835
NOVEMBER 1947 Table I. Calculated and Observed Temperature Increases for Reaction of 0.1% Oxygen with Hydrogen in Gases Test
Gas
Mean Specific Heat C p . (cal./mole)
Temperature Increase CalcuOblated served C. c.
Temperature Yield
,
%
tenaciously a t the catalyst surface. Small amounts of ethylene when present in neutral atmospheres are hydrogenated on the catalyst, thereby liberating heat, so that presence of this gas causes the instrument t o indicate and thus interferes with the oxygen determination. A similar behavior is to be expected with other unsaturated hydrocarbons. It appears feasible in this manner to determine small amounts of ethylene and possibly of other unsaturated compounds in oxygen-free gases with the instrument. APPLICATIONS OF THE INSTRUMENT
In industrial analysis and control the instrument may be employed either as a null-indicator or as a continuous recorder of oxygen Concentration. In the former case, the instrument shows no reading as long as the oxygen concentration of the test gas re-
mains below 0.0017,; should the oxygen concentration rise above that value, through some failure, the instrument would begin to indicate, and at a predetermined value of the oxygen concentration controls are energized which shut off the process or correct the failure. I n the latter case, the instrument records the oxygen concentration continuously but may likewise act as controller when the oxygen concentration rises above or drops below a desired value. The instrument has been used wit,h satisfactory resu!ts both 88 analyzer and controller in numerous projects that have hem carried out in this lahoratory. LITERATURE CITED
Berkman, S., Morrell, J. C., and Egloff, G., “Catalysis, Inorganicand Organic,” Chapter 7, New York, Reinhold Publishing Corp., 1940. (2) Gmelin, P., Ann. Physik, 76, 198 (1925). (3) Gmelin, P., in Eucken-Jakob, “Der Chemie-Ingenieur,”Vol. 2 . Part 4, pp. 324-51, Leipsig, Akademische Verlagsgesellschajt.
(1)
1933.
(4) Larsen, A. T., and V h i t e , E. C., J. Am. Chem. Soc., 44, 20 (1922) ( 5 ) Malmgren, G., U. S. Patent 2,139,902 (1938). (6) Muller, R. H., ISD. EXG.CHEM., ANAL.ED.,13, 667 (1941). (7) Pauling, L., m‘ood, R. E., and Sturdivant, J. H., J . Am. Ckenr’ Soc., 68, 795 (1946). (8) Rein, H., Schr. deut. Akad. Luftfahrtforsch., 1939, 1; Arch. ge* Physio2. (Pfliiger’s),247, 576 (1942). RECEIVED March 12, 1947. Presented before the Division of dnalytica! CHEMICAI. and Micro Chemistry a t the 109th Meeting of the AMERICAS Atlantic City, S . J. SOCIETY,
Simplified Instrument for Wide-Range Dielectric Constant Measurement ROBERT B. FISCHER D e p a r t m e n t of C h e m i s t r y , University of Illinois, Crbunn, 111.
A simplified instrument is described for dielectric constant measurement UP liquids, using a 6E5 electron-ray tube as combined crystal-controlled oscillator and oscillation indicator and a 25Z6GT voltage doubling rectifier. Dielectric constants from 1 to 7 may be directly measured by a conventional capacitance substitution method with a sensitiiity of at least 0.01 k unit. Relative dielectric constants from 1 to 80 or more may be measured by a method in which the substance being measured is placed within the coil with a sensitivity of about 0.2 k unit. 3Ieasurements may be completed by either method in a bery few minutes. Rapid, simple methods of dielectric constant measurement will increase the analytical usefulness of this physical constant. Further work is in progre-.
D
IELECTRIC constants of materials have been of interest primarily as an aid in determining molecular structures. Certain polar groups on organic molecules contribute significantly to the dipole moments of the molecules as shown by the dielectric constants. Dielectric properties are also of some value in analytical work, as every pure compound exhibits its characteristic dielectric constant. Although this physical property is used in analysis and in identification, further use might be made if the measurements could be performed easily and rapidly over a wide range of dielectric constants. Since the dielectric constant is by definition a ratio of capacitances, it follows that measurement of this constant is essentially
a measurement of electrical capacitance. Both bridge methods and resonance methods have been used for this purpose. The instrument described herein makes use of a modified resonance method. Alexander (1) and Bender (2) have developed the use of an electron-ray tube as an indicator of high-frequency resonance. Jensen and Parrack (5) have presented a novel application of an oscillator to conductometric titrations, using the titrated solution as a variable load on the oscillator resonant circuit. The instrument presented in this report modifies and add3 to bot11 these ideas with the specific purpose of making wide-range dielectric constant measurements powible with a single, easy-to-ust
836
V O L U M E 19, NO. 1 1
instrument. The usual resonant method, as typified by the Bender instrument, is very satisfactory for materials with low dielectric constant-e.g., 1 to 7-while an instrument for general applicability should be capable of measuring constants anywhere from 1 to 80. This instrument as described is for use with liquids only. DESCRIPTION OF IR‘STRUMENT
4 circuit diagram is given in Figure 1. X 6E5 electron-ray tube, which consists of a triode and a cathode-ray tube combined, is the heart of the instrument. The triode section serves as the high-frequency oscillator, and the electron-ray section is connected as an indicator of resonance. The triode oscillator circuit is of the tuned-grid, tuned-plate type with a piezo-active quartz crystal as the grid portion of the resonant circuit and as the frerluency-controllinp device.
