# Determination of pH in Powdered Concrete Samples or in Suspension
## 3 Results and Discussion  
We analysed two concrete solutions, one denoted with pH-Low and another with pH-High. Capacitance between parallel plate electrodes was measured with these two suspensions in separate channels. The measured results are shown in Figure 5.  
We selected three frequency points (Figure 5), and the results demonstrate that for each frequency, capacitance of pH-Low concrete solution was higher than that of the pH-High concrete solution. The obtained capacitance was in the range from 86.3 pF to 92.5 pF. It can be noticed that capacitance values decreased with increasing frequency, due to the same behaviour expressed by the dielectric constant of the fluid. It is known that the value of the dielectric constant decreases from the static state (low frequency range) through all four polarizations, up to the highest frequencies. The capacitance of the parallel plate electrodes capacitive structure can be calculated by  
\[C=\frac{\varepsilon_{0}\varepsilon_{r}\,A}{d}, \tag{1}\]  
where \(\varepsilon_{0}=8.85\times 10^{-12}\frac{F}{m}\), \(\varepsilon_{r}\) is the relative permittivity of the fluid inside the channel, \(A\) is the surface area of the parallel plate electrodes and \(d\) is the distance between them. If we apply the dimensions of our structure in the mentioned equation, the total capacitance is 92.93 pF (for a relative dielectric constant equal to 1, which is well in agreement with results of measurements presented in Figure 5). It can be concluded that a less alkaline solution (pH-Low) will have higher capacitance, taking into account that the solution has a higher relative dielectric constant. Namely, the concrete belongs to dielectric materials which are characterized by the fact that valence electron shells are nearly full [22]. The relative permittivity of concrete is larger when it is easier for the material to polarize, meaning that the ions are mobile and there is little crystallization. The dielectric constant can be increased by adding chlorides as well as additional amounts of water. The relative permittivity of concrete is also dependent on its aggregates. Using the proposed microfluidic device it is possible to quickly provide a binary response: (a) this is a pH-Low concrete solution, which would indicate an alarm for intervention in that construction, or (b) this is a pH-High concrete solution, an indication that there is low risk of pH-related corrosion in the concrete and the concrete structure should have appropriate integrity. In order to explore the influence of temperature, the presented platform was heated at 35 \({}^{\circ}\)C, as can be seen in the infrared image shown in Figure 6a. Figure 6b presents the so-called Bode plot, i.e., measured impedance as a function of frequency (both in logarithmic scale). Figure 5: Measured capacitance for two different concrete solutions at room temperature. It can be concluded that the pH-High sample has higher impedance due to the inverse dependence between capacitance and impedance. It is important to note that relative permittivity of concrete decreases with increasing temperature, because the amount of water is reduced during this time. This will result in a slight decrease in the output capacitance with increasing temperature. For example, when the temperature was increased from 25 \({}^{\circ}\)C to 35 \({}^{\circ}\)C, the capacitance decreased for approximately 3%. For determination of a more precise pH value we propose an IDC-based structure. This platform can also be exposed to liquid medium, but we successfully applied concrete in powder form to the upper side of the gold interdigitated electrodes system. The capacitance as a function of frequency was experimentally measured at room temperature for both 4- and 5-fingered gold electrode designs, and results are presented in Figure 7.  
As can be seen in Figure 7, both IDC-based capacitive sensors showed similar behaviour: The capacitance increased with increased alkalinity.