INDUSTRIAL AND ENGINEERING CHEMISTRY
February, 1929
q a ,where m is the molecular weight and d the density). DeSmedt34 next found that this equality was not general, for with longer molecules 6 was greater than a. However, by using the formula 6 = 1 . 3 3 q m / n . d , where n is a whole number representing the association or polymerization of the liquid, the two expressions could be brought into equality. Thus he found n = 2 for ethyl acetate, 2 for butyric acid, 3 for paraldehyde, and 3 for benzyl benzoate, in agreement with the association values determined by physical chemical methods. K a t ~ 3in~ a new series of experiments announced that in many cases the diameter of the diffraction ring was the same for an unpolymerized substance and its polymerizate as follows: Styrol Indene Chinese wood oil IsoDrene
a = 5.9 a = 8.5 a = 5.6 a 6.0
-
Dimethylbutadiene a = 6.5 Erythrene a E 5.6
Metastyrol Para-indene Polymerized Natural rubber Isoprene rubber Methyl rubber Erythrenerubber
a a
--
5.9 6.4 a 0 5.7 a = 6 0 a 6.0 a 0 6.7 a 5.6
-
It may be noted incidentally that the last three values afford a splendid method for identifying the source of a synthetic rubber, as Katz has pointed out. On the other hand, Katz found the following changes during polymerization :
-
Ethyl cinnamate
a
5.9
Polymerized
Methylethylketone Methyl acrylate
a = 6.5
Polymerized Polymerized
-
= 6.9 ar 4.8 a1
---
ai = 12 8 ar 5.6 a 7.7 a1 8.6 a¶= 5 0
In his original communications Katz failed to note one important fact, already observed by the writer with linseed and Chinese wood oils-namely, that upon polymerization an additional new ring of small diameter (large spacing) often appeared as a criterion. Hunemorder36 observed this also for metastyrol. I n a new and remarkable contribution, Krishnamurti37 shows that in all cases where the surface tension method indicates association the inner ring is very prominent. The spacing calculated from it may be substituted for the value of V a i n the Ramsay-Shields equation with greatly improved results. Here again there is occasion to wonder that highly polymerized substances give the same pattern as monomeric parent molecules, and in any case that the dimensions or distance of nearest approach are so small. A few additional facts should be noted in this connection. Stewart and Morrow3*have been able to show liquid difFraction maxima whose spacings were a linear function of the number of carbon atoms in a chain of the normal aliphatic alcohols and acids, together with other rings representing the lateral separation of chains. The long spacings were absent in the normal paraffins, showing that the end polar groups exercised a powerful influence in the longitudinal arrangement, just as was found in the crystallized 24
Bull. Sc:. acad. roy. B d g . , 10, 368 (1924).
2. physik. Chcm., 126, 321 (1927); Kaulschuk, 192T, 215. Kaulschuk, 1917, 106. n Indian Jour. of Physics, 2 , 491 (1928). Phys. Rev., SO, 232 (1927); 31, 10, 174 (1928).
Jfl
Commercial Uses for Bentonite As a result of investigations conducted by the Bureau of Mines,
it is announced that extensive commercial uses will probably be found for bentonite, one of oldest and least known mineral substances. Certain bentonites have such strong affinity for water that they are capable of absorbing more than ten times their volumes of water. Owing to its peculiar physical properties, bentonite has been suggested as a component material in the
133
polyoxymethylene diacetates and dimethyl ethers. I n all cases the lateral spacings were nearly constant, as might be expected for carbon chains. Katz has found that for complex molecules the “amorphous” spacing corresponded to that of one of the side chains alone (for example, triolein = oleic acid). Contrary to the results of Katz,39 Clark and associate^'^ have found distinct though usually small differences in spacings with China wood oil and linseed oil having polymerization and drying. The following results on China wood oil were obtained by careful measurement of the photometric curves; they are expressed as percentage change related to the spacings of the raw liquid oil, calculated by the Bragg formula n X=2 d sin 8: INNER RINGOUTERRINQ 8.5 A. 4.4 A. Per cent Per cent Raw dry film (oxidation at room temperature) $6 +I Raw gel (heat-polymerized) +3 +2 +12 f16 Oil plus turpentine Same dry film f 13 +6 Raw liquid
Changes have been observed in cumar and other resins. Shellac yields a crystalline pattern of sharp rings which disappear leaving only an “amorphous” ring when the shellac is heated to the plastic polymerized state. With nitrocellulose films producing amorphous patterns it was possible to follow swelling and shrinkage as shown by the following: FRESH LIGHTAGED HEATAGED Untreated (dry) Dry
+ residual solvent
Dry f oil softener
A.
A.
A.
7.18 4.02 9.30 4.47 7.34 4.31
7.17 4.05 9.32 4.47 7.50 4.39
8.98 3.92 7.86 4.31 7.50 4.39
It is a t once clear that these “amorphous” patterns for liquids and gels must have a valuable significance. I n the last case of nitrocellulose it is not possible that the small numbers represent the size of a colloidal particle as it is known in viscosity and diffusion experiments any more than that the small unit-cell dimensions for rubber or cellulose are measurements of the true “endless” macromolecule of the polymer. Hence it is logical to treat these data for a subcrystalline state of matter in the same way that those for crystalline materials have been interpreted. Further careful data like those of Stewart may show in many of these polymerized substances the true molecular lengths, although polar end groups and chains of only very moderate length may be essential. Conclusion It may be readily admitted that x-ray studies of the problem of polymerization are only in their beginning. Some complexities-as, for example, small unit cell dimensionshave been added, but in general a real contribution has already been made as a basis for further advances, which will come as knowledge of interpretation increases and as experimental technic improves. 1)
2. physik. Chrm., 125, 321 (1927).
40
Nature, 120, 119 (1927).
manufacture of commodities, as diversified as paper, rubber, putty, phonograph records, pencil leads, and soaps. On the other hand, underground deposits of bentonite have caused great difficulties in the drilling of oil wells, it frequently becoming necessary to take special steps to combat the nuisance. Bentonite contains 75 per cent or more of the crystalline claylike minerals montmorillonite or beidellite. A large number of samples of bentonites from many sections of the United States were studied in the course of the investigation.
INDUXTRIAL A N D ENGINEERING CHEMISTRY
134
Vol. 21, No. 2
Auto-Ignition Temperatures of Flammable Liquids' Norman J. Thompson FACTORY MUTUAL LABORATORII$S, BOSTON,MA&%
T
HIS study covers one phase of a thorough investigation of the causes of fires and explosions in ovens used for drying japans, paints, and lacquers. The history of these explosions discloses some facts which are a t first difficult to understand. For example, there have been very few disastrous explosions in ovens of the direct-fired type, in which the heat is applied from a series of gas jets issuing from perforated pipes placed a t or near the floor level. What few explosions have occurred can usually be blamed on the carelessness of the operator in allowing the oven to become filled with the fuel gas before lighting. During recent years, in which indirect-heated gas-fired and electric ovens have become popular, the explosion records of these equipments, in which there is normally no spark or flame, show that when an explosion occurs it is liable to be extremely violent. Experiments a t the Everett Testing Station of the Factory Mutual Laboratories have shown that in an oven where there are gas flames we11 distributed at the floor level, thinner vapors, being much heavier than air, tend to settle toward the floor of the oven and burn quietly a t the gas flames; or, if the concentration is slightly above the lower explosive limit, flashes will occur but without producing explosive pressures. If the ignition occurs in a small pocket of rich vapor, the oven atmosphere being largely below the explosive concentration, the resulting disturbance will be more or less local and may not result in damage. If, however, the oven contains a large volume of rich explosive mixture, pressures will be built up sufficient to wreck the oven structure if not protected by adequate explosion vents. Experience with oven explosions indicates that when no temperatures exist in the oven much in excess of 800' to 900" F. lean mixtures of combustible vapor will not be ignited. Ignition apparently does not take place a t these temperatures until vapors have accumulated to a concentration closely approximating conditions favorable to maximum explosive pressures. The foregoing should not be considered as an argument for the direct-fired oven. There are many manufacturing reasons for favoring the use of other types. Positive, dependable ventilation of the proper amount to maintain atmospheric conditions below the explosive range is of great importance, regardless of the type of oven or the manner in which it is heated. However, it is recognized that the autoignition temperature of thinner vapors is an extremely important factor and as such i s worthy of careful study. Work of Previous Investigators Most of the early work was confined to studies of ignition temperatures of gases, particularly hydrogen. I n 1913, however, Holm98* studied the ignition of various liquids by allowing drops to fall on a porcelain surface placed in an electrically heated tube furnace. His experiments failed to show anywhere near the minimum ignition temperatures obtainable on account of the small area of the igniting surface. Constam and Schlapferlo used a platinum crucible heated by a sand bath, dropping the samples of liquid fuel into the crucible. The compositions of the fuels used are not accurately known ftnd the authors themselves admit that their results are somewhat in error. In connection with a study of liquid fuels, Moore" devised 8 method for determining spontaneous ignition temperatures making use of a so-called diffusion block of steel having a hole in the top machined to fit accurately a nickel or platinum crucible. 1 Received
August 3, 1928.
* Numbers in text refer to bibliography at end of
article.
Oxygen or air was passed through holes drilled in the block in order to preheat it before admission to the crucible, Values obtained by this method are generally high on account of the small volume of the device. The method has been somewhat of a standard and has been used with modification by many investigators, notably Tausz and S ~ h u l t e ,Sinnott *~ and Moore," Ormandy and Craven,28*28 and Tanaka and Nagai.20 Most of the data available from the use of this method give ignition temperatures of flammable vapors mixed with oxygen. Several investigators have determined ignition temperatures of alcohol and ether by various methods. Alilairels was able to ignite a mixture of ethyl ether and air at 190' C. White and Price,lBusing a glass-tube apparatus, found 187' C. for an ether-air mixture and 465' C. for ethyl alcohol-air mixture. Tizard and Pye,Igusing a compression method, ignited an etherair mixture at 227' C. and a heptane-air mixture at 280' C. These results, especially those of the ether mixture, do not check very well with the determinations previously made by Holm, which gave 510' C. as the ignition temperature of ethyl alcohol in air and 400' C. for ethyl ether in air. Moore determined the ignition temperature of ethyl alcohol in air to be 518' C. using the Moore ignition meter. Until recent years only limited data have been available giving ignition temperatures of petroleum products in air. Moore obtained temperatures from 361' to 390' C. for several motor petrols. Four years later a report of the Underwriters' Laboratories of Chicagoz7 gave the ignition temperature of a motor gasoline as 536' F. (280' C.). The recent work of Masson and Hamilton3' shows what abnormally high results can be obtained with an igniting surface of small area and with the vapor sample not well confined. A few auto-ignition temperatures determined in their apparatus are-carbon bisulfide 343' C., ethyl ether 487' C., amyl alcohol 519' C., and n-hexane 520' C. They are very much higher than those obtained in this study, and there seems to be no proportionate relation. For example, the ignition temperatures of amyl alcohol and hexane are given as approximately 520' C., but the writer finds them to be about 350' and 250' C., respectively; and Tanaka and Nagai, using the Moore meter and an atmosphere of oxygen, report 332' and 285' C., respectively. Therefore, in spite of the apparent simplicity and high accuracy claimed for the Masson and Hamilton apparatus, it would seem to be of doubtful value. Callendar32 has reported some very interesting work done in a research on "dopes and detonation" in motor fuels. He sought to determine temperatures of initial combustion, which was observed in several ways-formation of carbon dioxide by limewater, water vapor by absorption in phosphorous pentoxide, acids by litmus, and aldehydes by their reaction on rosaniline previously decolorized with sulfur dioxide. He found in practically every case that the mixture strengths of the flammable vapor with air were in excess of the theoretical proportions in order to obtain combustion a t the minimum temperature. A few of his results are as follows: n-hexane 265' C., benzene 670' C., xylene 540' C., methanol 440' C., ethyl alcohol 445' C., isoamyl alcohol 450' C., butyl alcohol 570' C., benzyl alcohol 390' C., and ethyl ether 145' C. It would be expected that determinations of temperatures of initial combustion would give values a t least not greater than the ignition temperatures as evidenced by actual flame, In some cases, however, Callendar's results are higher than were found by the writer. This leads to the speculation that Callendar might have obtained even lower figures throughout if his apparatus had been of different form. I n addition to the work in his original equipment where the combustion took place in a glass tube, Callendar made comparative tests using hexane on other surfaces. He found that carbon and copper gave slightly lower results than glass; castiron turnings and lead foil gave values of 255' C. and 225' C., respectively; with aluminum the value was about 284' C.; and with tin, nickel, and zinc there was no noticeable change. Platinum black, as might be expected, gave the greatest reduction, with a temperature of initial combustion as low as 170' C. Apparatus I n a preliminary investigation the writer found that an apparatus for the determination of auto-ignition temperatures
February, 1929
INDUSTRIAL AND ENGINEERING CHEMISTRY
135
Figure 2 illustrates an equipment in which glass was used should have certain features in order to get results a t minimum temperatures. Ignition was favored up to a certain point by as the igniting surface. (The general idea of this equipment increasing the volume of 'the vapor-air sample. Moreover, is similar to that of the Underwriters' L a b o r a t o r i e ~ . ~ ~ ) it seemed desirable to have the apparatus in such form that Whenever round flasks were used, they were so placed in the area of igniting surface would be large compared with the the solder bath that the bottom of the flask would be not volume of sample. Lower results were obtained when open- less than "4 inch (1.9 cm.) above the bottom of the solder ings to the sample chamber were reduced. It was desired container. The level of alloy in the pot was just enough to have an apparatus which could be readily constructed or to submerge completely the rounded part of the flask. For assembled with facilities generally available, which would the Erlenmeyer shapes the same distance was maintained give uniform and dependable results, and which would lend between the bottom of the pot and the bottom of the flask, itself readily to rapid work with a minimum of inconvenience but the flasks were submerged only to about two-thirds of their total height. Pyrex glass flask equipment was used in cleaning. The equipment shown in Figure 1 was first constructed throughout. The Erlenmeyers were of the narrow mouth by machining from a piece of solid copper bar stock 4 inches variety, both sizes having now been discontinued. The (10.2 cm.) in diameter and 4'/2 inches (11.4 cm.) long. A nearest stock size a t present is Pyrex flask No. 6 (125 cc.). cylindrical cavity with a rounded bottom was bored out of The round flasks used were Pyrex No. 3 boiling flasks, with the block and a small hole drilled communicating with the flat bottoms and vial mouths. The 150-cc. size in this shape outside to accommodate a pipe, by means of which the at- is no longer standard. mosphere ,in the cavity could be scavenged between tests. ' Temperature Measurement The top of the cylindrical cavity was closed by a metal disk, held down tightly by wing nuts and provided with a hole It was desired to obtain the temperature of the hottest for insertion of the fuel sample. This apparatus was later portion of the igniting surface as closely as possible without duplicated, using low-carbon machine steel throughout; and taking the slightest chance of reporting any value lower than was also modified by electroplating the interior walls with that corresponding to the hottest areas. Consequently, pure metallic chromium. The capacity of this apparatus temperatures were measured by placing a thermocouple was approximately 106 cc. junction of fine wire between the source of heat and the The apparatus shown in Figure 1 was later machined out nearest part of the ignition chamber. For thermocouples without increasing the depth so as to make the lower por- chrome1 and constantan wire of No. 25 B. & S. gage (0.455 tion of the cavity spherical with a diameter of 21/2 inches mm. diameter) was used throughout. I n the metal-block (6.4 cm.), the top or neck being left unaltered a t 13/4inches apparatus the thermocouple was located in a hole drilled (4.4 cm.) diameter in cylindrical shape. The purpose of just '/le inch (1.6 mm.) below the bottom of the cavity. this change was to note the effect of increased capacity The temperatures obtained a t this location differed very (140 cc.) and altered shape.
