INDUSTRIAL AND ENGINEERING CHEMISTRY
996
almost linearly proportional to the moisture content. The third period begins when the wetted area finally disappears and diffusion controls.
Nomenclature a = linear shrinkage coefficient I = linear dimension of object
V = volume of object = moisture content, % ' total water, dry basis Subscripts o = bone dry conditions 1 = initial conditions
W
VOL. 32, NO. 7
Literature Cited (1)
Comings, E. W., and Sherwood, T. K., IND. ENG. CHEM.,26, 1096 (19341,includes references to all of Sherwood's articles
on drying.
K., Zbid., 13, 427 (1921). (3) Luikov, A. V., Ibid., 27, 406 (1935). (4) Newman, A. B., Trans. Am. Znst. Chem. Engrs., 27, 310 (1931); 27, 203 (1931). (5) Running, T. R., "Graphical Mathematics", New York, John Wiley & Sons, 1927, (6) Turbitt, J. G., 5. M. thesis in chem. eng., Univ. of Wash., 1933. ( 2 ) Lewis, W.
D-Dichlorobenzene as a Vapor Fumigant Physical and Chemical Studies
F. R. DARKIS, H. E. VERMILLION, AND P. M. GROSS Duke University, Durham, N. C .
P
AM-DICHLOROBENZENE has been found t o be of value as a vapor fumigant (1,s) in connection with blue mold, a fungus disease of tobacco seedlings. Certain data pertaining to its physical and chemical properties which would be significant in its use as a fumigant were not available in the literature. This paper details the results of experimentation involving the determination of the effect of crystal size on the rate of evaporation, a study of methods for estimation of p-dichlorobenzene in the atmosphere, and the determination of the vapor pressure of the crystalline solid in the temperature range 10" to 50" C.
Effect of Crystal Size on Evaporation Rate pDichlorobenzene, a solid, is available on the market in various crystal sizes. No information was available in the literature as to the effect of crystal size and temperature on the rate of vaporization. Therefore the relative rates of vaporization for crystals of five different commercial grades or sizes were determined under constant and controlled conditions at 15", 21", and 30" C. (59", 69.8", and 86" F.). The screening specifications for the five different sizes are given in Table I. An illustration will indicate the significance of the size numbers in Table I in determining the dimensions of the crystals which are included within each size: In the case of size 1, 100 per cent of the crystals passed through a 6/e-inch screen, 92 per cent through a '/2-inch screen, and 80 per cent through a 1/2-inch and on a 3/&ch screen; no more than 5 per cent of the crystals will go through a '/(-inch screen. Ten samples of each of the five sizes of crystals were employed, and each sample contained 2 grams. The samples were laced in aluminum dishes, 58 mm. (2.28 inches) in diameter a n f 1 7 mm. (0.67 inch) in depth. This amount of p-dichlorobenzene did not cover the bottom of the dish with a layer more than one crystal deep, except in the case of crystals of size 10 where slight piling existed. The dishes were placed on a board, 39 X 23 inches, and arranged as shown in Figure 1. The distance between the centers of the dishes was 4 inches. The board was placed in a cabinet made of Celotex, the temperature of which was controlled within 1 0 . 5 " C. (0.9' F.). A stream of fresh air was led into the cabinet and carried through a maze of light bulbs to bring it to the desired temperature. The air passed through a chamber com-
prising the upper portion of the cabinet and impinged more or less directly on the contents of the dishes in rows 1, 2, and 3. Then the air was circulated by means of a fan over the contents of the dishes in rows 4 t o 10, and allowed to escape from the cabinet. The rate of loss of the p-dichlorobenzene was progressively less from rows 1 to 10 at each of the three temperatures used. This ahange in rate of loss was no doubt attributable to the fact that the air stream was increasingly nearer saturation with respect to p-dichlorobenzene as it approached the last rows. The flow of air over the crystals was continued until 70 to 90 per cent of size 1 had vaporized. The data on the weight and percentage loss for the ten samples of each crystal sine were averaged for each of the three temperatures, and the percentage loss with time was plotted as shown in Figure 2. These curves indicate, as might be anticipated, that the rate of loss is less for larger crystal sizes of smaller exposed surface per unit weight, and that it decreases a t the lower temperature. The crossing of the curves in the case of crystals of sizes S and 10 is probably attributable to the slight piling of sample 10 in the dishes. The data represented by these curves are significant for the use of p-dichlorobenzene as a vapor fumigant, for they show
1
1
2
3
4
5
6
7
FIGURE 1. ARRANGEMENT OF DISHESWITHIN THF. CABINET Figures in circles re resent crystal size. Rows of dishes are numbered 1 t o 10. I i r stream entered directly over row 1 and escaped past row 10.
