THE DIELECTRIC CONSTANTS OF SOME INOR- GANIC

RY HERMAN SCHLUNDT About four years ago I commenced the study of the dielec- tric constants of pure solvents. The work was undertaken at the suggestion of Professor Kahlenberg, and was carried out in the Laboratory of Physical Chemistry of the University of Wis- consin. The results obtained appeared in two numbers of the Journal of Physical Chemistry,* and in complete form as a Bulletins of the University of Wisconsin. The values of the dielectric constants of about sixty pure solvents were determined. The results obtained enabled me to point out some new relations existing between this physical con- stant and the constitution of the compounds investigated. Moreover, as the ionizing power of a large number of sol- vents chosen for the measurements had been studied by Kahlen- berg and Lincoln,+ Lincoln,s and Waldeq6 the results had a special bearing on the Nernst-Thomson rule, according to which a close parallelism exists between the dissociating power of solvents and their dielectric constants. A number of exceptions to the rule were pointed out, and a number of new examples were found which followed the rule. The inorganic solvents studied followed the rule in a general way, although the parallel- ism between the ionizing power and the dielectric constant was far from close in several instances. The dissociating power of inorganic solvents has been dili-

* An abstract of this paper was presented before Section C of the A. A. A. S. on Dec. 31, 1903. Jour, Phys. Chem. 5, 157,503 (1901). Science Series, Vol. 11, No. 6, pp. 353-389 (1901). *Jour. Phys. Chem. 3, IZ (1899). Ibid. 3, 457 (1899). 6 Ber. chem. Ges. Berlin, 32, 2862 (1899) ; Zeit. anorg. Chem. 25, 209 (Igoo). Die Zect ric Constants 0f Inorganic So Zven ts 123 gently studied since the appearance of my results, by Walden and his pupils.' Frankland and Farmer2 have made a thorough study of the and ionizing power of liquid nitrogen tetroxide. The,dielectric constants of the solvents studied by these investigators have, for the most part, not been determined. In view of this fact and the importance attached to the relation of the dielectric constants to the ionizing power of solvents, it appeared desirable to continue the study of the dielectric con- stants of pure solvents. The present report embrzces the results obtained for the dielectric constants of some halogen compounds of , arsenic, and . The values found for the dielectric con- stant of phosphorus, silicon tetrachloride, and nitrogen tetroxide are also included. Method and Apparatus The dielectric constants were measured by Drude's well- known method.3 The apparatus used in the former measure- ments4 served for this work. A vacuum tube containing hydro- gen was used to determine the settings for maximum resonance. This particular tube was selected from a lot of tubes belonging to the Department of Physics. Professor Stewart kindly placed at my disposal his extensive collection of Geissler and other vacuum tubes. I tested about thirty of them, but found none that gave better results than the hydrogen tubes. It was ob- served that the tubes of uranium glass, and the tubes containing fluorescent solutions were specially sensitive to the oscillations. They responded far more readily than the hydrogen tubes, but it is difficult to judge the position at which maximum resonance occurs. Two cells of the type used for the measurement of sub- stances having low or medium values for dielectric constants served for condensers. The apparatus was calibrated for each cell for values ranging from 2.26 to 20.5 with the standard