n
Figure 1-Copper-Block Apparatus
* Figure 2-Glass
Apparatus
'
136
INDUSTRIAL -4 ND ENGINEERING CHEMISTRY
little from those at the surface of the chamber directly above. The temperature gradients a t 538" C. in the vicinity of the thermocouple junction were determined to be 7.2" and 17.2" C. per inch (2.54 cm.) for copper and steel, respectively. A series of tests was made to determine the uniformity of temperature of the igniting surface for each form of a p paratus. I n the metal-block equipment a second thermocouple hole was drilled 1 inch (2.5 cm.) from the top of the block and terminating within inch (1.6 mm.) of the inside wall. With the blocks held a t a constant temperature, tests showed differences in temperature between the new thermocouple location and the bottom of the cylindrical cavity to be less than 5" a t 250" C.; a t 400" C., 12" and 6" for steel and copper, respectively; and a t 600" C., 22" and 9" for steel and copper, respectively. It was originally considered of great importance to have a large percentage of the wall at the maximum temperature, but it was later found that this was not necessary. I n fact, with the insulation removed from the outside of the copper block so that tbe temperature differences were over twice as great as those cited above, ignition temperatures were increased only by 1" or 2" C. The greatest differences in temperature occurred in the glass equipment. Solder is not a particularly good conductor of heat, and therefore, in order to obtain approximately uniform conditions at the bottoms of the flasks, it was necessary to keep the flask well away from the bottom of the solder pot. With the flask placed within l/4 inch (6 mm.) from the bottom of the solder pot, and temperatures taken a t a point l1/*inches (3.8 cm.) from the bottom of the pot and */,inch (6 mm.) from the side of the flask, it was found that a t 400" C. this point differed in temperature from a point at the center of the bottom of the flask by approximately 25" C. Even a t 250" C. the difference was about 15' C. Providing better insulation for the solder pot did not lower ignition temperatures appreciably, however. Therefore, the insulation shown on the sketch represents merely enough for personal comfort and to obtain the higher temperatures more easily with the ordinary burner equipment. During the course of the work the question was raised as to the effect of the asbestos tubing surrounding the thermocouple wire. Consequently, a thermocouple was made up and coated by baking with a black japan which had previously proved itself capable of withstanding heat up to over 400" C. Results obtained from this thermocouple resting against the flask wall and immediately alongside the thermocouple encased in the tubing showed no difference in temperature. For work below 200" C. a bath of pure lard oil was used; consequently, no electrical insulation was re quired for thermocouple wires. Procedure
The manner of making tests was practically the same, regardless of the form of apparatus used. The temperature was raised to a point a t which ignition could be readily obtained and the character of the ignition was noted, using different quantities of fuel. The fuel was expelled from a specially drawn-out dropper so that individual drops, equal approximately to 0.01 cc., could be obtained. Using what appeared to be the best quantity, the temperature was gradually lowered until ignition could no longer be obtained. This procedure was repeated, using more and less liquid, and the results reported are the lowest ignition temperatures obtained with the most favorable fuel proportions. After each trial, whether ignition was obtained or not, the chamber was scavenged, using the permanently connected bulb in the case of the metal-block apparatus and another bulb with a glass delivery tube for the glass flasks. After scavenging,
Vol. 21, No. 2
a t least a minute was allowed to elapse before the next trial in order for the air in the chamber to become well heated. The amount of liquid required vaned with the sample, but in most cases was approximately 0.01 cc. for each 50 cc. of chamber capacity. Notable exceptions are methanol and ether, which require almost twice as much. The liquid was usually dropped in as rapidly as possible, but sometimes it was found better to expel the liquid violently so as to introduce it in the form of a spray. This was particularly true for ether when tested on the metal surfaces. When the apparatus was well above the minimum ignition temperature, ignition would take place almost immediately, but a t lower temperatures more time was usually required. The petroleum derivatives in particular sometimes required half a minute or even more. The ignition temperatures were determined always by the appearance of flame, whether the combustion was of an explosive violence or not. The interiors of the chambers were carefully watched through the mirror so that flame could be readily detected. Sometimes the flame is barely visible; therefore, it proved advantageous to darken the room so that the flames could be better observed. For example, isoamyl alcohol at points near the minimum ignition temperature burns with a faint blue glow which is not discernible in a well-lighted room. On the other hand, n-butyl propionate burns with a bright flame and with explosive rapidity until the temperature is lowered to a point where ignition is no longer obtained. At first the equipment was carefully cleaned after each test, but i t was later found that even if the walls were not entirely clean the results did not differ. If the interiors were allowed to become well covered with carbon, the ignition temperatures were in general slightly lower, but sometimes results under these conditions were higher. For the major portion of the work the equipment was cleaned between tests on different materials, but not between the trials on a single sample. The presence of sooty carbon, particularly in the glass flasks, may be thought to give lower temperatures, but i t was found that the effect was mainly one of visibility. With the room darkened, results generally could be obtained with an absolutely clean flask just as low as with a dirty one. Tests
