When you asked about food grade phosphoric acid, is that because you did find a supplier for it or you want to know if, once you find, you should use food grade ?
I guess food grade is always good but many people use acid that isnt food grade.. do check the msds of any product you do find (as advised in this thread:
The official "Is this chem ok to use? / Where can I find ...?" thread - FAQ - All your basic questions and answers - Welcome to the DMT-Nexus) .
Maybe going to bigger cities and checking out if there are any chemical / lab supply stores ? I've seen phosphoric acid being sold in big gardening stores (not in AUS, but just saying..) but I dont know whats the use they gave for it.
As for the friend swearing by it, there is a lot of hypothesis/supositions regarding extractions, but unless its tested, someone's opinion isnt necessarily right, even if they made some or other guide. The only real way to know is to make a side-by-side extraction with different acids to test the difference in yields, and AFAIK this hasnt been done yet (anybody up for it? ). Do note that im not saying you shouldnt do it, but just that doing it isnt guaranteed to give you better results and that people have been using other acids with very good results.
Do read up other teks just for the sake of seeing more or less the same process explained by different people so that you understand the process and arent just following some steps blindly. Nevertheless, to use vinegar you dont need to use a special tek of any kind, its just boiling the mimosa 3 or 4x with vinegar+water, straining/decanting/filtering each time and later putting the liquids together and discarding the solids. I guess this is the same as you would do with phosphoric acid, yes?
