Fuel Quality Monitoring by Color Detection

The quality of automotive fuels—gasoline, diesel, biodiesel, and ethanol—is discussed in the light of their specifications in different countries and regions and in terms of their adulteration, which has such a prejudicial effect on the production chain, distribution chain, tax revenues, the environment, and end consumers. Different ways of adulterating automotive fuels are analyzed, as are the procedures for their detection. Several analytical methods for monitoring quality and detecting adulteration have been addressed in the literature, emphasizing the determination of properties such as density, distillation curve, octane rating, vapor pressure, etc., by means of physicochemical methods and chromatographic and spectrographic techniques, to the detriment of colorimetric methods. This chapter looks at colorimetric techniques designed for quality monitoring and the detection of adulteration in fuels, especially simple, quick, low-cost procedures with potential to be used in the field.


Introduction
The quality of a fuel is associated with its fitness for use, the minimum requirements for which are given by its specifications, defined as a set of characteristics and their respective limits, which are required to ensure its good performance in engines. A comparison of the specifications for gasoline, ethanol, diesel, and biodiesel in different countries and regions is provided in the Supplementary Material to this chapter.
The failure of a fuel to meet any of its specifications is referred to as a nonconformity, which means the fuel is not fit for use. Nonconformity is not always due to adulteration, which is the deliberate and illegal addition of lower-cost substances to a fuel, usually with tax evasion, all with the purpose of increasing the profit margin by illicit means, resulting in reduced tax revenues, unfair competition, harm to the environment, and increased wear and tear of vehicle engines [1][2][3][4][5]. In fact, a fuel may be adulterated and yet stay within its specifications, in which case the only detrimental effect is on tax revenues [6]. It is a challenge to detect the adulteration of fuels because adulterants usually include compounds that are already present in the fuels themselves [2,6].
The illegal practice of fuel adulteration may impair the fuel's quality while also resulting in increased environmental contamination by polluting gases and the Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants. Meanwhile, in Japan there are two standards for fuels: the mandatory standards set by the Central Environment Council, under the Ministry of the Environment, and the Japanese Industry Standards. The former are statutory requirements and regulate the Act on Quality Control for Gasoline and Other Fuels, which has been enforced since April 1996 [19]. Tables 1-4 showing the gasoline, diesel, ethanol, and biodiesel specifications for each country or region are included in the Supplementary Material. Table 1. Gasoline specification. Table 3. Ethanol fuel specification. Analytical methods for monitoring gasoline and diesel adulteration were the target of a recent review [2], emphasizing the determination of their different properties, such as density, distillation curve, octane rating, vapor pressure, etc., using physicochemical methods and chromatographic and spectroscopic techniques. Nondestructive analytical methods for identifying off-spec levels of biodiesel in diesel-biodiesel blends and adulteration of the blend have also been covered in a recent review [20], focusing on first-order multivariate calibration models. In this chapter, different colorimetric techniques that can be used in fuel quality control are addressed, especially simple, rapid, low-cost methods that have the potential capacity to detect fuel adulteration in the field.

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 −1 sulfur) is naturally yellow in color, while S500 (500 mg kg −1 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][25][26].