IN COPPER-BLOCK APPmATus-The 106-cc. copper-block apparatus was first used, but it was soon found that copper had undesirable characteristics which more than offset any advantage which might be gained from its excellent thermal conductivity and the resultant uniform wall temperature. It will be noted in Table I that many of the figures are marked as "unreliable" and others are marked to indicate the reduction of copper oxide without ignition up to the temperatures given. This reduction was easily observed on account of the difference in color between the black copper oxide and the pure metal. I n almost every case it was difficult t o duplicate results exactly and for those marked "unreliable" tests could not be depended on to check a t all. I n fact, isoamyl alcohol was successfully ignited by copper a t 327" C. several times in one test, but this figure was never obtained again, even though the same conditions were maintained as far as possible and the utmost care was exercised to prevent any change in the purity of the sample. In working with the copper apparatus one has the uncertain feeling that, regardless of what results are obtained, he could by sufficient patience produce ignitions a t even lower values. It was thought that the lack of uniform results in the copper apparatus was due mainly to the varying character and thickness of the copper oxide present on the chamber wall. Consequently, it was decided to clean the block thor-
'
February, 1929
INDUSTRIAL AND ENGINEERING CHEMISTRY
137
Table I-Apparent Ignition Temperatures in Air PYREX150 cc. PYREX125 cc. PYREX150 cc. ERLENMEYER ROUND ROUND SUBSTANCE' O F . OC. OF. "C. OF. "C; 1122 636 639 1118 633 1128 Acetone 389 733 407 740 393 Allyl alcohol 764 353 667 356 661 349 673 Isoamyl alcohol 379 715 379 715 379 Isoamyl acetate 714 1076 580 582 1076 Benzene (Csz--O.o25%) 580 1079 81.7 436 444 817 436 Benzyl alcohol 831 461 862 460 860 460 860 Benzyl acetate 693 367 367 690 n-Butyl alcohol 366 692 793 423 426 792 422 798 n-Butyl acetate 799 426 433 803 n-Butyl propionate 428 812 825 441 434 820 Isobutyl alcohol 438 814 901 483 487 901 Tert-butyl alcohol 483 909 680 360 357 672 Butyl alcohol (tech. 356 675 378 712 407 722 Butyl acetate (techj 383 764 231 447 229 444 229 Butyl carbitol (tech.) 445 257 125 127 260 Carbon disulfide 127 261 366 186 188 370 188 Ether (CnHsOH-0.300%: 370 380 193 195 384 Ether (U. S. P.) 196 383 907 486 489 904 484 Ethyl acetate 913 799 426 439 789 421 Ethyl alcohol (95% U. S. 822 413 775 413 780 Ethylene glycol 416 775 739 393 402 747 Glycerol (tech.) 397 755 497 258 268 512 Gasoline (Socony) 267 515 248 479 254 488 n-Hexane 253 490 452 233 237 460 n-Heptane 238 459 526 274 297 556 Hexane ( ract.) 291 566 588 309 312 590 Pentane &act.) 310 593 470 878 474 Methanol 887 475 886 502 935 512 Methyl acetate 943 506 953 812 433 437 810 n-Propyl alcohol 818 432 852 456 458 855 Isopropyl alcohol 457 857 860 460 468 862 874 Isopropyl acetate 461 488 253 254 Turpentine 485 252 489 1026 552 1027 Toluene 553 924 496 507 929 o-Xylene 498 945 1002 539 553 1028 Xylene (tech.) 553 1028 488 253 256 Varnoline (Socony) 252 486 493 a Chemically pure, except as noted. U-Unreliable. D-Apparently reduces copper oxide without ignition up to temperature noted. No-KO ignition up to and including temperature noted.
oughly and to plate i t with chromium, which was known to withstand oxidation exceptionally well even a t high temperatures. Results obtained with the copper block chromium-plated on the inside walls were easily duplicated, but only in two cases-namely, allyl and benzyl alcohol-were the figures less than could be obtained otherwise. Moreover, the results were not in any direct proportion to values obtained with other surfaces, and therefore could not be used to predict the action of other metals. Consequently, it is not believed that the chromium-plated equipment would make an ideal apparatus for standard tests in spite of certain natural advantages, such as the extreme hardness of the surface and its resistance to oxidation. Nevertheless, the results with chromium should be interesting as they are probably the first that have ever been published. I N STEEL-BLOCK APPARATUS-The data obtained in the steel-block apparatus check very well in most cases with the figures secured with chromium. The greatest differences are with acetates, where considerably higher results were obtained with chromium as the igniting surface. The tests with steel show nothing unusual except that it is believed that the results are generally lower than have been obtained by previous investigators using this metal, Table 11-Apparent Ignition Temperatures in Air: Effect of Size and Shape ALLVESSELSPYREXGLASS 150 cc. 100 CC. 150 cc. 125 cc. Erlenmeyer Erlenmeyer Round Round O F . OC. O F . O-C. O F .. O C~. . O F~. . OC. -. Isoamyl alcohol 673 356 671 355 661 349 667 353 Butylalcohol (tech.) 675 357 675 357 672 356 680 360 Gasoline 515 268 517 269 512 497 258 267 Hexane bract.) 566 297 557 292 556 291 526 274 Varnoline 493 256 482 250 486 252 488 253 STEELVESSELS 106 cc. Cylindrical 140 cc. Round O F. O c. F. c. Gasoline 550 288 2 82 539 n-Heptane 484 251 484 251 n-Hexane 516 269 514 268 Hexane ( ract ) 593 312 573 301 Pentane kraci.) 596 313 580 304 Varnoline 522 272 512 267 n-Butyl alcohol 653 345 654 345
COPPER 106 cc. CYLINDRICAL O F .
