Baking soda is sodium bicarbonate. It's a base- and will create bubbles when mixed with acidic ingredients (such as cocoa powder, lemon juice, or buttermilk.) Baking powder is a mixture of sodium bicarbonate plus a few other ingredients, including something acidic, so that it will create bubbles in a dough as soon as water is added, so you don't need an acid in the recipe.
My box of baking soda says there is 160 mg of sodium per 1/8 teaspoon serving. I do not know if potassium bicarbonate works the same way or not.
Most "baking powders" out there contain corn starch, and many also contain aluminum. I use 1/2 teaspoon baking soda and 1/4 teaspoon cream of tartar as an alternative to 1 teaspoon baking powder. Some people use a different proportion of baking soda/cream of tartar.