Aim: To study the dissolution profile of a tablet formulation using a USP Type I (basket) or Type II (paddle) dissolution apparatus, and determine the percentage drug release at specified time intervals.
Apparatus Required
- Dissolution Apparatus (USP Type I — basket, or Type II — paddle, 6 or 8 stations)
- UV-Visible spectrophotometer
- pH meter
- Volumetric flasks (100 mL, 1000 mL)
- Pipettes and syringes for sample withdrawal
- Whatman filter paper / membrane filter (0.45 μm)
- Thermometer
Theory
Dissolution is the process by which a solid drug substance dissolves in a medium to form a solution. It is a critical quality control test for oral solid dosage forms (tablets, capsules). The rate and extent of dissolution directly correlates with bioavailability. USP <711> and Indian Pharmacopoeia (IP) define the apparatus, medium, speed, temperature (37 ± 0.5°C), and acceptance criteria for dissolution testing.
Dissolution media: 0.1 N HCl (pH 1.2, simulates gastric fluid), phosphate buffer pH 6.8 (simulates intestinal fluid), or purified water — specified in the individual monograph.
Procedure
- Prepare 900 mL of dissolution medium (e.g., 0.1 N HCl or phosphate buffer pH 6.8) and deaerate by heating to 41°C and filtering. Fill each vessel.
- Set water bath temperature to 37 ± 0.5°C. Verify temperature in all vessels.
- Set stirrer speed: 100 rpm for basket (Type I) or 50 rpm for paddle (Type II) — as per monograph.
- Place one tablet in each vessel (or in the basket for Type I). Start timer simultaneously.
- At specified time points (e.g., 5, 10, 15, 20, 30, 45, 60 min), withdraw 5 mL sample using a syringe from the midpoint of the vessel (not near the tablet or basket).
- Replace withdrawn sample volume with fresh medium to maintain 900 mL.
- Filter the sample. Measure absorbance at the drug’s λmax using UV spectrophotometer.
- Calculate drug concentration from a standard calibration curve. Calculate % drug release = (C × 900 / label claim) × 100.
Calculations
Standard curve: Prepare standard solutions of the drug (2, 4, 6, 8, 10 μg/mL) in dissolution medium. Plot Absorbance vs Concentration → y = mx + c.
Concentration from sample absorbance: C = (A – c) / m (μg/mL)
% Drug Release at time t: = (C × 900 / 1000) / (Label Claim in mg) × 100
Cumulative % Release: Account for previously withdrawn sample volume in the calculation.
USP Acceptance Criteria (Q value)
Most immediate-release tablet monographs specify: Q = 70–80% drug release at 30–45 minutes. Stage 1: 6 units — each must release ≥ Q + 5%. Stage 2: 6 additional units — average ≥ Q and none below Q – 15%. Stage 3: 12 additional units — average ≥ Q and not more than 2 below Q – 15%.
Frequently Asked Questions
What is the difference between USP Type I and Type II dissolution apparatus?
USP Type I (basket apparatus) uses a rotating cylindrical wire mesh basket containing the dosage form — used for tablets and capsules that float or disintegrate into large pieces. USP Type II (paddle apparatus) uses a rotating paddle positioned above the vessel bottom — preferred for most immediate-release tablets. Type II is more commonly used due to its simplicity and reproducibility.
Why is the dissolution medium deaerated before use?
Dissolved oxygen and air bubbles in the dissolution medium can affect drug release rates — air bubbles adhering to the tablet surface may create a barrier or interfere with the stirring pattern. Deaeration (heating to 41°C and filtering, or vacuum degassing) removes dissolved gases to ensure reproducible results consistent with pharmacopoeia standards.
What does Q value mean in dissolution testing?
Q is the specified amount (as a percentage of the label claim) of drug dissolved at a specified time, as stated in the individual monograph. For example, Q = 75% at 45 minutes means at least 75% of the labelled drug must dissolve within 45 minutes. The USP three-stage acceptance criteria define whether a batch passes or fails dissolution testing.
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USP Apparatus 1 (rotating basket) holds the dosage form in a 40-mesh basket, typically rotated at 50–120 rpm — preferred for capsules and floating dosage forms. USP Apparatus 2 (paddle) stirs the medium over the sinking dosage form, typically at 50–75 rpm — the default for tablets. Both use 900 ml vessels (500–1000 ml per monograph) maintained at 37±0.5°C in a thermostatic water bath.
Dissolution Acceptance Criteria (USP/IP)
Testing follows the three-stage scheme: S1 — 6 units, each not less than Q+5%; S2 — 6 more units, average of 12 ≥ Q and no unit below Q−15%; S3 — 12 more units, average of 24 ≥ Q, not more than 2 units below Q−15%, none below Q−25%. The Q value (commonly 75–80% in 45 minutes) is set by the individual monograph. For a compliant instrument, see our USP dissolution apparatus specification.
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