Fuel dyes

Fuel dyes are dyes added to fuels, as in some countries it is required by law to dye a low-tax fuel to deter its use in applications intended for higher-taxed ones. Untaxed fuels are referred to as "dyed", while taxed ones are called "clear".

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For example, in United Kingdom "red diesel" is dyed gas oil for registered agricultural or construction vehicles such as tractors, excavators, cranes and some other non-road applications. Red diesel carries a significantly reduced tax levy than un-dyed diesel fuel used in ordinary road vehicles. As red diesel is widely available in the UK, the authorities regularly carry out roadside checks to detect unauthorised use and penalties are severe.

The dyes used have to be soluble in hydrocarbon-based nonpolar solvents ("solvent dyes"), and therefore in the fuels they are added to. Red dyes are often various diazo dyes, eg. Solvent Red 19, Solvent Red 24, and Solvent Red 26. Anthraquinone dyes are used for green and blue shades, eg. Solvent Green 33, Solvent Blue 35 and Solvent Blue 26.

Due to technological requirements, it is advantageous to mix a liquid with a liquid instead of handling powdered dyes. The pure dyes are solid crystalline materials, therefore they have to be highly soluble, so concentrated solutions can be used instead.

In European Union, after August 2002, all EU countries became obliged to add about 6 mg/l of Solvent Yellow 124, a dye with structure similar to Solvent Yellow 56. This dye can be easily hydrolyzed with acids, splitting off the acetal group responsible for its solubility in nonpolar solvents, and yielding a water-soluble form. Like a similar methyl orange dye, it changes color to red in acidic pH. It can be easily detected in the fuel at levels as low as 0.3 ppm by extraction to a diluted hydrochloric acid, allowing detection of the red diesel added into motor diesel in amounts as low as 2-3%.

In United States of America, the Environmental Protection Agency mandates use of a red dye to identify high-sulfur fuels for off-road use. Solvent Red 26 is used in the United States as a standard, though it is often replaced with Solvent Red 164, similar to Solvent Red 26 but with longer alkyl chains. The Internal Revenue Service mandates use of the same red dyes, in fivefold concentration, for tax-exempt diesel fuels; their argument for the higher dye content is to allow detection even when diluted with "legal" fuel. Detection of red-dyed fuel in the fuel system of an on-road vehicle will incur substantial penalties.

Aviation gasoline is also dyed, both for tax reasons (avgas is typically taxed to support aviation infrastructure) as well as safety -- there being obvious and disastrous consequences of fuelling an aircraft with the wrong kind of fuel.

Some dyes required in some countries are listed here: