A dielectric spectrometer for liquid using the electromagnetic induction method

Hewlett-Packard Journal, April, 1997 by Hideki Wakamatsu

Key parameters of colloids are often directly related to or can be derived from permittivity or conductivity. Dielectric dispersion analysis (dielectric spectroscopy) yields insights into colloidal properties. A dielectric meter using a new sensing technique has been developed

Dielectric spectroscopy is useful for the characterization of colloidal dispersions. In the past, dielectric spectroscopy has been attempted using parallel metal electrodes to measure the permittivity and conductivity of colloids. However, it is difficult in practice to make these measurements in conductive solutions because large measurement errors can be caused by electrode polarization, which is a kind of contact impedance between the electrode and the solution. Electrode polarization behaves like a capacitance and masks the true properties of the solution at low frequencies.

The HP E5050A colloid dielectric probe was developed for colloidal liquid evaluation. Its electromagnetic induction technique eliminates the electrode polarization effect. The probe operates from 75 kHz to 30 MHz with the HP 4285A precision LCR meter and HP VEE software on an HP Vectra or compatible computer (Fig. 1). The HP VEE environment[1] provides easy operation, display, and analysis of the measurement data.

[Figure 1 ILLUSTRATION OMITTED]

Background of Dielectric Spectroscopy

Colloids are dispersion systems composed of a dispersed substance in particulate form in a continuous-phase dispersion medium. There are many types of colloid; some familiar examples are listed in Table 1.

Table 1
Familiar Colloids

Colloid              Dispersion           Dispersed Substance
                     Medium

Shaving Foam         Soapy Water          Air, Propane (gas)
                     (liquid)
Fog, Cloud           Air (gas)            Water (liquid)
Milk, Mayonnaise     Water (liquid)       Fat, Oil (liquid)
Butter, Margarine    Fat, Oil (liquid)    Water (liquid)
Charcoal             Carbon (solid)       Air (gas)
Blood                Serum (liquid)       Erythrocyte (microcapsule)
Black Ink            Water (liquid)       Carbon Black
                                            (solid)

Since there are interfaces between the dispersed substance and the surrounding dispersion medium in a colloidal dispersion, there can be appended (nonintrinsic) dielectric relaxations--typically the permittivity decreases and the conductivity increases with increasing frequency--as a result of interfacial polarization caused by charge buildup on the boundaries between the different materials. The analysis of these dielectric relaxations based on an appropriate theory of interfacial polarization provides valuable information on the structural and electrical properties of colloidal particles.[2]

The frequency characteristics of the permittivity and conductivity of colloidal solutions are especially informative. Fig. 2 shows some examples. A practical means of measuring these characteristics--that is, practical dielectric spectroscopy--would be a significant contribution to the study of the stabilization of dispersion systems and product quality control.

[Figure 2 ILLUSTRATION OMITTED]

Dielectric Measurement of Colloidal Solutions

Traditionally, permittivity is measured with parallel metal electrodes. This technique can be used to measure nonconducting (nonionic) solutions such as oils or alcohols. However, in the case of salty or ionic solutions that have high conductivity, this method suffers from large measurement errors. Fig. 3a shows the mechanism responsible for the errors. Electrode polarization results from the electrical double layer between the metal electrode surface and the ionic solution. This becomes a serious problem at low frequencies because electrode polarization is a capacitance, so the contact impedance is large at low frequencies. If the total impedance consisting of the solution impedance and the electrode polarization impedance is counted as the solution impedance, a large error is incurred at low frequencies (Fig. 3b). In other words, the increase of the contact impedance at low frequencies masks the true properties of the solution.

[Figure 3 ILLUSTRATION OMITTED]

The second reason why it is difficult to make permittivity measurements is as follows. Often in aqueous colloid spectroscopy, the permittivity is a minor part of the admittance compared to the conductivity. This means that it takes a highly precise measurement to extract the small capacitive permittivity component from the almost entirely conductive admittance. For example, in 0.1% aqueous NaCl ([[Epsilon].sub.r] = 80, [Kappa] = 0.2 S/m), the magnitude of the susceptance (the permittivity part) is only about 1/500 that of the conductance (the conductivity part) at 100 kHz. If 1% accuracy of the permittivity analysis is required, the measurement system must measure the argument of the complex admittance accurately within 20 microradians.

Nonelectrode Electromagnetic Induction Method

If the electrode polarization could be removed, the true impedance of the solution could be measured. To eliminate the electrode polarization, a measurement technique without electrodes has been implemented. In this method, a closed-circuit current is made to flow in the solution by electromagnetic induction. This current is then measured, also by electromagnetic induction.