In a diesel-biodiesel blend
The level of biodiesel in a diesel-biodiesel blend is determined in a laboratory by observing the mid-infrared absorption of the C〓O bond present in the fatty acid methyl esters (FAMEs) that constitute the biodiesel. Although this test gives satisfactory results, it cannot be used in the field, which is a major drawback, as fuel distributors need to test the fuel when they are adding biodiesel to diesel on a commercial scale. Furthermore, the accuracy of the results will be compromised if the biodiesel itself is adulterated with vegetable oil or animal fat, whose constituent triglycerides also have C〓O bonds.
The conversion of esters into hydroxamates by their reaction with hydroxylamine [27][28][29], forming complexes of a red to violet color caused by the reaction with Fe3 + , has been used to confirm esters in organic analysis procedures. Figure 1 shows the reaction by which this complex is formed.   Silva et al. used this test to develop a new spectrophotometric method to determine biodiesel levels in diesel-biodiesel blends ranging from B0 (pure fossil diesel) to B5 (fossil diesel containing 5% biodiesel), in which the esters from the biodiesel present in the mixtures react with hydroxylamine chlorhydrate in an alkaline solution, forming alkaline salts from the hydroxamic acid, followed by acidification, to form hydroxamic acid, followed by a reaction with Fe 3+ ions. The complex formed, the intensity of whose color is proportional to the biodiesel content, is extracted with n-hexane/nheptane, forming an upper phase with a yellow, orange, or red color, depending on the biodiesel concentration, as shown in Figure 2. Absorbance can be measured using spectrophotometry at 420-440 nm. The linearity, limit of detection, limit of quantification, accuracy, selectivity, and specificity results indicate that the proposed method is adequate for analyses of biodiesel in diesel-biodiesel blends. A simplified method was proposed for qualitative analyses for use at filling stations [29,31].
Using the same reaction, Santos developed a simple, practical, rapid method that could be used in the field for semiquantitative identification by the naked eye of biodiesel in diesel-biodiesel blends ranging from 0% to 6% v/v biodiesel [30]. A similar method was developed by Leite and Fernandes for the 0-7% range [32]. What sets these methods apart from their predecessors is the separation of the biodiesel from the blend before the hydroxamic acid test. To do this, solid-phase extraction is conducted using a chromatographic column adapted from a 3mL disposable plastic syringe into which around 0.4 g silica gel is inserted, supported by a cotton plug (see Figure 3a). The diesel-biodiesel blend is introduced as if in frontal chromatography, pushed by a plunger, generating a less polar diesel fraction (F1) followed by a more polar fraction containing biodiesel (F2), displaced by the addition of ethanol (see Figure 3b).
By using seven reference samples of the diesel-biodiesel blend containing 0, 1, 2, 3, 4, 5, and 6% (v/v) biodiesel, the concentrations of which were confirmed by the laboratory reference method [33], a standard table of colors was prepared that could be used for the semiquantitative analysis of the percentage of biodiesel, as in Figure 2, since the intensity of the color formed by the ferric hydroxamate complex is proportional to the level of biodiesel present in the sample. By comparing the observed colors with the standard color table, it was possible to determine the biodiesel concentration semiquantitatively with the naked eye, with a relative error of around 1%. This could be done in the field to find out whether samples are off spec. The proposed method was used on 33 samples of diesel containing different biodiesel concentrations and was found to be equivalent to the laboratory reference method (EN14078) by Student's t-test, with 95% confidence [30].
Another method-also adaptable for use in the field-has been developed for determining biodiesel in a diesel-biodiesel blend and for identifying the presence of vegetable oil in this blend. After solid-phase extraction using a silica stationary phase (EFS 1 ), the fraction composed of biodiesel and potentially vegetable oil is then put through another solid-phase extraction with an aminopropyl stationary phase (EFS 2 ) to separate the biodiesel from any vegetable oil (Figure 4a). For use in the field, the stages involving a manifold for solid-phase extraction and nitrogen-supported solvent evaporation are replaced by the manual use of plungers in the solid-phase extraction cartridges, which has yielded viable preliminary results. Figure 4b illustrates the procedures that can be done in the field to obtain the complexes. Figure 5 also illustrates the use of the hydroxamic acid test on both fractions to confirm the presence of esters, which has also proven satisfactory for indicating adulteration with 1% or more vegetable oil in diesel containing 5% biodiesel [32].
Any sample identified in the field as potentially adulterated with vegetable oil could be sent to a laboratory for confirmation via more precise analytical techniques, like high-performance liquid chromatography. As such, both methods (semiquantitative identification of the percentage of biodiesel in diesel and the identification of any vegetable oil in the blend) are potentially effective for field analyses not only for their ease of use but also for their speed. Furthermore, the fact that the methods use low-cost, low-toxicity materials and reagents makes them even more attractive.
The sequence of operations for the hydroxamic acid test, illustrated in Figure 5, is easily adapted for field analysis.
Costa has also developed an alternative method for determining the level of biodiesel in fossil diesel using the hydroxamic acid test in association with image processing and a colorimeter and compared it with the standard method (EN 14078). After the hydroxamic acid test, the images were photographed, and the  Colorsys program was used to create calibration curves based on color vs. biodiesel concentration, enabling the determination of biodiesel content by the intensity of the color. It was found that the hydroxamic acid test associated with image processing was adequate for detecting and quantifying the biodiesel content of dieselbiodiesel blends because the results given by the method were statistically similar to those given by the reference method using infrared spectroscopy (EN 14078). Furthermore, it is easily adapted for field analysis (Figure 6) [34].  Solvatochromism is the term used to describe the phenomenon of color change of a solute when it is dissolved in different solvents, resulting in a change in the position of absorption or emission of its spectroscopic band. When these changes of position occur in the visible range, they exhibit different colors that can be detected by the naked eye and can be used as sensors to determine the composition of mixtures [59][60][61].
Fong and Xue have developed an optical sensor to detect biodiesel in dieselbiodiesel blends based on the solvatochromic behavior of the dye called Nile Blue chloride, which turns blue when dissolved in ethanol or methanol and pink when in contact with biodiesel. Figure 7 illustrates the change in color, enabling the direct, rapid detection of variations from 0.5 ppm to 20% v/v biodiesel in diesel-biodiesel blends [35].
As Nile Blue chloride is not soluble in nonpolar solvents, like diesel, it was encapsulated in an ethyl cellulose polymer, dissolved in methanol or ethanol in the form of a fine film that turns blue in alcohols, with absorbance at 610 nm, and pink when   in contact with biodiesel, with absorbance at around 520 nm. Exposing the sensor containing the Nile Blue chloride to diesel resulted in no change in color or change in its stability. When it was exposed to biodiesel, it changed from blue to pink, which can be attributed to the alcohols surrounding the sensor being replaced by FAMEs from the biodiesel, resulting in a solvatochromic effect. The sensor provides a quick, direct method compared with methods involving gas chromatography or infrared spectrophotometry, presenting a linear response between color intensity and biodiesel concentration for low concentrations, between 0.5 and 30 mg kg −1 . For high concentrations of biodiesel (up to 20% v/v), absorbance may be linearly adjusted by a logarithmic function [35].

