Although cream of tartar and tartaric acid are close relatives, there are some important differences between the two. Make sure that you don't simply substitute equal amounts of one for the other, as the results will be quite disappointing.
Identification: Tartaric Acid
Tartaric acid (or, in scientific terms, dihydroxy-succinic acid) is a salt found in plants. It can also be produced by fermenting grapes or other substances such as tamarind and pineapple in a container (for example, a wine cask). A white crust called argol often forms during the process, and this can be precipitated to make tartaric acid. Tartaric acid was first synthesised in 1769 by Carl Wilhelm Scheele, who used boiled tartar and chalk, combined with sulphuric acid, to arrive at the final product.
Tartaric acid has been in use since Greek and Roman times, and for good reason. The culinary arts, photography, tanning, ceramics, mirror making, and medicinal remedies make use of tartaric acid. It has an intensely sour and tart flavour, and is a primary acid in wine.
Identification: Cream of Tartar
Another important use of tartaric acid (or potassium bitartrate) is the production of cream of tartar. Cooks should be grateful, because tartaric acid is much more expensive and hard to find. Cream of tartar is basically a weakened form of tartaric acid. After tartaric acid forms, it is mixed with potassium hydroxide to neutralise the acid. This mixture results in cream of tartar, which is not actually a cream, but a crystalline powder.
Baking commonly calls for cream of tartar. For example, making a meringue pie requires cream of tartar (or a close substitute, such as vinegar or lemon juice), since you need it to stabilise beaten egg whites. Cream of tartar also prevents crystallisation of syrups. A combination of cream of tartar and baking soda is all you need to make homemade baking powder.
Cream of tartar is also an excellent cleaning agent. Mixing cream of tartar and lemon juice results in a great copper cleaner. Hot water or hydrogen peroxide mixed with cream of tartar can help remove stains from aluminium pans.
If your recipe calls for tartaric acid and you don't have it, using cream of tartar might work. For every teaspoon of tartaric acid, replace with two teaspoons of cream of tartar. However, using tartaric acid will produce better results.