I Zeit. anorg. Chem. 29, 371 (1902); Zeit. phys. Chem. 39, 220 (1901). Jour. Chem. SOC. 79, 1356 (1901). Zeit. phys. Chem. 23, 267 (1897). Jour. Phys. Chem. 5, 157 (1901). 124 Herman Schlundt liquids recommended by Drude. The capacities of the cells differed somewhat. Both were used for each of the solvents in making the measurements. The values given below represent ' the average of the two independent determinations thus obtained. Whenever the two values for any one substance differed from each other by 5 percent or more, the measurements were re- peated. The methods of preparing and rectifying the various sol- vents will be found under each particular solvent in the state- ment of results given below. In most cases the specific con- ductivity of the solvents was measured. The Kohlrausch method for measuring conductivities *of electrolytes was followed. The range of the apparatus at my disposal for these measure- ments was rather limited. Specific conductivities less than 2 X IO-^ could not be determined with accuracy. Experimental Results . -The sample used for the measure- ments was Kahlbaum's preparation. It was redistilled, boiling at 73.9" C under a pressure of 738 mm. The value found for its D. C. at 18" was 3.72. This value is somewhat higher than the one obtained when this substance was formerly measI1red.I The present value, 3.72, is probably nearer the truth than the older value of 3.36 at 22' C. Phosphorm Tribrmide. - The sample was prepared by dropping bromine slowly on red phosphorus. The crude product was poured off from the red phosphorus remaining and was rectified by several distillations. The sample which served for the measurements had its boiling-point at I 73.5" under a pressure of 757 mm. Its density at 20") as compared with water at 4O, was found to be 2.8856. Its specific conductivity was less than I X IO-^. The value found for its D. C. at 20" was 3.88. Phosphorus . - This compound was prepared by slowly adding a solution of yellow phosphorus in carbon disul- phide to the required amount of dissolved in carbon disul-

Jour. Phys. Chem. 5, 512 (1901). Dielectric Constants of Inorganic Solvents 125 phide. The greater portion of the carbon disulphide was then distilled off, Upon cooling, crystals of phosphorus triiodide formed in the residual solution. The mother-liquor was poured off, and the crystals dissolved in a fresh portion of carbon did- phide, which was distilled off in part as before. The mother- liquor was again poured off from the crystals, and the adhering solvent removed by warming the crystals to 40" under dimin- ished pressure, The red crystals of phosphorus triiodide were protected from coming in contact with moist air. The melting- point was found to be 55" C. The D. C. of the solid sample at 20" was found to be 3.66. For the liquid at about 65" C the value 4.12 was found for the D. C. The positions for maximum resonance were well defined. Arsenic Trichloride. -The D. C. of this compound in the liquid state had been previously measured.I Its D. C. was again determined and found to be 12.6 at 17", which agreesfairlywell with former value of 12.35 at 219. Kahlbaum's sample was used for the measurements. It was redistilled. Its boiling- point was 129.3" under a pressure of 753 mm. Its specific con- ductivity was 3.8 X IO-^. The D. C. of the solid compound was found to be 3.6 at about - 50" C. To solidify the sample the cell was introduced for a short time into a cooling mixture of solid carbon dioxide and ether kept in a small Dewar test-tube. The temperature of the bath was nearly - 55". C. After the cell had assumed the temperature of the bath it was taken out and quickly placed in the apparatus and a setting made. After cooling it again, another setting was made. This operation was repeated until IO settings were obtained. Arsenic Tribromide. -The sample used for the measure- ments was Kahlbaum's preparation. It was redistilled under a pressitre of 20-22 mm. The melting-point of the crystals was 31" C. The liquid had a faint yellow tinge. Its specific con- ductivity was less than 2 X IO-^. The value found for the D. C. of the liquid at 35" was 8.83. The solid gave the value 3.33 at 20".

Jour. Phys. Chem. 5, 512 (1901).

x I 26 Herman Schdundt

Arsenic Triiodide. -Kahlbaum's sample was recrystallized from . Small red crystals were obtained, whose melting- point was found to be 146'. The specific conductivity of the liquid at 150' was 3.1 X IO+. The value 5.38 at 18" was found for the D. C. of the solid sample. The D. C. of the liquid, at about I~o', was 7.0. Antimony Tyibyomide. - The compound was prepared by slowly adding bromine to a quantity of finely powdered anti- mony somewhat greater than the proportion required by the formula. The product was purified by several distillations. The boiling-point of the sample was 286.4' under 736 mm pres- sure, and its melting-point was 93". The liquid had a slight yellow tinge. The specific conductivity at 100' was 5 x 10-5. The D. C. of the liquid at about 100' was 20.9. The solidgave the value 5.05 at 20'. Antimony Tyiiodide. - The sample was prepared by bring- ing together finely powdered antimony and in small quan- tities and gently warming. The product was distilled twice. Dark red crystals melting at 167' were obtained. The specific conductivity of the liquid at 175' was 1.1 X 10-4' The D. C. of the liquid at about 175' was found to be 13.9. For the solid at 20' the value 9.1 was found. Phosphorus. - A sample of Kahlbaum's yellow phosphorus which had been kept in the dark was freed froin adhering water by means of filter-paper. It was melted under chloroform, and by drawing up about a cubic centimeter of it in a medicine dropper between two layers of chloroform, it was easily trans- ferred to the measuring cells without taking fire. The chloro- form rises in the stem of the cell and is removed with a little filter-paper. The D. C. found for the liquid at 45" was 3.8. The D. C. of the solid sample at 19' was 4.24. A second sample was prepared by redistilling the Kahl- baum sample. The distillate was collected under water. It was transferred to the cells as before, but it was melted under water instead of chloroform. The water must be carefully removed, as a small quantity of acid is formed in the transfer of the sam- Dielectric Constants of Inorganic Solvents 127 ple to the cell. The presence of the small amount of acid had no effect on the positions of maximum resonance when the phosphorus was liquid. But upon solidifying, the plates of the cell evidently came in contact with the acid, as the positionsfor maximum resonance were not well defined. The energy was absorbed in the cell. The fact that an excellent electrolyte was in contact with the plates accounts for this absorption. Upon expelling the water by heating to about IOO', the solid sample gave a well defined position for maximum resonance. For the liquid sample at 45" the value 3.85 was found for the D. C. The solid at 20' had the value 4.02. By keeping a layer of water a few millimeters in thickness over the liquid phosphorus, it may be cooled to 20' without solidifying. The value found for the D. C. of the liquid at 20' was the same as the value at 45', .which was 3.85. Silicon Tetrachloride. -The sample was prepared by pass- ing dry chlorine over amorphous silicon heated to about 400". The product was redistilled ; its boiling-point was 57-50under a pressure of 749 nim. The D. C. found, at 16', was 2.40. Nitrogen Tetroxide. - The sample was prepared by heat- ing lead nitrate. The gas was passed through a phosphorus pentoxide tower before it was condensed. The D. C. found for the liquid at 15' was 2.56. The liquid was solidified by im- mersing the cell in a Dewar test-tube containing Thilorier's mixture. The snow-white crystalline mass was probably at -40' C when the settings were made. The D. C. of the solid was 2.60. One of the samples used for the measurements had a green color. The value of the D. C. was the same as that of the yellow samples. Upon solidifying this sample, the crystals at first had a green tinge, but upon further cooling they became pure white. Summary of Results The foregoing values are summarized in Table I and for more ready comparison of some of the values Table I1 was pre- pared. 128 Herman Schlundt TABLEI. _____ Substance Formula D. C. to

Phosphorus trichloride PCl, 3.72 18 PBr, 3.88 20 Phosphorus triiodide PI, 4.12 I< 65 I< . solid 3.66 20 Arsenic trichloride ASCI, 12..6 I7 $1 I1 solid I1 3.6 -50 Arsenic tribromide AsBr, 8.83 35 ll L( 11 solid 3.33 20 Arsenic triiodide ,4sI, 7.0 150 I< (l solid l< 5.38 I8 Antimony tribromide SbBr, 20.9 IO0 I1 " 1, solid 5.05 20 Antimony triiodide SbI, 13.9 I75 li 11 l1 solid 9. I 20 Phosphorus p4 3-85 45 (6 ii liquid 3.85 20 (1 11 solid 4-I 20 Silicou tetrachloride SiCl, 2.40 16 Nitrogen tetroxide NP, 2.56 I5 81 11 solid I1 2.6 -40

