Low Frequency Behavior of Strong Univalent Electrolytic Solutions with a Highly-Sensitive Impedance Method
Authors: Su Vin-Cent , Ho Kung-Chu , Ye Jia-Hao , Kuan Chieh-Hsiung 
Published: February 2020

INTRODUCTION

This paper investigates the strong univalent electrolytic solutions with a low-frequency highly-sensitive impedance method. The results show better repeatability and accuracy, especially in the concentrated cases. Electrolytic behavior reveals strong frequency dispersion relation at low frequencies. With a fixed concentration, acidic electrolytes represent largest conductivity due to extra assistance of proton transfer between hydronium ions and water molecules. All electrolytes appear the relaxation frequency at each peak value of dielectric loss due to relaxing total polarization, which are strongly correlated to the ionic mobility calculated conventionally by the interpolation method in the infinite dilution environment. The relaxation frequency of acidic electrolytes also becomes higher because of the proton transfer, which results in the higher conductivity as well as the lower bulk resistance in the acidic electrolytic solutions.

EXPERIMENTAL AND DISCUSSION

Fig
ure
1(
a) shows the microfluidic device and the measuring system including a function generator (AFG-3252) and a lock-in amplifier (SR-830). F
ig
.
1(
b) shows the corresponding electrode's double layer effect between the electrode- electrolyte interface[
1
]. Fig. 1(c) is the device photo. The microfluidic device contains a precision cut square reservoir on the resin surface, which can holding 13.5-μl-fixed amount solutions. The system was enclosed in a shielding box which prevents the interference and ensures the system grounded properly.

Figure 2 shows the standard deviation (SD) versus frequency of NaCl solutions, which the concentrations range from 1×10-5 M to 1×10-3 M, computed from ten measurements. All experiments in this paper are under 100 mVp-p bias and room temperature of 298 K. As shown in the figure, the SD of NaCl solutions becomes the smallest one at the highest concentration[
2
]. Therefore, various strong electrolytes were prepared at 1×10-3 M to carry out the following experiments. Fig. 3(a) shows the frequency dispersion of AC conductivity for six electrolytes of 1×10-3 M, which include two acidic electrolytes (HCl, HBr), two basic electrolytes (NaOH, KOH), and two neutral electrolytes (NaCl, KBr). AC conductivity of all solutions increases with increase in frequency. At lower frequencies, the ions face the highest resistivity because difficult to pass through the electrode region. At higher frequencies, improved mobility of carriers is responsible for the conduction mechanism because the fast periodic reversal of electric field make ions behave less diffuse in the electric field direction, leading to the mobility improvement. In addition, AC conductivity of acids is much higher than neutral and basic electrolytes. Furthermore, with a higher conductivity, the frequency-independent region in the AC conductivity clearly occurs at a higher frequency. Fig. 3(b) shows the frequency-independent region in terms of the associated conductivity (σ) versus all samples. Also shown in the figure is the calculated bulk resistance (Rb) through the intercept on real axis at low frequencies of Nyquist plots. The Rb decreases when σ increases, and the tendency of conductivity is: acidic electrolytes (HCl, HBr) > neutral electrolytes (NaCl, KBr) > basic electrolytes (NaOH, KOH). The highest conductivity of acids can be ascribed to proton transfer existing between hydronium ions and water molecules[
3
]. Moreover, the AC conductivity of HBr is higher than HCl. Similarly, the AC conductivity of KOH is higher than NaOH as well, which the results are correlated to the ionic mobility been calculated with interpolation at infinite dilution in aqueous solution[
4
].

Figure 4 shows the frequency dispersion curve of the dielectric constant (ε’) for different electrolytes. The ε’ shows strong dispersion relation at low frequencies and approaches almost a constant value at higher frequencies. The low-frequency behavior can be explained as that not only the ions accumulating at the electrode-electrolyte interface but also aligning themselves in the microfluidic channel along the electric field direction. As the frequency is increased, the ions cannot follow such a rapid inversion in electric field and hence diminish their contribution to the total polarization. Due to the extra assistance of proton transfer behavior, the acidis represent larger dielectric constant as compared to other kinds of strong electrolytic solutions. Fig. 5 shows the frequency dispersion curve of dielectric loss (ε’’). As indicated from the figure, ε’’ increases up to a specific frequency after which it decreases. All six electrolytes appear the relaxation frequency at each peak value of ε’’, which presents the relaxation of total polarization. The relaxation frequency of acids occurs at a higher frequency which is also attributed to the proton transfer, which enhances the conduction as well as the lower resistance in the acids. Moreover, the trend of relaxation frequencies for six electrolytes are strongly correlated to the above discussion regarding that the enhanced conductivity.

Figure 1