## 3 Results and Discussion
### Bioimpedance Measurement Prototype Validation  
The proposed measurement system has been initially validated considering the magnitude and phase response of the electrical impedance of distilled water at various temperature levels. Figure 4 shows the frequency response of the distilled water impedance. One can observe that such a response resembles that of an RC low-pass circuit. High impedance values are found at lower frequencies due to the limited availability of charge carriers. In fact, distilled water exhibits dielectric properties and can be modelled as an equivalent capacitor in series with a resistor. The impact of temperature on the measurements is primarily observed in the low-pass frequency band. The impedance variation regarding the temperature from 10 Hz to 10 kHz has a mean value of \(-53.3\ \Omega/\text{$\mathrm{\SIUnitSymbolCelsius}$}\). However, temperature has a lesser impact on the impedance phase measurements. Figure 5 presents the complex impedance measurements for NaCl saline solutions; it is assumed that NaCl solutions have a simpler electrical model than milk. One can observe that the presence of ions modifies the frequency response and reduces the impedance magnitude in comparison to Figure 4. A decreasing behaviour of the impedance magnitude up to \(10^{3}\) Hz is observed in the results in Figure 5A. At this point, one can notice that the concentration percentage modifies the curve's offset, and the greater the concentration, the closer the curves. The NaCl concentration directly affects the impedance magnitude in an inverse way. In addition to that, the decay of the impedance magnitude with increasing frequency from 10 Hz to \(10^{3}\) Hz is coherent with an impedance electrode response. Thus, impedance measurements should be performed at least one decade greater than \(10^{3}\) Hz. Figure 5B shows the same information, but from another perspective. The inverse behaviour of the impedance magnitude with respect to the NaCl concentration is clear. One should notice that from concentrations greater than \(1.5\,\text{mol}\cdot\text{L}^{-1}\) no significant impedance magnitude variations can be expected. Figure 5C,D show the impedance phase measurements. The predominance of negative angle values is coherent with the presence of capacitive loads. In this experiment, parasitic capacitances are mostly present at the electrode impedances. At higher frequencies, the phase values turn to positive, which is compatible with an inductive load response. Cabling inductance can interfere with the measurements at a frequency range of MHz. From Figure 5D, one should notice that the NaCl concentration affects the impedance phase somewhat. Such a result is coherent with the saline solution's electrical characteristics. Increasing the concentration does not add more reactance to the total load impedance. From this experiment, one can observe that the proposed impedance measurement method works properly within the frequency range from \(10^{3}\) Hz to \(10^{5}\) Hz. Figure 4: Impedance measurements in distilled water at different frequencies and three temperature levels. When a potential difference is applied to a pair of electrodes in contact with a liquid medium, the ions are forced to move because an electric field is present in the sample volume. The electric potential is distributed along the sample, from higher values close to electrode 4 (see Figure 2) to lower values close to electrode 2 (reference). The points within the sample that have the same potential values form the so-called equipotential surface. Figure 6A shows the experimental setup used to evaluate the equipotential lines along a measurement plane at a height of 20 mm from the container bottom. Millimetre paper has been fixed at the top of the container to control the measurement position. Figure 6B shows the copper electrode used to evaluate the potential measurements. Figure 7 shows graphically the voltage magnitude measured at 24 points inside the NaCl solution at a height of 20 mm from the container bottom.