Increase in Concentration of Insecticide in Freon-I 2 Accompanying Transfer or Discharge of an Aerosol-Producing Solution C. M. SMITH
L. U. S.
AND
Bureau of Entomology and Plant Quarantine,
D. GOODHUE Department of Agriculture, Beltsville, Md.
dm, consisting of solution only will cause ii corresponding change in the valur~of ic, as given by the equation
In the transfer or discharge of solutions in liquefied gases used for tho production of insecticidal aerosols, a concentrating effect occurs because of escape of solvent from the solution to maintain the high vapor density. A mathematical treatment of this effect is given, and experiments are described b y which it was confirmed for solutions in Freon-12. In that case a discharge of 90% of the liquid phase raises the concentration of the remaining solution by 870 of its value.
But
T
H E iiiaectioidal aerosol, produced when a, solution of insecticides in :t liquefied gas is released into the air ( 3 ) , has met an urgent military need, especially for disinfesting airplanes and for overseas use. The combination of pyrethrum :tnd sesame oil in dichlorodifluoromethane (Freon-12) produces a very effective nonflammable insecticide that is nontoxic to man and animals. I n the manufacture and packaging of this solution, certain questions have arisen concerning the physical properties of yolutions in liquefied gases. A mathematical treatment of one of these problems, which has t'o do x ith change in concentration due to transfer or discharge of insecticide, and confirmatory experimental results are presented in this paper. At 80" F. (26.7" C.) the density of saturated Freon-12 vapor is 0.0377 gram per cc. As liquid is withdrawn from an aerosol contairictr during use, appreciable quantities of Freon evaporate from the qolution remaining in the conhiner to maint.ain t h i j high V:LpOT concentration in t,he increasing space not occupied by the liquid. As a result the concentration of the remaining solution gi~iiduallyincreases as the container empties. While thiJ change is le+ d o u s than if the solution progressively weakened, still the need for con-ervation of insecticide suggests that some consider:*tioii be given t.o the matter, especially in connection with park:ging the solution.
From 1 and 3
(2)
1--r
Proceeding to definite integrals, Therefore, since
ui =
In
W
=
In
m I c and W = 3f.C
from which
If -If81s knoivn, as it was in some of these laboratory experiments because of the manner of filling, m, a t any stage is calculable from it and the weight, Q, of liquid withdrawn, for from Equation 2
By the aid of values calculated from this equation, the per-
c-c
wntage increase in concentration, 100 __- , can be plotted MATHEMATICAL DEVELOPMENT
g a i n s t loo,' ~
A n approsimate estimate of the magnitude of this effect a t any givctn fixed temperature can be obtained by means of the calculus,
if it is assumed that the densities of both gaseous and liquid plia+es remain constant. This is obviously not strictly true, but it v.ill be shown later that the departure from exactness i h incon;equential. The mathematical development follow : I,&
iVf
the percentage of solution withdrawn, as is illus-
8
trated in Figure 1 for a solution containing 5% of sesame oil in Freon-12. For the construction of this graph D, was taken as 0.0377 gram per cc., D, as 1.291 grams per cc., and T therefore as 0.0292. It shows that a quantity of liquid equal to 22% of the original liquid content must be withdrawn before the concentration of insecticide rises 1%, that removal of 80% causes a 5% rise, and that 92.5% delivery gives only a 9% rise. If, as will more often be the case, the total content of t h e container, M,rather than the liquid content, .%fa, is known,-Equatioii 4 can be converted, because of the relationship
V = volume of container, in cubic centimeters .I/ = weight, of initial total content, in grams .\I, = weight of initial liquid content, in grams C = initial concentration of insecticide in the liquid, in grams per gram W = initial weight of insecticide in the liquid, in grams D. = density of insecticide solution, in grams per cubic centimeter Do = density of solvent, vapor, in grams per cubic centimeter T = the ratio Do/?. Q = weight of liquid ivithdrawn (no vapor being- allo\ved to escape:ir n = weight, of total contents after withdrawing Q, in grams i)zs = weight of liquid contents after withdrawing Q , in grams c = concentration of insecticide after withdrawing Q , in grams per gram I ( ' = weight of insecticide in container after withdrawing &, in grams
M.