TABLE I. Size 1 Not Not belowabove
Sieve Opening Inch
1/z
on a/a
0.0787 0.0787 on 0 . 0 3 3 1
0.0331 0 . 0 3 3 1 on 0 . 0 1 6 5 0.0165
.. .. ..
..
5
.. ..
15
..
70
.. .. .. .. ..
.. .. .. .. ..
.. ..
.. .. .. .. .. .. 70 .. .. .. .. .*
.. .. ..
..
..
... ... ... ., . ... ... .. * ... ...
1/4
1 / 4 on 0 . 1 3 2 0.132 0 . 1 3 2 on 0.0787
fourth of that between crystals of sizes 1 and 3 at 30" C. Size 10 These curves indicate that the Iiot Not smaller the crystal size, the below above less is the effect of an increase in temperature on the rate of .. .. ... .. .. .. ... .. vaporization. Knowledge of .. .. ... .. this type is important in choos*. .. ... .. .. .. .. ing the proper size of crystals .. .. io0 .. to be employed in order t o .. .. .. .. .. .. 95 .. build up specific vapor con70 .. ... .. .. 15 .. .. centrations under the particu.. .. 40 7.5 .. .. ... 5 lar conditions of use of the vapor fumigant. I n the case of each of the sizes of crystals 1,6, and 10 a t 21" C., the percentage losses for rows 1, 2, and 3 were averaged separately and plotted against time in Figure 3. This graph also shows similar data for rows 4, 5 , 6, and 7 and for rows 8, 9, and 10, respectively. The curves in Figure 3 indicate the consistency of the results as related to crystal size. It is desirable to show this, since the results are purely relative and the vaporization conditions, both with respect to air velocities and degree of saturation of the air with p-dichlorobenzene, differ in different regions of the board. The curves indicate that the more directly the air comes in contact with the crystals, the greater the rate of vaporization, and that as the air becomes laden with p-dichlorobenzene, the rate of vaporization is reduced.
CRYSTAL SIZES OF DICHLOROBENZENE Size 3 Size 6 Size 8 Not hTot Not Kot Not h-ot below above below above below above Per cent through screens
.. ..
100 92 80
5/ E
'/z
947
INDUSTRIAL AND ENGINEERING CHEMISTRY
JULY, 1940
.. .. ..
.. ....
..
.. .. .. ..
.. .. ..
15
.... .. ..
Estimation of p-Dichlorobenzene in the Atmosphere I n connection with field studies i t became desirable to estimate the concentration of p-dichlorobenzene in the atmos-
/
///
/
Af$/ /n"/d
2
1
2
J 3
FIGCRE 2. Loss
TEMPERATURE 15°C.
0.6
4
5
6
YllQLlE 7 8
9
10
II
12
13
CRYSTALS
1 TO 3
8-ROWS
0-CRYSTAL SIZE N0.I 0-CRYSTAL SIZE N0.3 C- CRYSTAL SIZE N0.6 A-CRYSTAL SIZE NO 8 *-CRYSTAL SIZE NO.10
I
0
21-c.
TEMPERATURE
0 - R O W 5 4 TO
14
7
6 TO IO
.-ROWS
15
CRYSTALS (SIZES1, 3, 6, 8, 10) AT THREE TEMPERATURES us. T I h m
OF
the pronounced influence of crystal size and of temperature on the rate of vaporization. Thus a t 15", 21", and 30" C. the size 1 crystals differ greatly in their vaporization rate relative to the smaller sizes. The slopes of the curves in Figure 2 at 1 and 2 hours show that the difference in the rates of vaporization between crystals of sizes 3 and 6 and sizes 6 and 8 is only about half of that between crystals of sizes 1 and 3 a t 1.5' and 21 " C. Furthermore, this difference is only about one
I
2
3
4
5
6
7
8
9
IO
II
I2
I
LOSS OF CRYSTALS (SIZES 1, 6, 10) FROM DIFFERENT SECTIONS OF THE BOARDAT 21' C. us. TIME
FIGURE 3.
948
FIGURE4. ARRANGEMENTFOR
INDUSTRIAL AND ENGINEERING CHEMISTRY
THE
ESTIMATION OF
DICHLOROBENZENE CONTENT OF
THE THE
FIELD
VOL. 32, NO. '7
THE
AIR
OF
SEEDBEDS IN
phere of treated seedbeds directly in the field. Four methin the case of p-dichlorobenzene. The sensitivity was inods were tried, one of which proved more suitable than the creased by substituting a Leeds & Northrup portable galvanothers (Figure 4). ometer (No. 2400-a) and an appropriate switching arrangeThe combustion method outlined by Van Winkle and ment in place of the regularly employed Weston meter and its Smith (7) was first tried. This consists of drawing an air variable shunting resistor. Although this greatly increased stream containing p-dichlorobenzene through a quartz tube the sensitivity, the actual determination of the amount of pheated to approximately 1000' C. (1830' F.), absorbing the dichlorobenzene present in the air was made difficult owing to chloride combustion compounds in an alkaline sulfite soluseveral complicating factors. They were ( a ) the large contion, and determining the chloride by the Volhard volumetric trol reading given by the galvanometer when air alone was method. To obtain results of 95 per cent accuracy or better, drawn through the instrument, ( b ) the small magnitude of the i t was found necessary to draw the atmosphere containing the reading caused by a given mass of the p-dichlorobenzene, and p-dichlorobenzene slowly through the combustion system and (c) the effect of the moisture content of the air on the galvanabsorbing medium; this procedure limited the number of deometer readinks. The third factor was largely eliminated by terminations that could be made. It also necessihted the drying the air stream with sodium hydroxide before it entered removal of samples of large volume from the atmosphere of the instrument. This was done without affecting the p-dichlorobenzene content. Under these circumstances the inthe seedbed in order to increase the accuracy of the method. A method utilizing the principle of the dew point was next strument gave fair results if the partial pressure of the pemployed. This involves the determination of the temperadichlorobenzene in the air mixture was above 0.4 mm. Howture a t which a film forms on a cooled mirror surface due to ever, the concentrations in the atmosphere of the seedbeds the deposition of droplets or crystals from the saturated vapor were such as to give partial pressures of 0.1 to 0.2 mm. At next to the surface. p-Dichlorobenzene formed a series of these low values the readings on the modified instrument were crystals on the surface which continued to grow larger instead not reliable, and therefore it did not prove useful for estimaof forming a uniform coating on the mirror. This made i t tions in seedbeds. difficult to fix the temperature a t which condensation occurred The fourth method involved condensation of the p-dichlorowith accuracy. I n consequence this method was abandoned. benzene from the atmosphere. This was done by cooling a A third method which, it was thought, might be utilized tared condenser to about -70' C. (-95' F.) and weighing the p-dichlorobenzene directly. Air samples were drawn from for the rapid estimation of p-dichlorobenzene vapor in air was the use of a combustible gas indicator. An instrument was the atmosphere of the seedbeds by aspiration a t the rate of 18 available which had been calibrated for the determination