"C.
729 621 1010 1200 860 1108 637 1100 1020 861 1200 613 805 460 275 390 398 1100 975 965 960 534 510 481 585 862 855 1040 897 1200 1100 508
649 387 327 543 649 460 598 356 593 549 461 649 323 429 238 135 199 203 593 524 518 516 279 266 249 307 461 457 560 481 649 593 264
No 1200 534
649 279
No 1200 U
D No D D U No D No U U
No D D D
u
D D No
No
LOW-CARBON STEEL106 cc. CHROMIUM 106 cc. CYLINDRICAL CYLINDRICAL O F . C. F. C. No 1235 668 No 1200 649 378 713 738 392 856 458 672 356 1051 566 861 461 668 1235 No 1200 649 428 803 816 436 479 1129 610 895 697 369 653 345 1044 562 841 449 1036 558 447 836 862 46 1 902 483 1120 604 1113 601 885 474 349 661 938 503 708 376 454 234 234 453 27 1 133 272 133 372 189 193 379 384 196 196 385 1055 568 547 1017 850 454 392 737 975 524 891 477 932 500 861 461 559 293 288 550 513 267 269 516 483 251 484 251 313 596 312 593 595 313 313 596 947 508 886 474 606 1123 569 1056 447 837 861 461 1180 638 1174 634 1126 608 1133 612 518 270 503 262
1175
N o 1200 522
635 649 272
1175 1240 523
635 671 273
EFFECTOF SIZE AND SHAPE-Before making other series of tests i t was considered best to determine a t first hand the effect of such factors as size and shape. Since these factors are most readily tested with glass flasks, i t was decided to use this material in a bath of molten solder. The results are shown in Table 11. I n some cases results in the smaller sizes were lower than in the corresponding larger size-for example, gasoline and hexane in the round flasks. This led to the suspicion that the composition of the glass had more to do with the results than the size. Accordingly, tests were made in six Pyrex flasks of the same shape and capacity, using motor gasoline, pure n-hexane, and pure n-butyl alcohol. The results are given in Table 111. It is evident that there is sufficient variation in the composition of the glass to give appreciably different results. Flask E seemed to be the most favorable to ignition, especially for gasoline and hexane. Butyl alcohol evidently is not as much affected by the nature of the igniting surface. Table 111-Apparent Ignition Temperatures in Air: Comparison of Six Pyrex Glass Flasks (Each 125 cc., Round Bottom, Pyrex No. 3) FLASK GASOLINE n-HEXANB n-BuTuL ALCOHOL F. * C. O F. C. OF. C. A 508 264 484 251 697 369 B 502 256 261 492 703 373 C 524 273 507 264 700 37 1 D 509 265 483 251 698 370 497 258 E 479 248 693 367 F 507 264 488 702 253 372
It was next desired to find out the effect of volume and possibly shape, maintaining as closely as possible the identical composition of igniting surface. Consequently, the steel block was enlarged to 140 cc. capacity, as has been previously described under Apparatus. Table 11, comparing the results obtained with those arrived a t in the original steel apparatus, showed that the larger chamber generally produced slightly lower ignition temperatures, and it is not thought that the difference in shape was responsible for the difference in results, especially since in each case the apparatus was closed a t the top with a steel disk having the same opening to the atmosphere-namely, 6 / 8 inch (1.6 cm.) diameter.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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EFFECT OF REDUCING OPENINGTO ATMOSPHERE-AShas been mentioned before, i t was known that lower results could be obtained in a chamber in which the free opening to the atmosphere was reduced. This was demonstrated in a striking manner with a glass test tube, approximately ll/z inches (38 mm.) inside diameter and 10 inches (25 cm.) in length. Tests were made in this tube immersed in a solder bath so as to heat a section enclosing about 125 cc. capacity, the top of the tube being left entirely open to the atmosphere. With this arrangement ignition temperatures obtained with hexane (technical), motor gasoline, butyl alcohol (technical), and pure isoamyl alcohol were 373", 331", 451", and 441' C., respectively. *F
.C
IX10(619)
I I 1 I I 1 GASOLINE-BENZOL MIXTURES
The tube was then drawn down so that the portion immediately above the surface of the solder bath was constricted to an opening of l/z inch (13 mm.) diameter. After this change the materials were tested in the same order as before and gave the following results, respectively: 286", 269", 361", and 352" C. RESULTS IN GLASS FusKs-The data obtained using glass flasks are presented in the first three columns of Table I. A comparison of the figures will show that the results obtained in the 125cc. round flask are generally lower than in the other two, but this is believed to be due mainly to the composition of the glass in this particular flask. Ignition Temperatures of Benzene-Gasoline Mixtures with Air ROVND PYREX S T ~ 106 ~ Lcc. CHROMIUM 106 cc. 125 cc. GASOLIXE F. c. F. a C. P. OC. Per cent 100 497 258 550 288 559 293 553 289 566 297 90 278 561 294 572 300 80 532 608 320 597 314 70 60 553 289 50 720 382 792 422 865 463 40 743 395 921 494 992 533 33 20 822 439 1059 571 1086 586 0 1076 580 N o 1200 649 1235 668
Table IV-Apparent
