All baking soda is aluminum free. Baking soda is sodium bicarbonate, which is extracted from the earth. There is no aluminum in it nor is aluminum added to it. There has been some confusion on this topic because the packaging of some baking sodas indicate that they are aluminum free.
Bob’s Red Mill baking soda specifically states that it is aluminum free, while Arm & Hammer baking soda does not. However, both products are pure sodium bicarbonate and contain no aluminum.
Baking powder is a different story. Baking powder is a manufactured product which contains sodium bicarbonate, an acid and a starch. Some baking powders may contain aluminum.
Both baking soda and baking powder are called leavening agents and are used to make baked goods rise. Baking soda must be used in a recipe that also contains some kind of acid, such as vinegar or lemon juice. The combination of the acid and the sodium bicarbonate creates the chemical reaction that produces carbon dioxide bubbles and makes the cake or bread rise. Baking powder contains an acid, so mixing it with a liquid that does not contain an acid will cause the chemical reaction to occur.
Do not substitute baking soda for baking powder. If you don’t have baking powder, you can make your own by mixing two parts cream of tarter with one part baking soda.