First of all Welcome to the Nexus!
If you want pure morphine fumarate you`d probably need to do stoichometric calculations and measure pH as benz mentioned, though possibly simply doing FASA would work (never tried or heard of anyone trying, and not chemistry-savvy enough to look at molecule and predict if it would work). If it works, im neither sure how much fumaric acid contamination that would yield or if that would be a problem anyways. Methanol would dissolve both fumaric acid as well as morphine fumarate, so it most likely wouldn`t precipitate like many alkaloids do for FASA.. But you could still evaporate it to retrieve your alkaloids, if you added the right amount of fumaric acid to prevent excess of it once methanol is evapped. Please make sure to evap it completely and not breathe fumes as methanol is toxic.
The thing is, morphine is an addictive substance with a certain specific (un)safety profile. It`s not a substance we usually talk about, though given a certain context, I don`t see a problem. But you have given no such context, so I don`t think it`s good for this thread to continue until you do.
Of course we all have the right to do whatever we want to do if it doesn`t hurt others, and I don`t want to seem like im judging you for using morphine, specially because I can`t know your reasons and context until you state it... So you can post that kind of info if you feel will help. We rather here in the Nexus avoid promoting/discussing such substances unless there`s a specific reason for it, there`s an appropriate context involved and if it is clear that people understand the risks involved and are aware of the possible alternative options and how to quit the use of such substances if they so desire.
Here`s a couple of threads that are related to what I just mentioned:
https://www.dmt-nexus.me...aspx?g=posts&t=25363 (Policy regarding discussing different drugs in the Nexus)
and
https://www.dmt-nexus.me...aspx?g=posts&t=25676 (Quitting opiates/Addictive substances)