This test is to determine the rate of Dissolution of Aspirin Tablets 75 mg. This can give important information about the bioavailability of Aspirin.
Dissolution of Aspirin Tablets 75 mg Apparatus
The complex dissolution tester consists of six 1 litre USP vessels held in place by selfcentring vessel locking rings. The vessels are maintained at the correct working temperature by a thermostatically controlled water bath and each may be fitted with either stainless steel paddles or baskets. The height of this is adjustable.
Dissolution of Aspirin Tablets 75 mg Procedure
 Select the test from the main menu and use the keypad to select the required rpm and test duration. Ensure the water bath temperature is 37^{o}C.
 Individually place 3 to 6 tablets into a dissolution vessel containing 900 mL buffer or water.
 Samples may be withdrawn at 5 minutes, at the end of 10 minutes. Samples should be taken from a point half way between the surface and the tip of the paddle or bottom of the basket, not less than 10mm from the vessel wall.
 Determine the amount of active ingredient in the sample using UV analysis. Pass if more than 70% has dissolved by 10 min. To pass all must comply. If one fails a further 6 tablets may be tested.
Stock solution should be prepared by dissolving 1.8mg/18mg of pure aspirin powder in 1mL or 10mL DMSO
Dissolution of Aspirin Tablets 75 mg Calculations
Preparation of Aspirin 10 mM stock solution
Weigh and Dissolve 1.8 mg of Aspirin in 1mL DMSO
Preparation of 6 different concentrations ( 0.5 mM, 0.3 mM, 0.2 mM, 0.1 mM, 0.05 mM, 0.02 mM) of Aspirin solutions from 10mM stock solution
The amount of stock solution that would be needed to prepare 1mL of 0.5mM solution
The above value means that 50 μL of stock solution (10mM) should diluted with 950 μL of distilled water in order to make 0.5mM solution.
Table 1. Preparation of various concentrations of Aspirin solutions from 10mM stock solution
Table 2: Calculation of Amount of Aspirin in different dilutions
10mM stock solution contains 1.8 mg in 1mL i.e 1800 μg in 1000μL
50 μL will have 90μg
Table 3: UV spectroscopic analysis of different dilutions of Aspirin
Draw the linear regression graph between absorbance (270 nm) Vs various concentrations of Aspirin
Draw the linear regression graph between absorbance (270 nm) Vs various concentrations of Aspirin

 Graph 1: An example of linear regression graph.
 You should draw the graph based on your absorbance values
 Y = absorbance
 X = unknown concentration
In the above experiment we have tested the dissolution of 75 mg of Aspirin tablet in 900 mL distilled water.
If dissolution of 100% Aspirin takes place and is released into 900 mL distilled water
Concentration of Aspirin (75 mg) in 900 mL solution
If dissolution of 10% Aspirin takes place and is released into 900 mL distilled water

 Concentration of Aspirin in 900 mL solution
 Repeat the same Molarity calculations
 Molarity = 0.0462 mM
 From the Table 2 it is know that 0.5 mM concentration of Aspirin will have 90ug of Aspirin/mL
 0.462mM (100% drug release) = 83.16ug/mL
 83.16ug ——— in 1mL
 ? ——— in 900 mL
 Answer is 74844ug = 75mg
 Now find out the absorbance value (Y) of unknown concentration (X) of Aspirin in 900 mL distilled water. This will be done by taking a sample of solution and testing its absorbance in UV spectrophotometer.
 Once the Absorbance value is known from the spectrophotometer, X can be calculated.
 For example the absorbance value (x) = 0.260
Dissolution sample at 10 mins  0.260 
From the linear regression graph

 Y = 0.7265x + 0.0097
 R^{2 }= 0.9965
 0.260 = 0.7265 x + 0.0097
 X = 0.2600.0097/0.7265 = 0.2503/0.7265 = 0.3445 ~ 0.35mM
 As we know that 0.5mM contains 90 µg/mL
 Then 0.35mM contains = 0.35 x 90/0.5 = 63µg/mL
 If 63µg in 1mL
 Then 900 mL contains = ?
 900 x 63 /1 = 56700 µg ~ 56.7 mg
 So for 100% dissolution
 75 mg —— 100%
 56.7mg ——– ?
 56.7 x 100/75 = ~ 75%
 So the % dissolution is 75% after 10 minutes.
Tablet passed the Dissolution test.