I soak mine in pure water (reverse osmosis) in a glass container.
Don't soak them in plastic. At the beginning I soaked them in a small yellow container and in a few weeks the partial turned yellow!!! I saw this one morning and wondered what the heck is that???, then realized the yellow came from the plastic. If the plastic had been clear I wouldn't have noticed the leaching so this was a blessing in disquise! After switching to a glass container the coloring (and plastic from the plastic container) went away in a couple of days.
Since then I have used only glass, and always pure water, nothing else.
Having a denture brush is helpful. They are only about 3 bucks. I brush the partial and denture before and after soaking and that's all I do to them. This works very well.