rate of change (dC/dt) by Cs − C; the second divides the interval's midpoint concentration by Cs − C. They are not two ways of writing the same number, so the calculator keeps them in separate columns and never substitutes one for the other. The reported average comes from the midpoint column.
dC/dt needs a following reading to subtract from, and the final observation has none. Its cell shows “—”. Its midpoint is still calculated, using the previous reading, which is the convention the supplied practical table uses.
Not necessarily. A corrected reading that falls between two samples gives a negative slope and therefore a negative k. It usually points to sampling or dilution variability rather than to drug leaving the solution. The calculator flags it and keeps the value — it never deletes or takes the absolute value of your data.
By default the rows between the first and the last, reproducing the supplied practical table: the first observation is the start of the run (often C = 0, which makes its midpoint simply half the first reading) and the last row's midpoint repeats the row above it. Switch “Average over” to “All observations” to average every row instead — the two answers are genuinely different, so the calculator shows the choice rather than making it silently.
Because that is how the practical sheet is laid out: the midpoint is the numerator of the second k, and the denominator is always the row's own corrected reading subtracted from Cs. Mixing the two is the most common error in this table.
No. Every calculation runs at full precision and rounding happens only when a value is printed, so switching between scientific and decimal notation cannot change the answer.
Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy