Electrical Decomposition of Sulfur
Hexafluoride D.4VID EDELSON, C. A. BIELING, AND G. T. KOHR'IAN Bell Telephone Laboratories, Murray H i l l , -V. J .
T
HE great interest recently shown in the use of sulfur hexa-
low pressures. Thionyl fluoride, SOFn, was also observed in t h e v experiments, and a solid deposit on the potassium bromide cell windows n-as noted. This was identified as potassium fluosilicate, the silicon apparently being derived both from the ceramic insulator and the silicone rubber gaskets in the cell. An experiment performed under the much milder conditions of electrodeless discharge, using a glass-sided cell wrapped with tinfoil connected to a high frequency spark coil, yielded S P Fas ~ the only lower fluoride of sulfur.
fluoride as a high dielectric strength gas has been accompanied by serious consideration of the possibility of the production of toxic and corrosive products in the event of electric breakdown of the gas. Previous papers (%',8) have dealt with breakdown by arc or corona and the corrosive and toxic properties of the products. This investigation was undertaken to formulate a mechanism for the decomposition as well as to determine the products formed under varying conditions of electric discharge, including conditions considerably milder than those which have been previously investigated-e.g. , stress below the corona point. PRODUCTS OF DECOMPOSITION
The method of infrared absorption analysis was used to establish the identity of the lower fluorides of sulfur produced by the decompoeition. A special cell having a gold-plated metal body and a sparking electrode introduced through a ceramic insulator (Figure 1) was constructed for these measurements. This made it possible to decompose the gas directly in the cell and t o avoid losing some of the more labile products during transfer from a separate sparking chamber t o the cell. The spectrograms obtained were compared with the spectra of the lon-er fluorides, some of which are available in the literature [SsF1o,SF,( 5 , 6 ) ]and some of which were determined in these laboratories (SZF?,SF2). Fluorine, which has no infrared spectrum, was not obsxved directly, but was inferred from the nature of the solid products formed in some of the experiments. I?+-
SPARKING
*TO
PUMP
Figure 2. Portion of Plane Electrode Showing Deposits Opposite Point Electrodes A. B.
-
K B r WINDOW
I
Under the most drastic conditions of decomposition investigated-a high current, 60-cycle arc-SFz was the major product. Current-limited sparks, both with copper and gold electrodes, gave similar results, as did an experiment on glow discharge a t
Full size 7.5 X
A still milder test-exposure to electrical stress slightly below the corona point between point-plane copper electrodes for about a year-showed no detectable change in the composition of the gas phase. That some decomposition had taken place, however, was indicated by the formation of dark spots of copper sulfide on the plane electrode opposite the points surrounded by a more diffuse film of copper fluoride (Figure 2). These solid products were identified by means of chemical spot tests and election diffraction studies. I n an additional experiment, a double helical coil of fine copper wire wound on cable paper was placed in the test chamber. The conductors were connected t o external terminals so that insulation resistance measurements across the paper could be made. The results are shown in Figure 3. The decrease in insulation resistance is a definite indication of the absorption products by the paper. After the test, water extractions of the paper showed positive tests for fluorides and sulfides. I n none of the experiments performed were SeFlo or SF,ever observed as decompoqition products.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1953
TABLEI I. APPEARANCE POTENTIALS Ion
E.V.
OF POSITIVE IONS FROM
Ion
SFe
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that the activated complex (SzFlo)*does not execute a radiation transition t o the ground state but instead disproportionates:
E.V.
(SzFlo)* + S2Fz
MECHANISM OF DECOMPOSITION
*
.iny proposed mechanism to account for the observed products of the decomposition must take into account the ions and radicals formed during the discharge and the possible ways in which they may recombine to give stable products. Dibeler and Mohler (3) reported the appearance potentials of the positive ions, and their results are given in Table I. The negative ions were measured in this laboratory by Ahearn and Hannay ( I ) , who observed very strong resonance capture of electrons to form SF6- and SFs- at about 2 e.v. This is the mechanism which is responsible for the high dielectric strength of sF6; the capture of electrons t o form heavy ions reduces their mobility and increases the potential a t which avalanche and breakdown occur. This effect has been independently observed by McAfee ( 7 ) . At higher voltages there is formation of these ions t o a lesser extent because of secondary processes which liberate slow electrons, which in turn undergo resonance capture. F- and Fz- ions are also observed. Inasmuch as the negative ions appear at voltages below those a t which positive ions are first observed, the following reactions nre postulated a t low voltages-e.g., near the corona point.
