is crucial in baking powder. Upon dissolving in batter or dough, the tartaric acid that is released reacts with baking soda to form carbon dioxide that is used for leavening. Since cream of tartar is fast-acting, it releases over 70 percent of carbon dioxide gas during mixing. Household use Potassium bitartrate can be mixed with an acidic liquid such as lemon juice or white vinegar to make a paste-like cleaning agent for metals such as brass, aluminium, or copper, or with water for other cleaning applications such as removing light stains from porcelain. This mixture is sometimes mistakenly made with vinegar and sodium bicarbonate (baking soda), which actually react to neutralize each other, creating carbon dioxide and a sodium acetate solution. Cream of tartar was often used in traditional dyeing where the complexing action of the tartrate ions was used to adjust the solubility and hydrolysis of mordant salts such as tin chloride and alum. Cream of tartar, when mixed into a paste with hydrogen peroxide, can be used to clean rust from some hand tools, notably hand files. The paste is applied and allowed to set for a few hours and then washed off with a baking soda/water solution. After another rinse with water and thorough drying, a thin application of oil will protect the file from further rusting. Slowing the set time of plaster of Paris products (most widely used in gypsum plaster wall work and artwork casting) is typically achieved by the simple introduction of almost any acid diluted into the mixing water. A commercial retardant premix additive sold by USG to trade interior plasterers includes at least 40% potassium bitartrate. The remaining ingredients are the same plaster of Paris and quartz-silica aggregate already prominent in the main product. This means that the only active ingredient is the cream of tartar. Cosmetics For dyeing hair, potassium bitartrate can be mixed with henna as the mild acid needed to activate the henna. Medicinal use Cream of tartar has been used internally as a purgative, but this is dangerous because an excess of potassium, or hyperkalemia, may occur. Chemistry Potassium bitartrate is the United States' National Institute of Standards and Technology's primary reference standard for a pH buffer. Using an excess of the salt in water, a saturated solution is created with a pH of 3.557 at . Upon dissolution in water, potassium bitartrate will dissociate into acid tartrate,