# Determination of pH in Powdered Concrete Samples or in Suspension
## 3 Results and Discussion
Note also that the sensor with 5 fingers had higher values of capacitance, the result of a larger active area. The IDC-based structure with 4 fingers has a capacitance equal to 0.52 pF for pH-Low concrete powder and 0.47 pF for pH-High concrete powder, at 8 MHz. We tested two concrete samples, one with a pH value equal to 10.79 (pH-Low, below boundary value = 11) and another one with a pH value equal to 12.24 (pH-High, higher than boundary value = 11). These precise pH values were measured by means of HQ440d Benchtop Multimeter (Hach, CO, USA). This small difference in pH values of tested samples caused the small difference in the capacitance value, but even in this case the difference is enough to make decisions if it is a pH-Low  
Figure 6: (**a**) Infrared image of the microfluidics-based platform at an elevated temperature, and (**b**) Bode plots for pH-Low and pH-High concrete suspensions. Figure 7: Capacitance as a function of frequency for pH-Low and pH-High concrete powder with (**a**) four and (**b**) five interdigitated fingers/electrodes. or pH-High sample, which has huge practical importance. The difference in capacitance was even higher when the IDC-structure with 5 fingers was used. These values are 0.93 pF for pH-Low, and 0.54 pF for pH-High concrete powder (at 8 MHz). This difference is a consequence of an additional \"mini\" capacitor created between the fourth and fifth fingers in the structure with five gold electrodes, but also the interaction of this fifth finger with all others, bearing in mind that electrical field lines are closed among all fingers in this coplanar structure through the sample of concrete from the top side. When this IDC-based platform was covered with concrete samples in powder form, the relative permittivity between the electrodes of the planar capacitive structure was increased. The real part of the complex relative permittivity of concrete is in the range from 4 to 12 [23], compared to the dielectric constant values in air, which is equal to 1. Therefore, when the concrete powder particles enter between electrodes, it causes the capacitance to increase. It can be concluded from Figure 7 that in the high frequency range capacitance starts to increase, which means that resonant frequency will occur at frequencies which are out of the measuring range of the applied Impedance Analyzer HP4194 (\(\leq\)40 MHz). The proposed platforms can be used on site, near to the construction structure, using our in-house developed portable and low-power microprocessor system for remote measuring of environmental parameters [24], including pH value. That means instead of a bulky Impedance Analyzer (in the laboratory), on site we can use our small, portable, impedance/capacitance meter, along with the presented sensor platforms in this paper. Thanks to the selected fabrication technology, the proposed platforms are reliable and have long-term stability. Related to the reusability, for the microfluidics-based platform it is necessary to remove the previous suspension and to fill the channel with another one, whereas the IDC-based platform needs to be cleaned with a brush and covered with new powdered concrete samples. The presented sensor lifespan is mainly determined by the endurance of the gold layer. The interdigitated sensors platform is more prone to the appearance of scratches in the gold electrodes (Figure 8) due to using powdered concrete which limits its usage to approximately hundred times. The microfluidics-based structure does not have this limitation bearing in mind that the suspension is used and the applied pressure on the syringe during the filling channel is not high.