This test is to determine the dissolution rate of the Aspirin. This can give important information about the bioavailability of Aspirin.

**Apparatus**

The complex dissolution tester consists of six 1 litre USP vessels held in place by self-centring 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.

**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 - Samples may be withdrawn at 5 minutes, at the end of 10 minutes.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

**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 = 0.260-0.0097/0.7265 = 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.