Fuel dyes

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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".

For example, in United Kingdom the "red diesel", gas oil for heating, is significantly cheaper than heavier-taxed diesel fuel, but it is a different fraction, containing more sulfur. Using it in diesel engines is therefore damaging both to the environment and to the tax collectors, therefore the authorities want it deterred; addition of a dye is one of the methods.

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 here 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.

Some dyes required in some countries are listed here:

Country Fuel Dye
Austria Heating oil any red dye
France Gas oil Solvent Red 24
France Marine diesel Solvent Blue 35
Estonia Heating oil Automate Red NR or similar + Solvent Yellow 124
Estonia Agricultural diesel Automate Blue 8 GHF or similar + Solvent Yellow 124
Germany Heating oil Solvent Red 19 and similar
Greece Heating oil any red dye
Greece Marine diesel any black dye
Ireland Gas oil Solvent Blue 79
Ireland Kerosene Solvent Red 19 and similar
Italy Heating oil Solvent Red 161
Italy Gas oil Solvent Green 33
Portugal Agricultural diesel Solvent Blue 35
Portugal Heating oil Solvent Red 19 and similar
Spain Agricultural diesel any red dye
Spain Heating oil any blue dye
Sweden Heating oil Solvent Blue 35, Solvent Blue 79, Solvent Blue 98
United Kingdom Gas oil ("Red Diesel") Solvent Red 24, quinizarin
United Kingdom Rebated kerosene Coumarin
European Union many rebated Solvent Yellow 124 ("Euromarker")
USA low-tax fuels, high-sulfur fuels Solvent Red 26, Solvent Red 164

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