R2
1
from minimum to maximum. The state of oscillation is marked by a decrease in plate current. The shado;. angle in the electron-ray tube is a function of the potential difference between the ray control electrode and the target. The circuit diagram shows that the latter potential exceeds the former by whatever voltage drop takes place in the L-C circuit and in Rz, which is in turn a function of plate current. Thus the graph of Figure 2 also represents a plot of shadow angle versus capacitance, and the shadow angle is an indicator of the state of oscillation. The point at JT-hich the “eye” suddenly opens as the condenser capacitance is slowly increased may conveniently be used as the reference point in all measurements. Since the dielectric constant is the ratio of the capacitance of a condenser with the material to the capacitance of the same condenser with vacuum (same as air for practical purposes), it is necessary merely to measure the capacitance of Ct with air and again with the substance as dielectric, in terms of Cl or C,, to get the constant. As long as straight-line capacitance condensers are used, the ratio of dial divisions may be used without getting any actual capacitances. I t is advisable to measure the capacitance increases of Cs in going from “plates-open” to “platwclosed” positions with each substance as dielectric, so that any stray lead capacitances will be of no concern. This method is largely COnVentiohal and serves well for liquids up to about 7 in dielectric constant, but it is not very good above that because the total capacitance of the cell becomes too large for sharp oscillating characteristics. ;in entire measurement requires only a minute or tn-o. Other types of cells, or a much smaller cell requiring only a very small amount of sample, may be used with this oscillator. A neT7 principle is introduced to make the measurements for larger constants. I n the resonant formula,j =
L
110 V.A.C.
Figure 1. Circuit Diagram of Instrument KI. 40,000-ohm resistor RB. 150,000-ohm resistor R3. 290-ohm line cord resistor S W . Single-pole, single-throw toggle switch X. Quartz oscillating crystal, about 3500 kilocycles RFC. Small radio frequency choke VI. Type 6E5 electron-ray tube K . Type 2526GT rectifier tube CI. 150-mmfd. straight-line capacity variable condenser Cs. 10-mmfd. straight-line capacity variable condenser Cs. 15-mmfd. variable condenser mounted flexihly for u8e i n small beaker as dielectric cell CL. 0.001-mfd. fixed condenser Cr. 0.01-mfd. fixed condenser Cs, Ci, Ca. 16-mfd. electrolytic oondensrra, 450-volt I,,. 38 turns of cotton-covered w.ire wound on lower portion of ’/a inch diameter test tube
The circuit oscillatei ivhenever the plate-parallel resonant circuit (L1, C1, C2, C3) is tuned to the same frequency as the fixed quartz crystal. That condition exists !\-hen the following rcquirement is fulfilled: j = -in xvhich j i s the crystal fre2lrdrq quency, L is the inductance, and C is the capacitance of the parallel resonant circuit. The value of C in this circuit is the summation of C,, C,, C,, and the capacitance between turns in the coil. The manner in which such a circuit goes into and out of oscillation is shonn by the curve of Figure 2, in n-hich plate current is plotted againit capacitance n-ith the latter being varied
1
it is 2iTdLC’ seen that a change in either L or C may be offset by a corresponding change in C t o bring the circuit hack to the original resonant frequency. A change in L may be produced by introducing the substance being measured into a large test tube upon which the coil is wound. This is in effect changing the permeability of the material within the coil, which in turn changes the inductance value of the coil. Furthermore, placing the substance xithin the coil alters the capacitance between the windings of the coil. The factors affecting the electrical behavior of a coil are complex, but it is established herein that other factors may be kept constant, so that the dlelectric constant of the inserted material is the main variable factor involved. This method is proposed as a means of measuring dielectric constants. The method is somewhat empirical, and a calibration curve of several compounds of knovn dielectric constant must be prepared for each instrument. The experimental procedure consists of measuring the condenser dial differences, D, of the standard condenser (C1 or C,) with air and with the substance in the coil form tube, for a few standards to prepare the calibration curve and then for the unknovn, the condenser being adjusted to
‘MIN’
CAPACITANCE
MAX’
Figure 2. Oscillator Curve
.
