where GR represents the global response (as defined in the Section 2.3),
and [S], [HNO3] and [V] are the concentration of surfactant (Triton X-114
or Triton X-100), the concentration of HNO3 and the volume of aqueous
solution employed for emulsification, respectively.
From the application of such derivates, two systems with three
variables and three linear equations were obtained. The resolution of the
systems was performed to find the critical conditions for each surfactant
by simple application of the least-squares methodology.
As mentioned previously, the values of the variables found for the
critical point are the optimum values for each variable, once they
represent the point of the function where the global response is
maximized. In fact, this point must represent the best experimental
condition for the detergent emulsion formation aiming the Cu, Fe and Pb
determination in the naphtha samples, taking into account the individual
characteristics of the three metals. In the experiments using Triton X-100