**2. Material and methods**

#### **2.1 Reagents**

For the accomplishment of this research, the following reagents were used: formaldehyde PA 37% (Impex), 99% chromotropic acid (disodium dihydrate salt, Sigma-Aldrich), and 98% magnesium sulfate (Sigma-Aldrich), all of analytical grade. Distilled water was used for the preparation of all solutions.

#### **2.2 Equipment used in the experiment**

Spectrum spectrophotometer SP 1102; Nova thermostatic shower, model NI 1254; pH meter Hanna, model pH 21; analog rotational viscometer MDJ-1; Centribio centrifuge, model 80-2B; and analytical balance Edutec model FA-2104 N.

*Cosmetovigilance in Hair Straighteners: Determination of Formaldehyde Content… DOI: http://dx.doi.org/10.5772/intechopen.82842*
