Baking powder is generally
just baking soda mixed with
an acid, and a number of
kitchen acids may be mixed
with baking soda to simulate
commercial blends of baking
powder. The most common
suggestion is to use two
parts cream of tartar with one
part baking soda. Where a
recipe already uses
buttermilk or yogurt,
baking soda can be used
without cream of tartar ( or with less). Alternately, lemon juice can
be substituted for some of the liquid in the recipe, to provide the
required acidity to activate the baking soda.