Free and total glycerin
Free or total glycerin in biodiesel can be determined by spectrophotometry, since the reactions shown in Figure 8 result in a colored compound, as initially described by Greenhill [37]. Several publications have used similar methods, taking advantage of kits used to analyze triglycerides in blood [36,[38][39][40]. In the presence of adenosine-5'-triphosphate (ATP) and the enzyme glycerol kinase (GK), glycerol produces diphosphate adenosine-5'diphosphate (ADP) and glycerol-3-phosphate (G-3-P; reaction 1), which, in the presence of glycerol phosphate oxidase (GPO) and oxygen, produces dihydroxyacetone phosphate (DAP) and hydrogen peroxide (H 2 O 2 ; reaction 2). Then, in the presence of a peroxidase, an oxygen acceptor (OA), and 4-aminoantipyrine (4-AAP), water and a colored compound (CC) are formed (reaction 3): To determine total glycerin, it is necessary to use solid-phase extraction in an aminopropyl cartridge to separate the biodiesel (FAMEs) from the fraction containing the free glycerin (FG), monoacylglycerols (MAGs), diacylglycerols (DAGs), triacylglycerols (TAGs), and combined glycerin, as shown in Figure 9a [36]. To determine free glycerin, the biodiesel (FAME) is separated and isolated from the fraction containing the free glycerin (FG) by solid-phase extraction in a silica cartridge (see Figure 9b).

Ethanol
Budag et al. have used four dyes as solvatochromic probes to study the effect of adding ethanol to pure gasoline. The dye 2,6-di(4-tert-butylphenyl)-4-[2,4,6-tri (4-tert-butylphenyl)pyridinium-1-yl]phenolate (t-Bu 5 RB), for example, has a greenish blue color in gasoline, is violet in ethanol, and is bluish green in a mixture of 25% ethanol in gasoline, which means ethanol in gasoline can be detected by the naked eye. The development of analytical methods to determine fuel quality based on these promising probes has been discussed. Based on the spectroscopic determination of the maxima on the UV-visible spectra, the respective transition energy can be calculated, which is related to the composition of the fuel, opening up the potential for the development of an analytical method [41]. to make quantitative analyses of bioethanol and mixtures of bioethanol with gasoline and water in an experiment based on a constructivist approach for undergraduates in a chemistry class [43].
Solutions of the dye in ethanol-water and anhydrous ethanol-gasoline mixtures evidenced solvatochromic effects, with the color changing according to the composition of the mixture, as shown in Figure 11.
The results of using solvatochromic effects of different dyes in mixtures, especially involving additized or altered fuels, seem extremely promising for the development of alternative colorimetric methods with field applications.

