If a recipe calls for baking soda, it can be replaced by two to three times as much baking powder. Potassium bicarbonate can also be used, although this ingredient typically can only be found at a pharmacy, a drug store or a store that sells supplements.
Baking soda releases carbon dioxide, which helps leaven baked goods, as it neutralizes acidic ingredients. When using a substitute, it may be necessary to replace acidic liquids such as buttermilk or citrus juice with plain milk and water. For lighter, fluffier results, it's best to incorporate as much air into the batter or dough as possible by sifting the flour or vigorously whisking the eggs.