EFFECTOF BENzENE-Table Iv shows the results of tests made to ascertain the effect of benzene in increasing the ignition temperature of a mixture with gasoline. These results are probably more interesting in the field of internal-combustion engineering than from the standpoint of fire protection. However, they do show that very little is to be gained as far as rise in ignition temperature is concerned until the mixture is more than one-third benzene. From this proportion to the point where the benzene is slightly over half of the mixture the ignition temperatures increase much more rapidly. It is interesting to compare these data with figures obtained by Moore, using various petrol-benzene mixtures in an atmosphere of oxygen. His work showed very little
Vol. 21, No. 2
difference in ignition temperature from straight petrol u p to a point where the mixture contained about 80 per cent of benzene, after which the curve rose very rapidly in almost a straight line to the ignition temperature of pure benzene. The writer's data are presented in Figure 3. Discussion
This work has shown that there is a decided advantage in using glass rather than metal as the igniting surface, not only from the ignition values obtained but also on account of the greater facility with which results are secured. For example, ability to duplicate results is most pronounced when using glass, practically as good with chromium, somewhat less with steel, and in some cases results could not be duplicated a t all with copper. The principal objection to the chromium surface is that ignition does not take place a t as low temperatures as with glass and very few practical conditions can be imagined where there would be any interest in the ignition of flammable vapors or gases by this metal. The chromium-plated apparatus does have the advantages, however, of being easily cleaned and extremely resistant to oxidation. The behavior of the copper apparatus is so erratic that the data have no significance except for practical conditions in which copper metal is a factor. One very peculiar result obtained with copper was the figure of 461" C. for the ignition temperature of pentane (practical). This is entirely out of line with the results obtained in any other way, but ignition did not take place at any lower temperature in spite of several attempts. Unquestionably the greatest practical interest attaches itself to steel, because this material, or variations of the same elements in other steels or in the form of cast iron, is widely used for processes in which combustion takes place or may take place. Results can be duplicated and cleaning of the apparatus is relatively easy, except when used a t temperatures much above 400" C. The principal arguments in favor of glass are the comparative ease of replacement and the generally low results obtained. However, the latter is not necessarily always an advantage for practical purposes. Glass has been used for the study of ignition temperatures by several investigators, although the first record noted of its use in the form of a flask immersed in solder was in a report of the Underwriters' Laboratories of Chicago. This combination is particularly good because solder, soldering pots, and Pyrex flasks are universally available, a t least in this country. No doubt just as good work could be done with a flask surrounded by air and heated electrically, but there might be some difficulty in keeping the temperature of the flask as uniform and also in knowing just where the hottest portions might be located if there should be any difference in temperature. There has recently been proposed to the American Society for Testing Materials as a tentative standard method of test for determination of autogenous ignition temperatures an equipment consisting essentially of a Pyrex glass flask of Erlenmeyer shape, capacity 160 cc., a solder bath in which the flask is to be immersed up to two-thirds of its height, and a temperature-measuring device which may be a protected thermocouple or a thermometer. It is stated that the thermocouple (or thermometer) shall be placed and held firmly in the bath in such a position that its end shall be inch (6 mm.) from the bottom of the bath and l/4 inch (6 mm.) from the side of the flask. No statement is made as to how far the bottom of the flask shall be placed above the iron pot used as a solder container. Several objections to the tentative method have become apparent in the course of this work. I n the first place the manufacture of a flask of this approxi-
INDUSTRIAL AND ENGINEERING CHEMISTRY
February, 1929
mate size and shape has been discontinued a t the recomCHEMICAL SOCIETY,together mendation of the AMERICAN with all other 150-cc. sizes in conical or round flasks except one round extraction flask with an extremely wide neck. The position chosen for the location of the temperaturemeasuring device is such that temperatures may be obtained less than the hottest portions of the flask area. This is especially true if the bottom of the flask is located close to the bottom of the iron pot. With this distance cut down to inch (6 mm.) or slightly less, the temperatures existing at the center of the bottom of the flask may be 10" to 15" C. higher than would he shown a t the location advised in the specifications. It would seem that measurements of temperature should be taken to show results not less than the highest igniting-surface temperatures which may be obtained. Moreover, in order that some degree of uniformity can be obtained, the minimum distance from the bottom of the flask to the bottom of the iron pot should be specified. Furthermore, a standard method should contain a definition of autogenous ignition temperature. For example, if the temperature is to be determined by the appearance of flame, then the user should look for the flame and disregard any puffs of smoke which may be obtained a t lower temperatures. Conclusions