TABLE11. I Trichloride Tribromide Triiodide

Liquid ~ Solid ~ Liquid Solid Liquid Solid -___~______1 Phosphorus ' 3.72 I - I Arsenic I 12.5

Antimony ~ J 33.2 I :::4' I 20.9 5.05 13.9 ! 9.1

Discussion of Results Table I1 shows that the value for the dielectric constants of the compounds in the solid state is less than the value for the liquid. This drop in the value occurs upon solidification. The

These values for are taken from a former paper. Jour. Phys. Chem. 5,512 (1901). Dielectric Constants of Inorganic SoZvents 129 molecular rearrangement which occurs as the substances solidify has a marked effect on the value of the dielectric constant. Sub- stances which become crystalline in structure upon solidifying generally show a considerable change in value of the dielectric constant, Attention has been called to this fact by other inves- tigators, and the great difference in values of the dielectric con- stants of ice and water is generally cited as an example. Amor- phous substances, as a rule, show no marked change in the value of the dielectric constant upon liquefying. The ionizing power of most of the solvents in the tables has been studied. Walden' found that phosphorus trichloride, phosphorus tribromide, and silicon tetrachloride did not possess ionizing power. These substances have low values for their di- electric constants, and hence follow the Nernst-Thomson rule. The ionizing power of the trichloride of arsenic has like- wise been investigated by Walden,z and Tolloczko3 and Walden have studied the ionizing power of antimony trichloride. Their results show that antimony trichloride is an excellent ionizing agenr, and that arsenic trichloride has marked ionizing power. Phosphorus trichloride possesses no ionizing properties. The dielectric constant of antimony trichloride is relatively high ; that of phosphorus trichloride is quite low ; arsenic trichloride occupies an intermediate position in ionizing properties and in the value of its dielectric constant. Recently Walden4 has studied the dissociating power of arsenic tribromide. Tetraethyl-ammonium iodide, when dis- solved in arsenic tribromide, yields a solution whose niolecular conductivity at a dilution of 500 liters is 19.4 at 33". The same salt when dissolved in arsenic trichloride has a molecular con- ductivity of 54.3 at a dilution of 480 liters, at 25". The dielec- tric constant of the tribroniide was found to be less than the value of the trichloride. These solvents then follow the Nernst- Thomson rule.

Zeit. anorg. Chem. 25, 209 (1900). 1. c. Zeit. phys. Chem. 30, 709 (1899). * Zeit. anorg. Chem. 39, 380 (1902). 130 DieZectric Constants of Inoyganic SoZvents Frankland and FarmeII have thoroughly studied the solvent . and ionizing power of nitrogen tetroxide. They found that it did not possess ionizing power. The low value of the dielectric constant shows that it follows the Nernst-Thomson rule. The ionizing power of phosphorus, phosphorus triiodide, arsenic triiodide, antimony triiodide, and antimony tribromide, have to my knowledge, not been investigated. On the basis of the values found for the dielectric constants, phosphorus, phos- phorus triiodide, and arsenic triiodide should possess little or no ionizing power according to the Nernst-Thomson rule. Antimony tribromide and antimony triiodide should possess marked ion- izing properties. The value of the dielectric constant of silicon tetrachloride lies between the values of carbon tetrachloride and tin tetra- chloride. The values however differ but slightly : - Carbon tetrachloride D. C. = 2.18'at 17' C I6 Silicon 2.40 " 16' C Germanium " ? I( Tin 3.2' " 22' C A sample of germanium tetrachloride was not available. Its dielectric constant will probably lie between the values found for the chlorides of silicon and tin. I I desire to express my thanks to Professor 0. M. Stewart for placing at my disposal for this work an induction coil be- longing to the Department of Physics. Chemical Laboratory, Universil.v of Missouri, December, 1903.

Jour. Chem. SOC. 79, 1356 (1901). Drude. Zeit. phys. Chem. 23, 309 (1897). Schlundt. Jour. Phys. Chem. 5, 512 (1901).