## 3 Materials and Methods
### Methods  
#### 3.2.1 pH Sensor Characterization  
The pH of a solution under study can be determined after a meticulous calibration. In order to characterize the pH sensor, the potential difference over the two electrodes must be measured, and related to the hydrogen-ion activity of reference solutions.  
Three pH buffer solutions (Mallinckrodt Baker, Hampton, NJ, USA, manufacture date 31 August 2020) with a 2-year validity certification (at 25 \({}^{\circ}\)C) and traceable to NIST standards were used. The standard buffer solutions employed in the calibration of our instrument were: 1. Biphthalate; 2. Phosphate; and 3. Borate. Correspondingly, the pH and certified range values for the buffer solutions were 4 [3.96, 4.04]; 7 [6.96, 7.04]; and 10 [9.99, 10.01]. Utilizing the batch information of our buffer solutions, the pH reference values for our calibration procedure were: 3.99, 6.98, and 10.00.  
The characterization method is divided into two phases. In phase one, the sensitivity of the pH sensor is determined with a conventional voltage meter, such as a multimeter. It is important to note that the instrument input impedance should be high, and the noise performance optimally below the mV range for accurate calibration. In phase two, the signal conditioning stage, as described in this work is employed, in order to quantify  
Figure 4: (**a**) The pH instrumentation for global VF applications consists of six main components: (1) Glass electrode; (2) Signal conditioning module; (3) Bluetooth communications module; (4) Integration breadboard; (5) Signal acquisition and processing module; and (6) Power supply. (**b**) Representative photo.  
the modified (i.e., including bias and amplification effects) sensitivity of the pH sensor. Hereafter, the methods for both phases are described.  
Phase I does not include the signal conditioning stage. First, the pH sensor is attached to the measuring device using the BNC connector. For this phase, a multimeter (Steren, Ciudad de Mexico, Mexico, MUL-605) is employed to determine the voltage, irrespective of polarity, as a function of buffer solution. Next, the calibration curve that describes the linear behavior of the sensor is calculated by using regression techniques. As seen from Equation (4), the derived slope will serve to relate pH as a function of (sensor) voltage.  
In Phase II, the signal conditioning stage is included. Nonetheless, the above methodology remains practically the same. One of the differences is that in the first step, the pH sensor is attached to the signal conditioning module using the BNC connector. Then, the circuitry of Figure 3 is employed to measure the voltage as a function of buffer solution. Next, the calibration curve that describes the linear behavior of the sensor is obtained by employing regression techniques. Here another difference is in place. The pH is still related to a modified potential difference, which includes the effects of the signal conditioning stage: bias and amplification. The modified potential difference is obtained by dividing the output voltage by the gain and subtracting the bias voltage. The derived slope will serve to relate pH as a function of (instrument) voltage. Finally, it is important to note that, irrespective of the calibration method, the overall cleanliness and mechanical stability, while measuring, of the working electrode are key factors to obtain accurate calibration results.