INDUSTRIAL A N D ENGINEERING CHEMISTRY ‘d from edihration curves obtained with mixomposition. BIBLIOGRAPHY
:enbua, and Roswell, IND.ENO.CHBM..ANAL.ED.. 940). , 8, 224 (1936).
Sohicktans, Bur. Standards J. Research, 7, 851 lohnson. U.5. Patent 2,251.185 (July 29, 1941).
rrrronr,
_.R..IND.END.CHEM.,24,482 (1932).
Fenske, Lsdoski. and Tongberg, Zbid..30. 297 (1938). Fenske, Quipple. and Tongberg.Zbid.,24,408 (1932). Feneke, Tongberg, and Quiggle, Zbid., 26. 1169 (1934). Fenake, Tongberg. Quiggle, and Cryder. Zbid., 28, 644 (1936). Haehmuth and Tooke. Notl. Petroleum News, No. 18, R-142 Ilnlm
Leoky and Ewell, IND.ENO.CIIEW..ANAL.ED.,12,544 (1940). Leslie and White, Bur. Sto&rds J. Researdr, 15, 211 (1935).
Cell
(13) (14) (15) (16) (17) (18) (19)
Vd. 119 No. 9
Mair, Glasgow, and Rossini,Zbid.,27,39 (1941). Marshall. IND.E m . CHEM.,20, 1379 (1928).
Marshall and Sutherland.Zbid.. 19. 735 (1927). Marshall. Walker. and Baker. Can. J . Research. 15B, 1 (1937). Podbielniak. IND.EN(?.CWEM.. ANAL.Eo., 13,639 (1941). Rose and Welshens, IND.ENG.CHEM..32,668 (1940).
Scott. T m m . Zmt. Chnem. Envrs. (London), 13, 211 (1935). (20) Selker. Burke. and Lankelma. IND.ENO.CHBM.,ANAL.ED., 12,
352 (1940). (21) Stedmsn. Can. J . Reaea?&, 15B, 383 (1937). (22) Tongberg, Lawroski, and Fenske, IND. Em. Cas% 29, 957 (1937). (23) Tongberg, Quiggle, and Fenske. Ibid.. 26. 1213 (1934). (24) Ward, C. E., U. S. Bur. Mines, Bull. 600 (1939). (25) Whitmore. Femke, Quiggls. Beratein. Carney, Lawroski. Popkin. Wagner, Wheeler. and Whitaker. I.Am. Chem. Soc.. 62, 795 (1940). P~ESENTED before the Division 01 Petroleum Chemistry at the 108th MselLing of the A u e ~ i o & C ~~E X K A So< L:IETI,New York. N.Y.
for Determination of Dielectric Propertie:s of Liquids Method for Calibration L. J. BERBERICH, Insulation Department, Westinghouse Research Laboratories, East Pinhburgh, Pa.
A M P L E and,reliable,cell fo,r measuring the dielectric p r o p e r t m of liqmds consists pnncipally of two glass tubes and two electrodes in the farm of concentric nickel or Monel cylinders. Its dimensions arc not critied and, using glass, nickel or Monel, tungsten, and copper, it can be constructed by m y laboratory equipped with glass-blowing facilities. The cell has been found very satisfactmy for measuring the dielectric constant, dissipation factor, and resistivity of liquids. Since only small quantities of specimens are available in the investietion of many synthetic liquids, it is significant that this cell requires only 10 to 15 cc. of liquid. Moreover, it is applicable over a wide frequency range, both above and below mom temperature, as well as over a wide frequency range extending to 500 kc. Many of the cella now in use require the use of organic insulrttion under the liquid level. Organic materials may he either Blightly soluble or absorb some of the liquid being measured, which usually causes contamination of subsequent samples. Porosity of the metals used in Some cells has caused similar difficulties. Such cells are usudly very difficult to clean properly. Copper and brass have long been known to he undesirable metals for usc as electrodes because of their catalytic and other effects on many liquids. Plating copper 01 brass with such metals as rhodium, nickel, ohromium, and gold has not always proved satisfactory. Such plstings rarely remain continuous over long periods of time. Plating produces particularly poor results in cases where the various parts of the cell are either screwed or soldered together.
