**2. Colorimetric methods for monitoring fuel quality in laboratories**

The colorimetric assays used on a routine basis in fuel quality control laboratories in Brazil are visual color, ASTM color, and the copper strip corrosion test.

In Brazil, hydrous ethanol should be colorless, while anhydrous ethanol has an orange dye added to it. This difference is designed to prevent the kind of fraud known as "wet ethanol," whereby fraudsters acquire anhydrous ethanol directly from the plants that produce it, evading taxes, then mix it with water, and sell it on as hydrous ethanol. Regular gasoline, additized gasoline, and premium gasoline can be told apart visually by their color: regular gasoline ranges from colorless to yellow, while the other two are colored with a dye, which may be of any color but blue (which is reserved for aviation fuel), but which is normally green for additized and purple for premium [11, 21]. All the dye does is enable the two products to be distinguished from one another; it has no effect on their specifications. Meanwhile, diesel S10 (containing 10 mg kg<sup>−</sup><sup>1</sup> sulfur) is naturally yellow in color, while S500 (500 mg kg<sup>−</sup><sup>1</sup> sulfur) is dyed red to prevent incorrect refueling with this diesel, whose high sulfur level means it could cause damage to vehicles with more sophisticated engine technologies like selective catalytic reduction or exhaust gas recirculation systems.

The standard test method for the ASTM color of petroleum products (ASTM 1500:2012) is used on samples of diesel S10 (which contains no dye). The test involves transferring a sample of the product to a test tube and comparing its color with the color of standard colored glass disks, which represent a range of values from 0.5 to 8.0. A standard light source is used in this test [22]. Corgozinho et al. have developed a method using partial least squared regression with molecular absorption spectrophotometry to determine the ASTM color of diesel in the 0–6.5 range, offering greater precision than the standard colorimetry test [23].

Corrosiveness is determined using the copper strip test in Brazil (ABNT NBR 14359), the USA (ASTM D130), and Europe (EN ISO 2160). It consists of immersing a standard copper strip in a sample of biodiesel in a corrosion bath for 3 hours at 50°C. The strip is then rinsed in solvent and compared with an ASTM copper strip corrosion standard, which has different colors that correspond to different levels of corrosion, classified from 1 to 4 [24–26].
