After a box of baking soda is opened, it is only good for up to 6 months. Baking soda has the tendency to absorb odors and moisture. To prevent this from happening, store the baking soda in a clean, dry jar.
To test the potency of an opened box of baking soda, add 1/4 teaspoon baking soda to 1 tablespoon of vinegar. If it bubbles, it is still good to use for baking.