Combine these ingredients thoroughly and store in jar. Makes 1 1/4 cups of baking powder. The arrowroot repels moisture, so it keeps well. To make baking powder for immediate use, combine 1/4 teaspoon cream of tartar with 1/4 teaspoon bicarbonate of soda. This is the equivalent of 1 teaspoon of commercial baking powder and has 225 milligrams of sodium. To make sodium-free baking powder, eliminate the sodium bicarbonate and substitute potassium bicarbonate which is available at pharmacies.