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" or "white".
The dyes used have to be soluble in the fuels they are added to and therefore in hydrocarbon-based nonpolar solvents ("solvent dyes"). Red dyes are often various diazo dyes, e.g., Solvent Red 19, Solvent Red 24, and Solvent Red 26. Anthraquinone dyes are used for green and blue shades, e.g., Solvent Green 33, Solvent Blue 35 and Solvent Blue 26.
The pure dyes found in modern liquid petroleum dyes are essentially longer alkyl side chain forms of traditional dyes and normally multiple chain length variations of the chromophore are found within a typical commercial liquid petroleum dye. For instance, Sudan Red 462 is essentially Solvent Red 19, with the ethyl side chain replaced by either a 2-ethylhexyl or a tridecyl side chain. The longer branched side chains improve solubility dramatically, but in some cases the high solubility prevents the dye being isolated as a crystal, except at very low temperatures. The high solubility liquid dyes originated with Morton International and BASF (ACNA Italy) as the primary inventors. For instance, Morton International created Solvent Blue 98 as a high solubility form of Solvent Blue 35. BASF created Solvent Blue 79 as its high solubility form of Solvent Blue 35. In some cases it is possible, with normal solvents—e.g., xylene—to prepare stable (to -20C) solutions at 65% "solids" content. The original powder dye form of the chromophore would not be soluble beyond 2% in xylene.
Only a few refineries worldwide still use powder dyes for colouring fuels, as ultimately they are still lower cost per active molecule of dye chromophore than the modified forms. They have significant handling issues and health and safety issues that inherently arise from the handling of azo dyes (reds/yellows/green mixes). It is advantageous to mix a liquid with a liquid instead of handling powdered dyes into a liquid.
Aviation gasoline is dyed, both for tax reasons (avgas is typically taxed to support aviation infrastructure) as well as safety (due to the consequences of fueling an aircraft with the wrong kind of fuel).
Fuel dye in the European Union
After August 2002, all European Union countries became obliged to add about 6 mg/L of Solvent Yellow 124, a dye with structure similar to Solvent Yellow 56, to heating fuel. 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 the 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 such as boats. Red diesel carries a significantly reduced tax levy compared to 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. Unauthorized use incurs heavy fines but despite this spot checks have occasionally found as many as one in five motorists using red diesel.
Red diesel can also be used in road vehicles which are registered as SORN with the DVLA provided they are only used on private land. On 14 July 2014, the European Commission announced it was referring the United Kingdom to the European Court of Justice over the use of red-diesel in propelling private pleasure craft on water. It believes the UK is not properly applying EU regulations for the fiscal marking of fuels. 
On 18 November 2014, a new measure to combat fuel laundering should result in the illegal trade being "virtually eliminated" in the United Kingdom, according to the HM Revenue and Customs. A new dye will be introduced in April 2015 in the United Kingdom and the Republic of Ireland.
Carbon Offset Red Diesel
Carbon Offset Red Diesel is now available in the UK. It is an environmentally friendly alternative to regular red diesel. There is an extra cost incurred when purchasing Carbon Offset Red Diesel, however some suppliers of the fuel are donating the extra cost to projects aimed at lowering carbon emissions, meaning they make no extra profit from the sale of Carbon Offset Red Diesel.
Currently there are no naked-eye visible dyes in car fuels sold in Poland. However, during the time of Communist Party rule, the state-owned CPN fuel monopoly dyed leaded gasolines (marketed as "ethilins") in the following colors: 78 - blue, 86 - green, 94 - yellow, 98 - red. Diesel fuel, although unleaded, was also dyed a brown color.
Fuel dye in North America
In the 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, which is similar to Solvent Red 26 but with longer alkyl chains. The Internal Revenue Service regulation 26 C.F.R. 48.4082-1 mandates use of the same red dyes, in fivefold concentration, for tax-exempt diesel fuels such as heating oil; 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.
Processing fuel to remove the dye so it may be illicitly sold to motorists is a recognized criminal activity in Ireland and the United Kingdom. In Northern Ireland and the Republic of Ireland it has been a means of fund raising by illegal paramilitary organizations. In 2004, Northern Irish police discovered an illegal facility capable of removing the dye from 12 million litres of fuel per year. In 2009, customs officials shut down a plant capable of removing the dye from 6.5 million litres of fuel per year. In 2011, a plant capable of processing 30 million litres was discovered.
Some dyes required in some countries are listed here:
|Australia||Regular unleaded petrol||purple/bronze (Changed to red/orange in 2013) |
|Premium unleaded petrol||yellow|
|Austria||Heating oil||any red dye and Solvent Yellow 124|
|Canada||Off Road Fuel ( Agriculture, Construction, Mining etc..)||red/purple dye|
|Heating oil||any red dye|
|Finland||Heating oil||Furfural and Solvent Yellow 124|
|Diesel for construction and agriculture||Furfural and Solvent Yellow 124|
|France||Gasoil oil (heating/diesel off road)||Solvent Red 24 and Solvent Yellow 124|
|Marine diesel||Solvent Blue 35|
|Estonia||Heating oil||Automate Red NR or similar|
|Agricultural diesel||Automate Blue 8 GHF or similar|
|Germany||Heating oil||Solvent Yellow 124 + 4.1 gr/litre Solvent Red 19 or 5.3 gr tolyazotolyazo-ethylhexylbetanaphthylamine or 6.1 gr tolyazotolyazo-tridecylbetanaphthylamine & similar|
|Greece||Heating oil||any red dye|
|Marine diesel||any black dye|
|Ireland||Gas oil||green dye = Solvent Yellow 124 and Anthraquinone Blue dye equivalent to Solvent Blue 35|
|Kerosene||Solvent Red 19 and similar|
|Italy||Heating oil||Solvent Red 161|
|Gas oil||Solvent Green 32 or 33 and Solvent Yellow 124|
|Netherlands||Agricultural diesel||any red dye and Solvent Yellow 124 the additive Furfural is obsolete|
|Norway||Agricultural diesel||any green dye|
|Portugal||Agricultural diesel||Solvent Blue 35|
|Heating oil||Solvent Red 19 and similar|
|Spain||Agricultural diesel||any red dye + Solvent Yellow 124: Orden PRE/1724/2002 of the 5th of July.|
|Heating oil||any blue dye + Solvent Yellow 124: Orden PRE/1724/2002 of the 5th of July.|
|Sweden||Heating oil||Solvent Blue 35, Solvent Blue 79, Solvent Blue 98 and Solvent Yellow 124|
|Thailand||Gasoline 95||yellow dye|
|Gasoline 91||red dye|
|United Kingdom||Gas oil ("Red Diesel")||Solvent Red 24, quinizarin and Solvent Yellow 124|
|Rebated kerosene||Coumarin and Solvent Yellow 124|
|Europe||many rebated||Solvent Yellow 124 ("Euromarker")|
|United States||low-tax fuels, high-sulfur fuels||Solvent Red 26 3.9lbs per 1000 barrels, Solvent Red 164|
|Worldwide||Aviation gasoline 80/87||red dye|
|Aviation gasoline 82UL||purple dye|
|Aviation gasoline 100LL||blue dye|
|Aviation gasoline 100/130||green dye|
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