of liters per hour. The sample passed through a long tube (20 to 24 inches) containing flake sodium hydroxide to remove the benzene vapors. The principle on which this operates is esmoisture. Then it entered the tared condenser, which was sentially as follows: A sample of vapor-laden air is led over a cooled by immersion in a solid carbon dioxide-ethyl alcohol wire coated with a catalyst. This wire forms one arm of a bath. Preliminary experiments were carried out using bridge arrangement. A similar wire in vapor-free air forms another arm of the bridge. The combustion of the vapor in air over the 1 catalyst raises the temperature of the TABLE11. VAPORPRESSURE MEASUREMENTS first wire changing its resistance. The Weight of Barometric T e m p . of Vol. of Vapor Pressure Av. Vapor Temp. of p-Dichlorc1extent of unbalance of the bridge is reDry Air of CsHaCli Pressure Pressure Aspirator corded on a meter which is calibrated M m . Hg M m . Hg c. Liters Mm. Hg C. Gams directly in terms of the benzene vapor 2.935 0.232 752.99 28.4 10 0.0057 751.97 28.0 2.935 0.232 O'ii2 content of the air. 0.0057 31.6 2.915 1.65 ... . .. 751.7 30 0.0376 This instrument was modified as 751.1 32.0 2.910 1.51 0.0344 described below in the hope that it could 0.0397 754.2 29.0 2.932 1-73 ... 755.0 26.0 2.952 1.63 ... 0,0377 be made sufficiently sensitive to be used 0,0374 752.5 29.0 2.933 1.63 1.63 for the estimation of p-dichlorobenzene. 50 0.1802 753,ss 31.4 2.945 5.39 31.6 2.946 5.45 5:?;35 755.49 0.1817 Greater sensitivity is required because of the lower vapor concentration involved 0
0
JULY, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
weighed quantities of p-dichlorobenzene to check the accuracy of this method: Moist air (as present in the seedbeds) was drawn over the p-dichlorobenzene, through flake sodium hydroxide, and then through the tared condenser. The increase in weight of the condenser was usually greater than could be accounted for by the amount of p-dichlorobenzene used. This increase, due to moisture which was not taken out of the air stream, varied from 2 to 4 mg. per 18 liters of moist air. If the air stream was dried by P 2 0 5 tubes before passing over the p-dichlorobenzene and was then passed through the flake sodium hydroxide tubes before entering the condenser, approximately the theoretical increase in weight was obtained. These results are as follows: ' Condition of Air Stream Moist
p-Dichlorobenzene T-aporized into 18 Liters Increase in --eight of of Air, Grams Tared Condenser, Grams 0.0318 0 0337 0.0344 0 0384 0,0503 0.0466 0,0288 0,0287 0.0273 0.0273 0.0282 n.0287 0.0316 0.0310
Furthermore, i t was found that if the p-dichlorobenzeneladen air was dried by passing over P20jinstead of over sodium hydroxide, part of the p-dichlorobenzene was retained by the Pzo5. The following data illustrate this: p-Dichlorobenzene Taken U p by Air, Grams 0.0271 0.0284
Increase in Weight of Tared Condenser. Grams 0,0202 0.0216
These results showed that air with a moisture content a p proximating that found in the seedbeds could be completely dried over P206but that some of the p-dichlorobenzene would be retained by this drying agent. This made it impossible to utilize P205 in practice in seedbed determinations. On the other hand, while flake sodium hydroxide did not remove any p-dichlorobenzene from the air stream, i t allowed a small amount of moisture to pass through and be condensed in the tared condenser. I n view of these results the following procedure was employed in making estimations in the seedbeds. Sodium hydroxide was used for drying, and the small amount of moisture passing by i t was allowed for by means of a blank determination on the pdichlorobenzene-free atmosphere from an adjacent bed. The correction thus found was subtracted from the weight of the p-dichlorobenzene condensed out in the determination paralleling the blank. This method of freezing out p-dichlorobenzene in a tared condenser was found to be as accurate as the combustion method, outlined previously. It could not only be carried out more quickly, but was also more easily applicable to field conditions and was of sufficient accuracy for the present purpose.