Methanol
The maximum methanol content permitted in ethanol and gasoline in Brazil is 0.5% v/v, and the reference method for its detection uses gas chromatography [44].   In view of cases of ethanol adulteration with methanol identified in the country, a colorimetric kit has been designed for use in the field to identify this form of adulteration of anhydrous ethanol and hydrous ethanol and in gasoline with ethanol [45][46][47].
The procedure is based on visual detection of a violet blue color as a result of the reaction of aldehydes with Schiff 's reagent used in qualitative organic analysis to identify functional aldehyde groups [48,49]. The color is obtained using the aforementioned colorimetric kit by the following reactions: The nonappearance of a color indicates the absence of methanol. When the solution turns violet blue, this is a qualitative indication of the presence of methanol in the fuel. A comparison of the intensity of the color with that of a reference solution containing 0.5% v/v methanol (the maximum permitted) enables conformity/nonconformity with specifications to be determined in the field. Figure 12 shows the results of the samples of hydrous ethanol containing 0, 0.3, 0.5, and 1.0% methanol, showing the photos of the solutions after the test sequence. The higher the concentration of methanol in the sample, the stronger the blue of the solution. As expected, the blank, containing just ethanol, is colorless.  (pure ethanol, W30, W50, W70, W90). Adapted from Galgano et al. [43].

Figure 12.
Schiff test applied to samples of pure ethanol and ethanol contaminated with 0.3, 0.5, and 1.0% methanol.

Water
In Brazil, ANP Resolution #19, of 2015, states that hydrous ethanol fuel may contain up to 7.5% water [50]. Giordano et al. have developed a simple, rapid, portable, low-cost method that can be used in the field to indirectly determine water in ethanol fuel. The system is based on a colorimetric reaction between ethanol and ceric oxide (Ce(IV)), as shown in Figure 13. When a solution of ammonium cerium (IV) nitrate ([(NH 4 ) 2 Ce(NO 3 ) 6 ], which is yellow in color, is mixed with ethanol, an intense orangey-red color immediately appears because of the formation of Ce(IV)ethanol (1:1) complexes [51].
The device is made up of mechanically connected layers of acrylic, a sample reservoir, a light source, and a detector. A solution containing the reagents is added to the reservoir, together with aqueous solutions of ethanol (standards or real samples) prepared at 0.2-5.0% v/v and transferred to the reservoir. The volume introduced is approximately 70 μL. As soon as the solutions are mixed, the reactions take place, as indicated by a change in color from yellow to orangey red.
Real samples of ethanol obtained from filling stations in Brazil were tested. The water concentrations were compared with those obtained by Karl Fischer titration (ASTM E203, reference method), and the data obtained by the two techniques were found to be statistically equivalent by Student's t-test, at a 95% confidence level [51,52].

Methanol
The method for determining methanol in gasoline containing ethanol, applicable in the field, was described in 3.2.1.

