Brush out the soda before putting anything back in the case! On reflection, I'd advise a little research before trying something so seemingly benign as baking soda in the case; it's great around the house and in the laundry, but you don't want to have it react in some way with the materials the instrument(s) are made of.

I'd hate to be the cause of a thread on "dumb ideas for the care of musical instruments".