+ 8F
(13)
There are, of course, several other mechanisms by which SzF2 may be produced, but SzFlais not found as adecompositionproduct inspite of the many possible recombinations which would lead to it, thus indicating that a mechanism such as Equation 13 is operating. The remaining products must be accounted for on the basis of secondary reactions of SzFz. The thermal decomposition of SZFZ is well known (4).
S2F2
+
SFz
+ [SI
(14)
This reaction takes place a t temperatures above 100' C. This explains the observation of SFz as the principal product in experiments where the heat of the arc or spark xould easily decompose the initially formed SZF2. The other gaseous product, thionyl fluoride, is probably produced by the attack of SFZ on the silica-containing ceramic insulator. The other product of this reaction, silicon tetrafluoride, is readily absorbed by the salt windows of the cell to give the fluosilicate deposit; gaseous silicon tetrafluoride is not found in the cell. PROPERTIES OF DECOMPOSITIOK PRODUCTS
The corrosive nature of fluorine is well known, and no attempt is made here t o enumerate the ways in which it may attack electrical components. However, the lower fluorides of sulfur are not as well known, and their effects should be mentioned. A comprehensive review of the sulfur fluorides is available in the literature ( I , 9).
One should then expect the following types of recombination.
*
Since S2F10has never been observed, Reaction 7 is assumed to be negligible. Reaction 5 may account for the formation of copper fluoride in the prolonged test below the corona point. The formation of copper sulfide in the same experiment is probably more complicated, involving direct reactions of the ions or radicals with the metal. At the higher voltages a t which corona and breakdown actually occur, it is presumed that the positive ions listed in Table I will be present in addition to the negative ions. Recombinations such as the following are then likely to occur.
TIME IN HOURS
Figure 3.
X1( I
Insulation Resistance of Cable Paper Placed i n Electrically Stressed SFe
Decrease in resistance is due to absorption of decompsition products
Inasmuch as only SZFZ is observed under the mildest conditions of decomposition, it may be presumed that it is the primary product of the decomposition and t h a t all the other products found in the more severe tests are the result of secondary reactions undergone by SZFZ.Since S2F10is not found, it appears
SFz and S~FZ are, at room temperature, fairly stable toward the usual metals when pure. I n the presence of slight amounts of moisture, however, they are readily hydrolyzed to hydrogen fluoride and sulfur dioxide, which attack most metals. At elevated temperatures, even the pure gases are reactive, forming fluorides and sulfides of metals. Silica-containing ceramics, such as steatite, are slowly attacked by pure SZFzand SF, a t room temperature; when hot, the reaction
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INDUSTRIAL AND ENGINEERING CHEMISTRY
proceeds more rapidly. In the presence of moisture, silica is easily attacked by the hydrogen fluoride formed. It is presumed that under some conditions, even the silicone plastics and greases will be affected. Rubber is vulcanized by the the sulfur compounds and becomes hard and brittle; organic greases become discolored and hard, although pure paraffins have been found to stand up somewhat better. However, with the present availability of fluorocarbon plastics and greases, the choice of suitable materials for use with SF,, especially where there is a good possibility for breakdown, should not be too difficult. TOXIC EFFECTS
The toxic properties of fluorine are well established.
As far as
SF, and SzF2 are concerned, the rapid hydrolysis of these compounds in moist air to give hydrogen fluoride and sulfur dioxide indicates that any hazard would be due to these gases. Physiological tests made to date show no effects other than those which would be expected on this basis. Fortunately, all these compounds possess strong characteristic odors which render them easily detectable. However, there is some question as to whether the threshold of smell is below the toxic limit.
Vol. 45, No. 9
ACKNOWLEDGMENT
The authors would like to express their appreciation to H. V. Wadlow for microchemical analyses, to C. J. Calbick for electron diffraction studies, and to K. H. Storks, D. M. Dodd, and M. H. Read for infrared spectroscopy. LITERATURE CITED
Ahearn, 8 . J., and Hannay, K.B., J . Chem. Phys., 21, 119 (1953). Camilli, G., Gordon, G . S., and Plump, R. E., Trans. Am. Inst. Elec. Engrs., 71, 111,348 (1952). Dibeler, V. H., and Mohler, F. L., J . Research Natl. Bur. Standards, 40, 25 (1948). Dubnikov, L. &I., and Zorin, N. I.,Zhur. ObshcheE Khim., 17, 185 (1947). Edelson, Da\-id, J . Am. Chem. Soc., 74, 282 (1952). Lapemann, R. T., and Jones, E. A , . J. Chem. Phgs., 19, 534
(1951) .
