Low Frequency Analysis of Acidic and Neutral Electrolytes with a Highly-Sensitive Microfluidic Sensor
Authors: Ye Jia-Hao , Ho Kung-Chu , Su Vin-Cent , Kuan Chieh-Hsiung 
Published: February 2020

INTRODUCTION

The electrolytic behavior operated in low frequency regime are investigated through an high-sensitivity liquid sensor realized with a specific microfluidic device and high resolution instruments. The better repeatability and accuracy are confirmed at higher concentrations from 10-4 to 10-3 M, especially in lower end of the frequency. The dielectric constant decreases with increase in frequency, and finally approaches to a constant at higher frequencies. All solutions appear relaxation frequency at each peak value of dielectric loss. The relaxation frequency and AC conductivity of acidic electrolytes are always higher than neutral electrolytes due to the proton transfer mechanism. The results are consistent with the conventional mobility calculation.

EXPERIMENTAL

Figure
 
1
(a) shows the stereoscopic view of our device and the corresponding electric double layer between the electrode-electrolyte interface[
1
]. The measurement system including a function generator (AFG-3252) and a lock-in amplifier (SR-830), which can be selected to carry out the phase difference observation between the device and source signal. Finally, the system was enclosed in a shielding box which prevents the interference and ensures the system grounded properly. F
ig
.
1
(b) shows the standard deviation (SD) versus frequency of NaCl solutions from 10-4 to 10-3 M, computed from ten measurements. Clearly ,with a higher concentration, the SD of NaCl solutions becomes smaller. Fig. 1(c) is our device photo.

DISCUSSION

Figure 2(a) shows the frequency dispersion of AC conductivity for two neutral (NaCl, NaBr) and two acidic (HCl, HBr) electrolytes with concentrations of 1×10-4 M, 5×10-4 M, and 1×10-3 M. All experiments in this paper are under 100 mVp-p bias and room temperature of 298 K. For a given concentration, AC conductivity of all electrolyties increases with increase in frequency because ions cannot pass through the electrode region at lower frequencies, so the ions face the highest resistivity. At higher frequencies, the periodic reversal of the electric field represents so fast that ions behave less diffuse in the electric field direction, leading to the mobility improvement. Besides, the improved mobility of the carriers is responsible for the conduction mechanism, especially for the proton transfer in acidic electrolytes.

The AC conductivity of all electrolytes is also increased with the increase in concentrations under the same frequency, which results in higher charge density. At higher concentration, the frequency independent region in the AC conductivity occurs at a higher frequency. The conductivity of the acidic electrolytes (HCl, HBr) is always higher than the neutral electrolytes (NaCl, NaBr) in each concentrations. The highest conductivity of acidic electrolytes may be deduced to the proton transfer mechanism. Moreover, the AC conductivity of NaBr is higher than NaCl, the similar results was showm in HBr >HCl, which are strongly correlated to the ionic mobility been conventionally calculated with interpolation at infinite dilution in aqueous solution. Fig. 2(b) shows the bulk resistance (Rb) versus concentration characteristics of all samples. The Rb is calculated through the intercept on real axis at low frequencies of Nyquist plots[
2
]. As confirmed by the higher conductivity in the concentrated electrolytes, the carrier density increase, and hence the Rb decreases.

Figure 3(a) shows the frequency dispersion curve of the dielectric constant (ε') in log-scale. The initial value of ε' is large, then decreases with increase in frequency, and finally saturates at higher frequencies. At lower frequencies, a higher dielectric constant value occurring at a larger concentration implies that more ions are present since the dielectric constant measures stored charges. Due to the extra assistance of proton transfer, the acidic electrolytes represent larger dielectric constant as compared to other kinds of strong electrolytic solutions. Fig. 3(b) shows the dielectric loss (ε") in relation to frequency. As indicated from the figure, the ε" increases up to a specific frequency after which it decreases. All electrolytes appear the relaxation frequency at each peak value of dielectric loss revealing the presence of relaxing total polarization The relaxation frequency with larger concentrations occurs at a higher frequency owing to the stronger total polarization from the conductivity increase as well as the lower resistance. Fig 4(a) shows the higher relaxation frequency of acidic electrolytes (HCl, HBr) occurs than the neutral electrolytes (NaCl, NaBr) in the same concentration. This is also attributed to the proton transfer behavior[
3
], which enhances the conductive performance as well as the lower resistance in the acidic electrolytes. The trend of relaxation frequencies for all samples are strongly correlated to the enhanced conductivity due to high ionic mobility. Fig 4(b) illustrates the electrode's double layer and the corresponding equivalent circuit. Rb and Csol(ω) are the bulk resistance and the capacitance of electrolytes, respectively.

Figure 1