Cream of tartar is the common name for potassium hydrogen tartrate, an acid salt that has a number of uses in cooking.
Add cream of tartar to beaten egg whites to stabilize and give more volume. For 1 to 2 whites, add: 1/8 teaspoon cream of tartar. For creamier frostings, candy fillings and syrups and to prevent premature hardening add 1/4 teaspoon of cream of tartar per 1-1/2 cup of sugar. It inhibits the formation of crystals. A pinch of cream of tartar added to vegetables when boiling also helps reduce discoloration.
Cream of tartar is used commercially in some soft drinks, candies, bakery products, gelatin desserts, and photography products. It is the acidic ingredient in some brands of baking powder. Since it is slightly acidic, it can be used to clean brass and copper cookware.