A special Instrument
With the new BI-870 measuring of dielectric constants has never been so easy. Simply insert the probe in the liquid to be measured, adjust the two controls on the front panel and read the dielectric constant from the display. The BI-870 can accurately measure in low and high dielectric solvents, including mixed liquids and solutions.
Application
Good zeta potential determinations of suspended colloidal particles depend on an accurate knowledge of the dielectric constant of the liquid. For a range of particles of interest a number of different liquids may be used, including mixtures of various proportions creating the need for an accurate measurement of dielectric constant. While literature values of dielectric constants are suitable when using pure liquids values are not readily available for mixtures of liquids. There are no generally acceptable methods of calculating values for mixtures from pure component values. For example, weighting by mole fraction leads to errors of 17% and 6% for the two mixtures given in the table below. Weighting by volume leads to errors of -8% for Methanol/Water and 2.4% for Dichloro Methane/Benzene respectively. 
Operation
The BI-870 has two selectable sensitivity ranges: 1-20 and 1-200. Absolute accuracy is ± 2% while repeatability and linearity are better than 0.2%. The measurement signal applied to the outer cylinder of the probe is a low-distortion sine wave at a frequency of 10 kHz. The amplitude is approximately 7 volts rms on the 1-20 range and 0.7 volts on the 1-200 range. The frequency is crystal-controlled and is, therefore, stable to approximately 1 part in 10 5 . The dielectric constant of the liquid sample is determined by measuring the current between the outer and inner cylinders of the probe. With a stable voltage source and precisely known probe parameters, it is possible to display the dielectric constant directly. Calibration is simple using the back panel adjustment with a liquid of known dielectric constant.
Probe
The probe has an open structure and is easy to clean. It is constructed from two precision cylinders, machined from type 316 stainless steel. Six Teflon balls maintain the cylinder spacing. If the probe is used primarily with low-dielectric constant hydrocarbon fluids, we recommend cleaning by agitation in acetone or ethyl alcohol, followed by gentle drying with clean compressed air. It is important that the probe be cleaned before any residue dries on the cylinders. Since the probe is made of stainless steel and Teflon®, it may be cleaned in almost any solvent.
BI-870 Dielectric Constant Measurements for Common Liquids
| Material |
Temperature |
Measured
Dielectric
Constant
|
Published1
Dielectric
Constant
|
% Difference2 |
| Cyclohexane |
25 0C |
2.03 |
2.015 |
0.7 |
| Ethyl acetate |
25 0C |
6.05 |
6.02 |
0.5 |
| Dichloro methane |
25 0C |
8.92 |
9.073 |
-1.6 |
| Methanol |
25 0C |
33.2 |
32.63 |
1.7 |
| De-ionized Water |
25 0C
24 0C
23 0C |
79.3
79.9
80.2 |
78.54
78.90
79.27 |
1.0
1.3
1.2 |
Dichloro methane +
Cyclohexane4 |
25 0C |
5.35 |
N/A |
- |
Methanol +
Water4 |
25 0C |
61.2 |
N/A |
- |
1. CRC Handbook of Chemistry and Physics.
2. %Difference = [(Measured - Published)/Published] x 100.
3. Corrected for temperature to match 25°C measurements.
4. Liquids were mixed 1:1 by volume at 25°C. |
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