Denatured alcohol, also called methylated spirits or denatured rectified spirit, is ethanol that has additives to make it poisonous, bad tasting, foul smelling or nauseating, to discourage recreational consumption. In some cases it is also dyed. Pyridine, methanol, or both can be added to make denatured alcohol poisonous, and denatonium can be added to make it bitter.
Denatured alcohol is used as a solvent and as fuel for alcohol burners and camping stoves. Because of the diversity of industrial uses for denatured alcohol, hundreds of additives and denaturing methods have been used. The main additive has traditionally been 10% methanol, giving rise to the term "methylated spirits". Other typical additives include isopropyl alcohol, acetone, methyl ethyl ketone, methyl isobutyl ketone, and denatonium.
Denaturing alcohol does not chemically alter the ethanol molecule. Rather, the ethanol is mixed with other chemicals to form an undrinkable solution. For many of these solutions, there is no practical way to separate the components.
Denatured alcohol is used identically to ethanol itself except for applications that involve fuel, surgical and laboratory stock. Regular ethanol is required for food and beverage applications and certain chemical reactions where the denaturant would interfere. In molecular biology, denatured ethanol should not be used for precipitating nucleic acids.
In some countries, sales of alcoholic beverages are heavily taxed. In order to avoid paying beverage taxes on alcohol that is not meant to be consumed, the alcohol must be "denatured", or treated with added chemicals to make it unpalatable.
Denatured alcohol is not, in itself, a product that would be normally demanded if given the alternative of normal ethanol. Denatured alcohol and its manufacture are a public policy compromise. The supply and demand for denatured alcohol arises from the fact that normal alcohol (which in everyday language refers specifically to ethanol, suitable for human ingestion as a recreational drink or extractive medium for medicinal tinctures) is usually very expensive in comparison with similar chemicals, being highly taxed for revenue and public health policy purposes (see Pigovian tax). If pure ethanol were made cheaply available for fuel, solvents, or medicinal purposes, some people might ingest it.
Denatured alcohol provides a solution to permit industrial use and manufacture of ethanol, whereby cheap ethanol can be made available for non-consumption use without the risk of its being converted for consumption. The process creates an ethanol-containing solution that is not suitable for drinking, but is otherwise similar to ethanol for most purposes. As a result, there is no duty on denatured alcohol in most countries, making it considerably cheaper than pure ethanol. As a consequence, its composition is tightly defined by government regulations that vary between countries.
Despite its poisonous content, denatured alcohol is sometimes consumed as a surrogate alcohol. This potentially can result in blindness or death if it contains methanol. For instance, during the American Prohibition, federal law required methanol in domestically-manufactured industrial alcohols. On Christmas Day, 1926, and the two following days, which was roughly at the midpoint of the "Great Experiment" of nationwide alcohol prohibition, 31 people in New York City alone died of methanol poisoning. To help prevent this, denatonium is often added to give the substance an extremely bitter flavor. Substances such as pyridine are added to give the mixture an unpleasant odor, and agents such as syrup of ipecac may also be included to induce emesis (vomiting).
Different additives are used to make it difficult to use distillation or other simple processes to reverse the denaturation. Methanol is commonly used both because its boiling point is close to that of ethanol and because it is toxic. Another typical denaturant is pyridine. Often the denatured alcohol is dyed with methyl violet.
There are several grades of denatured alcohol, but in general the denaturants used are similar. As an example, the formulation for completely denatured alcohol, according to 2005 British regulations was as follows:
Completely denatured alcohol must be made in accordance with the following formulation: with every 90 parts by volume of alcohol mix 9.5 parts by volume of wood naphtha or a substitute and 0.5 parts by volume of crude pyridine, and to the resulting mixture add mineral naphtha (petroleum oil) in the proportion of 3.75 litres to every 1000 litres of the mixture and synthetic organic dyestuff (methyl violet) in the proportion of 1.5 grams to every 1000 litres of the mixture.
A specially denatured alcohol (SDA) is one of many types of denatured alcohol specified under the US Code of Federal Regulations Title 27 Section 21.151. A specially denatured alcohol is a combination of ethanol and another chemical, e.g. ethyl acetate in SDA 29, 35, and 35A, added to render the mixture unsuitable for drinking.  Often an SDA is used in cosmetic products but can also be used in chemical manufacturing, pharmaceuticals, and solvents. Another example is SDA 40-B, which contains tert-butyl alcohol and denatonium benzoate, N.F.. The use of denatured, non-beverage suitable, alcohol in the USA avoids excise taxes on alcohol.
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