A higher voltsac
DOWW
currents whieb msy flow over the solid iMUhtion used to maintain the spacing between electrodes. The edihration of a threeterminal cell, however, is somewhat simpler thau for B twcterminel cell. The added complications encountered in the use of a thre-terminal cell are justified in the highest precision work, especially at low frequencies. However, for measurements on liquid dielectrics, the emls resulting from the omission of the guard electrode are negligible, provided 8 properly constructed and calibrated two-terminal oell is used. A two-tenninal cell is pmtieularly useful for measurements above the audio-frequency range, because the difficulties of having to use a bridge equipped
factor cell made from solid nickel or
able. Other cells of merit are des&iberl in the literature. Three-terminal cells require the use of rather speoialised bridges equipped with B guard circuit in order to hold the third electrode, usually called the “guard electrode”, at the proper potential. The guard electrode performs two functions: it eliminates the effects of a fringing field at the edges of the measuring electmdc and it provides a means for by-passing any leakage
Figure 1. Two Principal Parts of Cell
September, 1945
ANALYTICAL EDITION
583
trode. The spacing between cylinders is about 0.023 inch. The capacitance of the empty cell is about 35 mmf. The latter can be increased somewhat if desired by increasing the length of the middle c linder. The gfks enclosure as well as the spacers between cylinders is made of low-loss Corning No. 707 glass. Pyrex is unsuitable because of its relatively high dielectric losses particularly a t temperaturea over 50' C. The Ant cells were made with quartz spacers which were very difficult to form. The substitution of Cornin No. 707 glana spacers has proved entire1 satisfactory A list of the materials used for t i e numbered items of Figure 2 is given in Table I. The dimensions of the cell are a matter of choice. As the cells are now being built, the over-all length is 8 inches, the diameter of the outer glass tube is 1.5 inches, and the diameter of the inner glass tube is 1 inch. The dimensions of the cylinders used are given in Table I. Figure 3 shows a number of cells in a shielded air oven which is used for measurements at room temperature and above. The ungrounded leads pssa through the side of the oven in quartz tubes and then to plu receptacles mounted on the outside of the oven. 4he various cells can be connected to the bridge in turn through the use of a coaxial cable. Measurements below room temperature are made by mounting the cell in a Dewar flask in which a carbon dioxide-alcohol mixture is used aa the coolant. Besides the normal measurements on liquids, the cell has also been used in following chan ea in electrical propertiea of reactants in a vessef during the course of the reaction.
I
C A L I B R A T I O N OF CELL
Figure 2.
Diagram of Cell
with a guard circuit operable a t such frequencies are avoided. In any caae, bridges suitable for use with two-terminal cells are more generally available. The simple two-terminal cell described in this paper has proved very satisfactory for dielectric constant and power factor measurements on a wide variety of liquids. It is particularly useful for studying synthetic liquids where in many cases only small amounts are available. Measurements can be made on aa little as a 10-cc. sample. The cell has been used over the temperature range of -60" to 150" C. and the frequency range from 50 cycles to 500 kc. Dissipation factor measurcments were ma'de with commercially available alternating current bridges in which the voltage on the cell varied from a fraction of a volt to 100 volts. The cell is connected to bridge by means of a low-loss polyethylene coaxial cable. The cable capacitance can be accounted for by the use of well-known substitution methods in making the measurements. In the case of resistivity measurements, voltages as high as 500 volts have been applied to the cell. Commercially available electronic resistsince bridges were used.
The cell can be used for dielectric constant, resistivity, and dissipation factor measurements. For' dielectric constant measurements, the effective air capacitance between cylinders must be found. This can be done conveniently by making a measurement on the empty cell and a second measurement with the cell filled with a liquid of known dielectric constant. The measured capacitance of the empty cell may be expressed
as follows:
c,
=
c.
+ c,
(1)
where C1is the measured capacitance of the empty cell, C. is the desired air capacitance and C , includea the capacitance of the glass spacers and the ieads. The measured Capacitance of the cell filled with the liquid of known dielectric constant may be expressed by :
c,
= t'C.
+ c,
(2)
where C, is the capacitance ?f the cell filled with.the liquid of known dielectric constant, e' is the known dielectric constant of the liquid, and C. and C, are the same as before. The values of C. and C, can easily be determined from Equations 1 and 2. C.P. benzene waa used as the calibrating liquid because of the abundance of data on its dielectric constant in the literature.
DESCRIPTION OF CELL
Figure 1 show a photograph of the two principal parts of the cell which are assembled by means of a ground-glass joint. Figure 2 gives the details of construction. The electrodes consist of three concentric cylinders machined from solid nickel or blonel bar stock. This ensures perfectly cylindrical and rigid cylinders which cannot be obtained by rolling cylinders from sheet material. The inner and outer cylinders are connected by means of a nickel ribbon, which is spot-welded to the cylinders on one end, and to one of the tungsten rods sealed through the glass at the other end. The middle cylinder, which is connected to the other tungsten rod, constitutes one electrode while the other two cylinders form the second elec-
Table I. Materials Used in Cell Item No.' 1
2
3 4
5 6 7 8 9 10
Material Corning 707 glass Corning 707 glsaa 6 4 / u inch 0 D X rl/u inch I.D. X la/is inch- lon nickel or Monel U/u inoh 0:D: X 4 I / u inch I.D. X 1/4 in!h long nic%el,orMonel 42/11 inch O.D. X I*/u inch I.D. X 1 8 / 1 6 inches long ruokel or Monel Corning 707 glnss Corning 707 nlnsa TungsGn Nickel Copper
For identification of items, refer to Figure 2.
