*3.2.2 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 ([(NH4)2Ce(NO3)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].

$$\text{Ce(IV)}\_{\text{(Coilow)}} + \text{C}\_{2}\text{H}\_{5}\text{OH}\_{\text{(coilowleio)}} \xrightarrow{\text{K}\bullet\text{q}} \text{Ce(IV)} + \text{C}\_{2}\text{H}\_{5}\text{OH}\_{\text{(coaug\\_rest)}}$$

## **Figure 13.**

*Colorimetric reaction between the reagent Ce(IV) and ethanol. Adapted from Giordano et al. [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].
