**Figure 6.**

*Schematic representation of procedure for taking pictures of samples (front and perspective views). Source: Costa [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 [59–61].

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].

### **Figure 7.**

*Change in color of Nile Blue chloride from blue (without diesel) to pink (exposed to biodiesel), due to solvatochromic effects. Adapted from Fong and Xue [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

**95**

*Fuel Quality Monitoring by Color Detection DOI: http://dx.doi.org/10.5772/intechopen.86531*

adjusted by a logarithmic function [35].

*3.1.2 Free and total glycerin*

(reaction 3):

**Figure 8.**

cartridge (see **Figure 9b**).

**3.2 Ethanol**

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<sup>−</sup><sup>1</sup>

For high concentrations of biodiesel (up to 20% v/v), absorbance may be linearly

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, 38–40]. 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 (H2O2; 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

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

*Glycerol reactions generating a colored compound analyzable by spectrophotometry. Adapted from Muniz et al. [36].*

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-Bu5RB), 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].

.