Detergent-dispersant additives
Gasoline is additized with detergent-dispersant additives, which are designed to reduce the formation of deposits in engines and valves, enhance car engine efficiency, and ensure a cleaner combustion process, all of which has a positive impact on the environment. Although different types of additized gasoline have been available on the market for a long time, there is no reference method for analyzing the detergent-dispersant additives they contain. Figure 14 illustrates the structure of some of the additives available on the market.   One factor that hampers the development of analytical methods for detergentdispersant additives is that they are sold as packages, with the additives already dissolved in solvents compatible with the hydrocarbon composition of the fuel, without indicating their respective composition.
Santos et al. have developed two laboratory methods for analyzing detergentdispersant additives in gasoline. One uses thermogravimetry to identify the type of additive in the gasoline, which is then quantified by size-exclusion chromatography with refraction index detector, while the other just uses thermogravimetry [53][54][55].
d'Avila and Souza have developed a method that can be used in the field for the qualitative and/or semiquantitative identification of detergent-dispersant additives in fuels and lubricating oils. It is based on differences in the chromatographic behavior, in nonpolar stationary phases like silica, of nonpolar fuels and oils that form the basis for lubricants, which, due to their hydrocarbon composition in nonpolar stationary phases like silica, are retained less than detergent-dispersant additives, which are polar. The basicity of amine groups in additives could be used to reveal their position in the chromatography column by the addition of different acid-base indicators, making the presence or absence of the additives in the fuels and lubricating oils unequivocal. The process could be used by customers, inspectors, producers, or even by distributors-i.e., by operators who are not necessarily technically qualified-because of the simplicity of the procedure and the interpretation of the results, as shown in Figure 15, for a gasoline additized with ethanol, as is commonplace in Brazil and in many other countries [56]. The stages are: 1. Extraction of the ethanol from the gasoline with a saline aqueous solution, as described in Brazilian standard NBR13992, which is available and executable at all fuel distribution stations to easily determine the ethanol content in the gasoline 2. Continuous introduction of gasoline without ethanol in the column made of a Pasteur pipette containing a polar adsorbent (e.g., silica), using a hypodermic syringe 3. Addition of an alcoholic solution of different acid-base indicators, followed by the addition of ethanol to remove any excess, revealing the presence of additive in the column due to the "turning" of the indicator and its change of color because of the change in pH caused by the presence of the additive in the column When the fuel contains a detergent-dispersant additive, a colored ring is formed when the indicator changes color (in Figure 15 exemplified by bromothymol blue), enabling qualitative identification by the naked eye. Semiquantitative identification may be possible, as the intensity of the color is proportional to the concentration of the additive in the fuel.

Adulteration
Lee et al. (2011) have developed a sensor to detect gasoline adulteration based on the solvatochromic behavior of a polydiacetylene (PDA) conjugated polymer embedded in matrix polymers of polystyrene and polyacrylic acid in the form of fibers. When exposed to pure and adulterated gasoline, they change to different colors and also change color in the presence of potential adulterants like paint thinner, methanol, and toluene (see Figure 16) [57].

Conclusions
The quality and adulteration of fuels have been the object of several publications, especially with the use of modern analytical techniques, such as chromatography, spectroscopy, and chemometric methods, but with very few examples of colorimetric techniques. Among the colorimetric techniques shown here, developed for fuel quality and adulteration monitoring, especially those that are based on simple, rapid, low-cost procedures with potential for application in the field, we highlight those traditionally used for the qualitative identification of functional groups in organic analysis. The colorimetric methods for detecting biodiesel and vegetable oil adulterants in diesel-biodiesel blends are based on the qualitative analysis of carboxylic esters using the hydroxamic acid test, which produces a colored ferric hydroxamate complex. The colorimetric detection of methanol in ethanol and in gasoline was also based on a reaction for identifying aldehydes by the reaction for the formation of a Schiff base, which is colored. These assays are based on spot tests developed and described by Fritz Feigl and colleagues, whose publications [27, 48, 58] describe an incommensurable archive of spot tests used in organic and inorganic analyses, which could serve as inspiration for the development of other colorimetric assays. Likewise, the solvatochromic effect of dyes and fibers, used as probes or sensors, has been used to detect biodiesel in diesel-biodiesel blends and water or ethanol in gasoline and even for detecting adulterants in gasoline. Publications that use solvatochromic effects for the analysis of fuels based on undergraduate experiments-showing their operational simplicity-are also highlighted [43]. The solvatochromic effect has been the target of several studies by Christian Reichardt and colleagues and also represents an opportunity for the development of new colorimetric techniques [59][60][61]. Photos after the exposure of the PDA fibers in polystyrene to commercial and adulterated gasoline, thinner, methanol, and toluene on glass (a) and on silica TLC plate (b). Adapted from Lee et al. [57].