Voi. 11, No. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
14
where tan 6 is the dissipation factor (approximately equal to the power factor far values of either less than 0.1) of the two dieleetries in parallel, C, and tan a, are the oapacitance and dissipation factor, respectively, of one of the two dielectrics, and C, and tan 62 are the capacitance and dissipation factor of the other dieleetrie. \men the emyty c d is measured. the dielectrie (essentially the glass spacers having eapwitance C. i, in parallel with an sir I. ? ~ ~ . ~ ~ . ~ .n ~ ~ , ~ ~ ~ For~ ~ the~ c a~ w of~ ~ the. cmpty cell ~ ~~~~~~
kC. t
tan 6, = (C,
tan 6.
c .
+ CJ tan 61 C.
where tan 6, is the total measured dissipation faotor of the empty cell and the other symbols are the same as before. The next step is to measure the cell filled with the unknoran liquid, which results in tan 82. Since the unknown liquid is now in parallel with the glass spacers, Equation 6 takes the following form for this ease:
c, tan 6, c.
.an
+ -t
m u mn 6, are the true C&PBOI~ILUL~B mu um~pmon factor, respectively, of the unknown liauid. Solvine Eauation 8 for tan S,, we have
wnem
Figure 3.
Four Cells in an
-
Air Oven
Pure benzene has a dielectric constant of 2.27 a t 25" C. I n determining the dielectric constant of an unknown liquid, let C, he the measured capaoitanoe of the cell containing the unknown liquid, The dielectric constant of this liquid, e'*, is then computed from the following simple relstion:
where the symbols are the same BY before, except that C, includes not only the c3paritance of the glass apneera and leads of the rill nronrr. but also thp cmscitawe of unv other l e d < or rables &nt&riiginto the messuFement.
In measuring resistivity or conductivity i t is necessary to know the ratio of the effective electrode area to interelectrode distance. This is called the cell constant and can he determined very simply from a measurement of the air capacitance, C., of the cell.
It can be shown by the use of the well-known formula for the capacitance of a cylindrical capacitor (as well as for m y other
capacitor of simple shape) that the cell constant and the air capaeitance are related through the following simple expression: Cell constant = C.JO.08854 = 11.29 C.
(4)
where C. is in mioromicrofarads and the cell constant in mutimeters. The resistivity then is expressed by: p
= 11.29
R ah-cm.
(5)
where p is the resistivity in o h - c m . and R the measured resistance the liquid in the cell. The leakkaget b u g h the %lass, which is usually found to be small compared with that through the liquid, is neglected in Equation 5: In general, no correction for the dissipation factor of the empty cell is neoessary for tempemtures of 100' C. and helow unless liquids of extremely low dissipation faotors are to he measured. The empty cell usually has a. dissipation factor of the order of 0.0005 at 100" C. and less at lower temperahmes. Thisiduences the results to 8. ne ligibly small extent in most practioal work. However, should it%e desirable to make this correction, it can be done through the u6e of the following expression for the dissipation factor of two dielectrics in parallel: tan 6 =
C, tan 6,
ci
+ Cn tan
+ c*
69
(6)
(C,
+ eo)tanC.
62
(10)
All the sum! ities on the right side _..._(___.-I _.-d o w n ; thus the true d~ssipatioo fartor of the liquid can easily be comDuted. Howew21, correction is rarely evrr necessary tn ordinary
,."...
CLEANING THE CELL
It is very important to dean the cell properly, for imperfect cleaning ean lead to very erroneous results, especially as to die sipatiou factor. New cells and those used with very badly d e teriarated oils or other liquids should he given a wash with 25% nitric soid solution. Caremust he taken not to leave thissolution in the cell long after the first bubbles appear; otherwise the elec. trodes and leads will hecome badly etched. This should be followed by swerd washes of distilled water, after which thf cell should he soaked for a few hours in 10% ammonium hydroxide solution. aeain followed hv several successive washinm with distilled wa&: Once the new cell has been cleaned as suggested, a less d m t i e cleaning procedure will suffice during normil use: The follow. ing procedure has been found adequate, provided no badly eon. taminated or deteriorated liquids are placed in the cell. ~
1. Fill cell with toluene or like solvent while hot and let it soak for 15 minutea. 2. Remove toluene and rinse electrodes with fresh toluene. using B wash bottle. 3. Repeat 1 and 2 in prder given. 4. Repeat 1 and 2, uslng acetone instead of toluene. 5. Place cell in 100' C. oven until all traces of solvent are re moved. ACKNOWLEDGMENT
The design of this cell has evolved over B period of years and various members of the Insulation Department of the Westinghouse Research Lsbaratories have contributed to it. LITERATURE CITED
(1) Balsbaugh, J. C., and Howell. A. R.. Rm. 86.Inrtrunenta. 10, 194 (1939). (2) Berberioh, L.?.. Tvans. Am. Inst. Elec. Ewn.. 55.264-8 (1936).