a37
NOVEMBER 1947 the oscillator resonance point for each reading. A complete measurement for a standard or an unknown may be made in a minute, and another few minutes are usually required to clean the permeability cell after each test. This method is shown to be satisfactory over a wide range of dielectric constant values. The
.2 0
"
IO
I
20
I
I
I
30 DIELECTRIC
I
I
40 50 80 70 C O N S T A N T FROM L I T E R A T U R E
I
80
Figure 3. Typical hleasurements by Wide-Range hIethod NO. 1 2 3
4 5 6
i 8 9 10
11
Substance Xylene Toluene Amyl acetate Benzyl alcohol Amyl alcohol n-Butanol Isohutanol n-Propanol Acetone Isopropanol Ethanol
No. 12 13
14 15 16 1i 18 19
21 2o
Substance Methanol
8070 ethanol in water
merous types of apparatus. For many materials conflicting data are to be found in the literature. The theoretical sensitivity of this method of measurement is very high and is governed by the relative capacitances of the standard and the cell condensers. Theoretically the measurements should be valid to more than the two decimals recorded in Table I with capacitances as listed in Figure 1. hctually, however, very careful purification of the specimen and extreme care in shielding of the cell and in temperature control would be necessary for a greater sensitivity. Twmty-one substances were tested by the method involving placing the liquid within the coil. This method does not give absolute dielectrip constant measurements, but it gives values that are here shown to be relative to the dielectric constant. The results plotted in Figure 3 show that there is a rather smooth relationship between the observed quantity, D, and the dielectric constant as taken from the literature for each silbstance. This suggests that a calibration curve of D us. k can be made from a few substances of known dielectric constant and any unknown values obtained by experimental measurement of D as interpreted by the calibration curve. Obviously the standards and the unknowns should all be measured in the same manner to ensure comparable results, and in any specific application it would be well to use standards with constants in the same range as the unknovms. If a calibration curve over the entire range of 0 to 80 is desired, it is suggested that distilled rvater, 50% ethanol in water, ethanol, amyl alcohol, and benzene be uwd as general standards.
66.570 ethanol i n water
55.5% ethanol i n water
46.2 % ethanol i n water 38.5% ethanol in water 27.5% ethanol in water 19.5% ethanol i n water 13.2% ethanol in watkr water
level of liquid in the tube is large'ly immaterial as long as it extends over the length of the coil rvinding, although it is desirable to use a constant level for each of a series of measurements. K i t h the tube described, 25 ml. are satisfactorv. Thus, there are two methods in which the dielectric constants may be measured with this instrument, one conventional by a direct capacitance substitution method for substances of k = 1 to about 7, the other a new method for substances of k = 1 to 80. In Figure 1, the 25ZGGT tube and condensers Cg and C7 are a voltage doubler rectifier system, which, along with filter condenser CSprovides plate and target voltage for the GE5 tube. Usual 110-volt alternating current line voltage is used as the rectifier input, while the output is about 250 volts direct current, twice the peak value of the input alternating current voltage. The heaters of the two tubes are connected in series Tvith a properly selected line cord resistor across the 110-volt alternating current line. Circuit components are not critical, and the values shoim have given good service. Some experimentation may be necessary on the number of coil turns required if a different diameter form is used. If the eye does not close sufficiently, a modification of R? will cure the trouble. Other frequencies may be used by inserting another crystal and adjusting the number of coil turns as necessary to bring the parallel resonant circuit into the proper range. I t is advisable to calibrate condensers C1 and CI in terms of each other, so that either or both may be employed in any particular measurement. RESULTS
I n Table I are given the results obtained on several liquids by the capacitance substitution method. This method gives an absolute measurement of the dielectric eonstant, and the experimental results agree well with the literature values. The literature data are somewhat erratic, as many investigators have contributed data taken under varying conditions and with nu-
Table I.
Typical aleasurements by Capacitance Substitution Method
Substance
k , Observed
Benzene Olive oil Castor Oil Mineral oil Oleic acid .4niline
2.28 3.13 4.67 2.16 2.54 7.25
k , Literature
2 . 2 8 (4) 3.11 (3) 4 67 ( 3 ) 2,13(3) 2 45(4) 7 . 2 (4)
-1linear relationship between the dielectric constant and the measured quantity is desirable. Khile the curve of Figure 3 is not lincar, it may be considered so over any interval of about 10 units with a generallv negligible error. hIost applications n ill probably involve a series of measurements within ranges no larger than this. Thus a linear calibration curve may be obtained Iyithin any range of about 10 k units by use of only two standards. The deviation of the experimental points from a smooth curve in Figure 3 is probably due in most cases more to uncertainty about the correct dielectric constant than to lack of reliability in the D readings. -1. mentioned above, the dielectric constant literature is frequently inconsistent. Again, the sensitivity in practice depends upon the particular coil and reference condensers used. I n the set used for this study the condenser dial scales were in 100 divisions in 180" with each reading to 0.1 division. This corresponds to 0.1 or 0.2 dielectric constant unit on various parts of the calibration curve LITERATURE CITED
(1) Alexander, F. C., Jr., Electronics, 18, N o . 4, 116 (1945). (2) Bender, P., J . Chem.Education, 23, 179 (1946). (3) "Handbook of Chemistry and Physics," Cleveland, Ohio, Chemical Rubber Publishing Co. (4) International Critical Tables, Vol. VI, New York, McGraw-Hill Publishing Co., 1900. ( 5 ) Jensen, F. W., and Parrack, A. L., IND. ENG.CHEM.,ANAL.ED., 18, 595 (1946). RECEIVED December 7, 194b
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