For a project I have to determine the drug concentration release profile of self-made methylphenidate HCl (MHCl) extended release tablets (hydrophilic matrix). We are thus doing a dissolution test as described conform the US/Eur. Pharmacopeia. However a problem we are facing is that we are not sure how to analyse the samples that we take from the dissolution test. This is because we are not sure what detection method to use. The problem is that we cannot directly get the right concentration for our primary detection method which is UV spectrophotometry. This is because MHCl has a very low extinction coefficient and thus a relatively higher concentration is needed to get the measurable absorption range of 0.2-0.8, but since the tablet dose is only 18 mg this is difficult to accomplish. We thought of doing two things: a solid phase extraction followed by UV or picric acid ionpairing followed by colorimetry. We asked our professor if the SPE followed by UV was possible, but he said that in this case it wasnt practical cause of the such low amount of drug in the tablets. So that leaves us with the picric acid analysis method. Problem is that we havent ever performed a colorimetric analysis before, though it looks pretty similar to spectrophotometric. From what we understand we need to measure the absorbance at a specific wavelength that the picric acid-MHCl complex is absorbing at. But how can we calculate the MHCl concentration from this? How much picric acid do we need to use to know for sure that all the MHCl has formed pairs with the picric acid? It's kind of confusing still and appreciate it if anyone could point us to some literature or give us insight on how to analyse these samples. Thanks in advance.
If you are insistent on a picrate salt, I'd suggest first taking your methylphenidate and forming a known concentration of the picrate by adding a known amount of picric acid to a solution in which you know the base would be in excess. This ensures that all of the picric acids forms the picrate salt, and you would therefore know the picrate complex concentration (assuming there are no other compounds in solution that might react with the picrate). You could then measure the absorbances at a wavelength at which picric acid has only a negligible absorbance, provided one exists. Note that if no such wavelength exists, matters become rather more complex, and you will have to look for changes in absorbance on either side of an isosbestic point.
An alternative reaction to the use of picric acid is to instead use 1-fluoro-2,4-dinitrobenzene (FDNB), which reacts with primary and secondary amines at room temperature to form a highly-coloured 1-amino-2,4-dinitrobenzene (ADNB). The spectral overlap between FDNB and ADNB is typically pretty small, so using an excess of FDNB usually doesn't greatly change the absorbance at the maximal wavelength of ADNB. Again, to at least know the absorbance of the methylphenidate ADNB, you would have to first perform the reaction with an excess of methylphenidate, and assume a complete reaction to the ADNB.