*3.1.1 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 [27–29], 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.

### **Figure 1.**

*Chemical reactions involved in the test for esters known as the hydroxamic acid test, used to test for the presence of biodiesel in fossil diesel. Adapted from Silva et al. [29].*

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**Figure 2.**

*Santos [30].*

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

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 Fe3+ ions. The complex formed, the intensity of whose color is proportional to the biodiesel content, is extracted with *n*-hexane/*n*heptane, 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

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**).

*Color scale of samples of fossil diesel containing biodiesel at 0–6% v/v, showing intensities proportional to the concentrations of biodiesel in the diesel, for semiquantitative analysis observable by the naked eye. Source:* 

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

was proposed for qualitative analyses for use at filling stations [29, 31].
