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Dissolution Testing: Salicylic Acid

Posted on | October 2, 2009 | 1 Comment

Dear DSN, I have read that they are getting rid of Salicylic Acid tablets for Calibration of dissolution apparatus. What is your opinion on this?

A few years back, a few individuals in the PhRMA study claimed that Salicylic Acid was not sensitive to mechanical parameters.  While it is true that Salicylic acid is not as sensitive to the mechanical parameters measured for that particular study, it is sensitive to many parameters that were not measured. These were not tested as part of the PhRMA study. It has some obvious uses that can help the pharma analyst ensure perfect operations of their apparatus.

For example, suppose you having a problem with Salicylic acid, paddles running low on paddles in one vessel? The most obvious parameter that jumps to mind is media deaeration. Let’s assume you are deaerating properly and are sure of the standard, because all the other vessels passed middle of the range? The vessels are brand new. The mechanical parameters are perfect. Baskets pass as well. What would you suggest?

First of all, you can eliminate the tablets, if all other vessels are passing mid range. Usually this is my first suggestion because of the subliming of salicylic acid. However, if all other vessels are passing and all tablets are from the same bottle. Then I would suspect the vessel. No one wants to admit their vessels are flawed. But ever glass vessel is hand made, and therefore, flawed.

Take it out and look at it. It may have a ridge, dent, or other defect. No matter how perfect, all are hand made and have subtle flaws. Because salicylic is non-disintegrating, it is sensitive to vessels flaws under paddle conditions. You won’t have this with baskets, because they are not in contact with the vessel. (However, the tablet is useful for identifying warped baskets, if you were running baskets.)

 If you haven’t already, make note of the vessel positions and replace the suspect vessel and keep all other vessels in the same position. Other possibilities exist as well. Since the tablets resides undisintegrated, on the antapex of the vessel, it is subject to vibration. Vibrational acceleration > 0.09 m/s is more likely to cause this rather than vibrational displacement . But most labs only measure displacement. Also low frequency vibration, below 20 Hz or at 130Hz, have both been identified to generate the most significant displacement values. But this is another subject altogether. Another option, if it were not one vessel but all, then I would suspect the media pH. Since Salicylic Acid is a pure substance without excipients, it reacts differently if the media is prepared incorrectly. True, this is not an instrument parameter, but it is an indication of the level of detail used to perform the test.

While Salicylic Acid will not tell you all about all of the mechanical parameters, it will inform you of most of the laboratory variables and proper preparation. An inexperienced chemist will show their worth by passing Salicylic Acid tablets. While it may not tell you what’s wrong with your apparatus, it will tell you a tron of information. The criticism that it will not tell you about mechanical parameters may have some merit. But Salicylic Acid will provide you with a wealth of knowledge. These factors are often the laboratory variables, such as vibrations, media pH, deaerations, basket deformations and other parameters that are not as easily measured.


One Response to “Dissolution Testing: Salicylic Acid”

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    October 3rd, 2009 @ 3:48 pm

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