As I’ve been doing a lot of baking, I decided to check up how to make my own baking powder. This way it is always fresh with good leavening power, and you can also avoid the aluminium (believed to contribute to Alzheimer’s) that is found in some commercial baking powders. [The good news is that the common brands in Singapore don't seem to use aluminium compounds; however do read the labels next time you are in the supermarket, it's quite interesting to see the difference in chemicals that are used by various brands.]
The basic recipe for homemade baking powder:
To make the equivalent of 1 teaspoon of double acting baking powder, mix 1/2 teaspoon of cream of tartar with 1/4 teaspoon baking soda (two parts of cream of tartar to one of baking soda). This single acting backing powder will work very successfully but you must remember that when you use it, get whatever you’re baking into the oven right away.
[from Diana's Desserts]
However, it’s very enlightening to read more about the difference between baking powder, baking soda, and how each of them function and react with other ingredients in the recipe. A kitchen chemistry lesson! See these:
Joe Pastry: Leavening Methods – Chemical, Mechanical, Microbial [fabulous resource on food science for cooks]
Diana’s Desserts: Leavening Agents for Quick Breads [very detailed on action of rising agents]
Wikipedia: Baking Powder [quick summary]
Cook’s Thesaurus: Leavens [with pictures]
Ochef: Does baking powder go bad or lose its potency?