On very rare occasions, I get a sore tongue, not burning and not white, just along the side or under the tongue, it gets sore and anything tart like lemonade makes it worse. The only thing I've ever used is powdered alum. My grandmother told me of this, and I think she used this spice when making pickles. It is a white powder and all I know is that it works every time and I only need to just dampen my finger in water, dip it into the powder and apply it to the sore area and it heals it after one application.
Hope this helps someone else, what an easy fix for something that can make your miserable, especially when trying to eat.