What to substitute baking soda with in a recipe that needs baking powder too?

I'm making chocolate muffins, but the problem is the recipe needs baking soda AND baking powder. Since I don't have baking soda, could I use all baking powder? Or could I leave it out? Or is there any substitute I might have in my house? Thanks!
Update: Here's the recipe: http://www.food.com/recipe/good-for-you-chocolate-muffins-364764
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