Just on the bottom for tin foil? Need a whole box of baking soda?
Not sure of the specifics of the chemical reaction, but the baking soda makes the water alkaline and the aluminum foil catalyzes the silver sulfides in the tarnish into hydrodgen sulphide gas. If it smells like rotten eggs, you're doing it right.
I use maybe 2-3 tablespoons of baking soda per quart of warm tap water, and often leave it to soak overnight (but green's boiling water would give faster results). It works wonderfully to clean things that are black with tarnish, but I still use Wright's Silver Cream to when I want to "finish" the silver off to a high polish.