1-Copper is not suitable as an igniting surface for general test work. 2-A chromium-plated apparatus has many advantages, but the results are not the same nor are they in proportion to those obtained with steel. 3-Steel has the most universal practical application for its results. However, an apparatus made of steel, or any other metal of sufficient volume to give low results, must be made to order in the machine shop. Such an apparatus is not so easily or quickly cleaned as one in which glass is used as the igniting surface. 4-Fksults can be duplicated more exactly with glass than with any of the other materials tried. There is little to be gained by increasing the size of the ignition chamber over 125 cc. A decrease in the size of the flask mouth lowers the temperature of auto-ignition. The temperature conditions are more uniform when using a solder bath if R round flask is used rather than one of Erlenmeyer shape. Consequently, for test work with glass, Pyrex boiling flask KO.3, capacity 125 cc., is recommended; and temperatures should be meas-
139
ured under the center of the bottom of the flask as close to the flask as is practicable (within '/16 inch or 1.6 mm.). 5-The lowest ignition temperatures are obtained with quantities of fuel such as to give a rich mixture in the ignition chamber. I n very few instances does ignition take place with quantities of fuel calculated to give mixtures leaner than the theoretical combining proportions, except a t temperatures much higher than the values presented in the tables. Therefore, it is concluded that relatively lean mixtures of flammable vapor and air cannot be ignited by hot metals a t the temperatures commonly prevailing a t burner boxes or electric heating units in japan ovens; but when vapors have accumulated to a rich concentration, ignition can and does occur with generally disastrous results. Bibliography 1-Krause and Meyer, Ann., 264, 85 (1891). 2-Askenasy and Meyer, Ibid., 269, 49 (1892). 3-Freyer and Meyer, Z . physik. Chcm., 11, 28 (1893); B n . , 26,622 (1892). 4-Gautier and Helier, Compf. rend., 122, 566 (1896). 5-Helier, Ann. chim. fihys., [71 10, 521 (1897). 6--Bodenstein, Z . phystk. Chem., 24, 665 (1899). 7-Falk, J . A m . Chem. Sac., 28, 1517 (1906); 29, 1536 (1907). 8-Dixon and Coward, J . Chem. SOC.,96, 514 (1909). 9-Holm, Z. angew. Chem., 26, 273 (1913). 10-Constam and Schlapfer, Z. Ver. deuf. In&, 67, 1489 (1913). 11-H. Moore, J . SOC.Chem. I n d . , 36, 110 (1917); J.Inst. Petroleum Tech., 6, 186 (1920~; Automobile Eng., 8, 245 (1918). 12-McDavid, J . Chcm. Sac., 111, 1003 (1917). 13-Anonymous, Engineering, 104, 692 (1917). 14-Sinnott and B. Moore, J . Soc. Chem. Ind., 89, 72 (1918). 16-Alilaire, Compf. rend., 168, 729 (1919). 16-White and Price, J . Chem. Soc., 116, 1248 (1919). c 17-Wollers and Ernke, K r u p p . Monatsh., 2, 1 (1921). 18-Ddiber, 2. Ver. deut. Ing., 65, 1289 (1921). 19-Tizard and Pye, Phil. Mag., 44, 79 (1922). 20-White, J . Chem. Sor., 121, 1688 (1922). 21-Alt, 2. Ver. deut. Ing., 67, 686 (1923). 22-Wartenberg and Kannenberg, Z . physik. Chem., 106, 205 (1923). 23-Tausz and Schulte, Mitt. chem.-tech. Inst. tech. Hochschule Karlsruhc 2 (1924); 2. Ver. deut. Ing., 68, 574 (1924). 24-Mason and Wheeler, J . Chem. Sac., 126, 1869 (1924). 25-Wheeler, Ibid., 125, 1858 (1924). 26-Ormandy, J . Inst. Petroleum Tech., 10, 335 (1924). 27-Underwriters' Laboratories, Miscell. Hazards, Repf. 1130 (1924). 28-Ormandy and Craven, J . Inst. Pefroleum Tech., 12, 650 (1926). J . Soc. Chem. I n d . (Japan), 29,266 (1926). ag-Tanaka-Nagai, 30-Garner and Saunders, Trans. Faraday Soc., 22,281 (1926). 3l--Brown, Univ. of Mich., En&.Res. Bull. 7 (1927). 32--Callendar, Engzneerrng, 123, 147, 182, 210 (1927). 33-Egerton and Gates, J . Inst. Petroleum Tech., 18, 244 (1927). 34-Masson and Hamilton, IND.END. CHBM.,19, 1335 (19271. 3S-Weerman, J . Inst. Petroleum Tech., 18. 300 (1927).
Correlation of Freezing Points and Vapor Pressures of Aqueous Solutions b y Duhring's Rule' Carl C. Monrad UNIVERSITY OF MICHIGAN, ANN ARBOR,MICH.
ARIOUS equations have been proposed for expressing the relationship between the temperature and pressure of saturated vapors. Of these by far the most useful is the generalization known as Duhring's rule. This rule states that, if the temperature of one substance is plotted against the temperature a t which another substance has the same vapor pressure, the curve produced is a straight line. Duhring expressed the relationship mathematically as
V
Ti-- Tz -
el - ez
1
Received August 2, 1928.
-
This was found to hold for a large number of substances with fair accuracy. The advantage of this rule over others, such as the RamsayYoung equation, is that the curve is a straight line. Two points are sufficient to determine the line completely. If additional data are determined, they are of use merely as checks. The reference substance should preferably be one that is chemically similar to the one under consideration. Thus, alcohols are best plotted against water, benzene against toluene, etc. For most purposes water is a suitable standard