Vapor Pressure of p-Dichlorobenzene A knowledge of the vapor pressure of a crystalline solid fumigant like p-dichlorobenzene is necessary if it is to be intelligently employed in laboratory and field experimentation. A survey of the literature showed that the vapor pressure of solid p-dichlorobenzene had been reported by only two au6). thors (4, Speranski (6) used a static method, employing an oil manometer, and made determinations on both solid and liquid phases. Roark and Nelson (4)obtained their figures from measurements by Smith on liquid p-dichlorobenzene above the freezing point. From the vapor pressure of the liquid over a range of temperatures the heat of vaporization was computed. To this was added the heat of fusion as estimated from Trouton's rule, and the curve was then extrapolated to cover the solid p-dichlorobenzene at room temperature (6). Since these two sets of values disagreed as much as 50 per
949
cent a t the lower temperatures, it was decided to make a direct experimental determination of the vapor pressure of the solid. For this purpose the following air current method similar to that described by Perman and Davies ( 2 ) was employed: Purified p-dichlorobenzene was sublimed onto glass wool in a 250-ml. bulb. The loss in weight of this bulb was determined when a known volume of dry air was passed through it. The bulb was immersed in a constant-temperature bath held a t 10" * 0.02", 30" j: 0.02", and 50" * 0.05" C. The vapor pressure, p, was calculated by means of the formulas: W X T X 760 X 22 4 p =-P x v, VI = M X 273 X P V VI where W = weight of p-dichlorobenzene lost T = temperature (absolute) of bath M = molecular weight P = barometric pressure, mm. H g at 0" C 1' = volume of dry air = V , ____ (p V , = volume of moist air = volume of p-dichlorobenzene vapor VI
+
-THao)
The results of the measurements a t the three temperatures are given in Table 11. The average vapor pressure values a t the three temperature points were fitted to a n equation of the type, -A logP = - + B T
The constants found for the equation are A = 3570.0" and B = 11.985. I n column 2 of Table I11 the vapor pressure values calculated by means of this equation are listed, and in column 3 the values of Smith as given by Roark and Nelson are shown for comparison.
DICHLOROBENZENE
TABLE111. VAPOR PRESSURE OF Vapor Pressure
Temp.
=
c.
0 5 10 15 20 25
Calcd. Vapor Pressure Mm. 0.089 0.139 0.234
0,388 0.633 1.02
by
Rosrk and Selson Mm. 0.08
..
0:39 0.64
1 .o
Temp.
c.
30 35 40
45 50
Calod. Vapor Pressure Mm. 1.60 2.47 3.77 5.74 8.55
Vapor Pressure by Roark and Nelson Mm 1.5 2.3 3.4 5.1 7.4
The agreement between the two sets of values is good except a t 50" C. where our results are about 1 mm. higher than those calculated by extrapolation by Smith. I n view of the agreement between these calculated values and our direct determinations, the data of Speranski appear to be unreliable.
Acknowledgment I n connection with these studies we wish to acknowledge the help of Marcus Hobbs and Helen Parks, both of this laboratory. Literature Cited Clayton, E. E.. Science, [N. S.]88,56 (1938). Perman, E.P.. and Davies, J. H . , J . Chem. SOC.,91,1114 (1907). P i n c k a r d , J. A., and McLean, R u t h , Phytopathology, 29, 216 (1939). Roark, R . C . , and K'elson, 0. -4., J. Econ. Entomol., 22, 385 (1929). Smith, C . M., private communication. Speranski, A., 2. p h y s i k . Chem., 51,44 (1905). Van Winkle, W.A . , and Smith, G. M c P , J . Am. C'hem. Soc., 42, 333 (1920).