Mcifee, K. B., unpublished work. Schumb, W. C., Trump, J. G., and Priest, G. L., IND.ENQ. CHEhf., 41, 1348 (1949). Trauta, A I . , and Ehrinann, K., J . prakt. Chem., 142, 79 (1935). RECEIVED for review February 3, 1953. ACCEPTED J u n e 11, 1953 Presented before t h e Conference on Electrical Insulation, National Research Council, Lenox, %lase.,October 1952, and before the winter meeting of the Aiiierican Institute of Electrical Engineers, January 1953.
ine
ercetin from
ar
E. F. KURTII Oregon Forest Products Laboratory, Corvallis, Ore.
HE flavonol, quercetin, is now extracted commercially from Howering buckxheat and the Chinese scholar tree, Xophom japonica, as the rhamnoglucoside, rutin, It may be produced by the hydrolysis of the rhamnoside, quercitrin, obtained from the bark of black oak ( 1 ) . Quercetin is a yellow, crystalline powder, almost insoluble in water, and its melting point has been variously reported to be between 312" and 317' C. Extensive literature has appeared sho\ving that it is nontoxic and has antioxidant and physiological propert,ies (2, 4, 7 , 9). A410ngwith other flavonoids it possesses vitamin P activity (8). Dihydroquercetin and quercetin are closely related coinpounds. The former is a white crystdline flavanone t,hat has two more hydrogen atoms in the molecule than quercetin. Quercetin is, therefore, an oxidized derivative of dihydroquercetin. The flavanone is a substantial constituent of the barks of certain trees, such as Douglas fir and Jeffrey pine. It has been claimed that dihydroquercetin, which is present also in the heartwood of Douglas fir, interferes with the pulping of this species with calcium bisulfite liquor. Therefore, the action of bisulfites on dihydroquercetin was explored and a process for the conversion of this abundant, raw material to quercetin developed. In this connection, it has been found that dihydroquercetin was converted to pure quercet,in rapidly and in high yields by the simple process of refluxing it with an aqueous solution of an alkali metal bisulfite or ammonium bisulfite ( 6 ) . Chemically pure quercetin in the form of yellow crystals melting a t 316" to 317' C. separated promptly from the hot aqueous solut'ion in the order of about a 90% yield, by weight. When calcium bisulfite liquor was reacted similarly with dihydroquercetin in an open vessel, a finely divided, insoluble crust of a calcium-quercetin complex was deposited that adhered tenaciously to the sides of the vessel. In the conventional calcium bisulfite pulping renction, therefore, digestion of wood chips may be hindered by t8heimpervious deposit of this complex and by the removal of the calcium ions from the reacting liquor. The difficulty encountered in pulping Douglas fir heartwood chips with calcium bisulfite is not experienced when ammonium bisulfite is used. This distinc-
tion between the two pulping processes may be caused by the difference in the end product of the two bisulfites and dihydroquercetin. The converaion of dihydroquercetin, which is readily soluble in aqueous sodium, potassium, or ammonium bisulfite solutions, to insoluble quercetin is unique in that the bisulfite reducing agents unexpectedly remove two hydrogen atoms from each molecule of dihydroquercetin. Although the mechanism of the conversion is not clear, the bisulfites apparently act in a catalytic capacity or are reduced to some lower oxidation state, for after the conversion of an initial quantity of dihydroquercetin to quercetin, the residual liquor may be used and reused, with only slightly re luced effectiveness in the conversion of further quantities of dihydroquercetin. In addition, the reaction appears applicable only to 3-hydroxy flavanoids that have a carbonyl group in the four-position, Under the same conditions, catechin from white fir bark, which has the same chemical structure as dihydroquercetin, except for the absence of the carbonyl group, is not affected by bisulfites. EXPERIBI ENTAL
The catalytic character of the bisulfite reagent is illustrated bv four siccessive runs made with the same bisulfite liquor. First, 5 grams of dihydroquercetin and 20 grams of sodium bisulfite were dissolved in 100 ml. water. The solution was refluxed a t atmospheric pressure for 15 minutes, a t the end of which time quercetin was separated by filtration from the hot solution in a yield of 50% by weight. After the residual liquor was refluxed for an additional 25 minutes, an additional 35ql, yield of quercetin was separated, making a total yield of 85%, based on the dry weight of dihydroquercetin. The product was bright yellow in color and melted a t 316' to 317" C. with sublimation. It gave an acetate derivative, melting a t 193' t o 194' C. A mixed melting point determination with an authentic sample of quercetin gave no depression in melting point. To the liquor remaining from the above procedure, another 5 grams of dihydroquercetin was added The resulting mixture
.
