Analysis of methylphenidate (Ritalin) HCl extended release tablets
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.