=
A1 - V D , -___ I - r
into the> equivalent form
c-c
.tnd 100 _ _ can be plotted against
'y,
the percentage
(I:
total contents removed. EXPERIMENTAL VERIFICATION
The errors due to the assumption t h s t the gas and liquid denhities are constant can be judged by a consideration of the possible departures of those values from constancy. The possible changei n 4 : b q denrity a e r e derived from mmwremrnts of the lowerine
A t :iny st:tge of emptying, the rveight of insecticide in the coilTTithdrewnl of an additional infinitesimal weight, I/'.
txiner is
355
356
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 16, No. 6
both sides were used. The exact amount of Freon needed for the solution was introduced to avoid fractionation by the removal of any excess. The results are shown in Table I.
J
10
20
30
40
50
60
PERCENT OF O R I G I I \ I A L S O L U T I O N
70
80
90
100
WITHDRAWN FROM CONTAINER
Figure 1. Increase in Concentration of Freon Solution of O i l y Insecticide as Contents A r e Withdrawn from Container
Table
I.
Lowering of Vapor Pressure of Freon-19 by Sesame Sesame Oil
70
Oil
I'apor Pressure Lowering M m . of HQ
2.5 5.0 10 15
6, 18, 23, 35,
7, av. 6 . 5 19, 18, av. 1 8 . 3 24, 28, av. 25 37, av. 36
of vapor pressure produced by dissolving various proportions of qesame oil in Freon-12.
The apparatus used t o make these measurements (Figure 2) consists of two identical containers with valves connected through a U-tube containing mercury, which acts as a differential manometer. Two small petcocks, one on each side of the manometer, are necessary to operate the apparatus. Three hundred grams of liquid Freon-12 were always placed In the container on the right and an equal weight of solution was made up in the one on the left. The connections were made to the manometer, and the vapors from the two containers were allowed to enter the manometer simultaneously until the total vapor pressure on each side was exerted. The whole apparatus was then submerged in a transparent water bath. Readings were made a t 80" F. after the system had reached equilibrium. The Freon-12 used contained some nonliquefiable gases, which interfered somewhat. To overcome this interference, containers of the same size v i t h the same volumes occupied by the liquid on
Since the total vapor presaure of Freon-12 is about 5000'mm. 7 of mercury, the degree of reproducibility shown is consideredivery good. Fifteen per cent of nonvolatile material has been found to be about the optimum that should be used in a n aerosol solution. Such a solution will have a vapor pressure about 36 mm. of mercury below that of Freon-12. Reference t o the equation of state derived for Freon-12 by Buffington and Gilkey ( 0 ) shows that this lowering of pressure produces a change in vapor density of only 0.0003 gram per rc., which for the authors' purposes can be considered negligible in comparison with t h e figure 0.0377 gram per cc. wed in constructing the graph. The possible change.: in liquid density were evaluat,ed by consideration of the values for density of solutions of sewme oil in Freon-12, determined at 80" F. by means of ,z small hydrometrr in a closed system. The liquid was placed in a prwsure test tube together lvith the hydrometer. The whole apparatus (Figure 3) vas srt in n glass water bath at 80" F. and the length of the emergrnt stem was determined with n cathcxtometer. The hydrometer was calibrated b y observing t h e length of the emergent stem above the Freon at several temperatures. The error of this calibration due to the increasing vapor density above the Freon was calculated and found to be negligible. rl curve was plotted from lvhich the densities of the various sesame oil solutions were determined at 80" F. (26.7" C.). The viilues obtained are shown in Table 11. Since 15rd of oil is taken as the optimum, the change in liquid density vi11 not exceed 0.048 gram per cc., which would produce a change of only about 0,3
c-c
L
Figure 3. Pressure Test Tube Assembly and Small Hydrometer 1. Heavy-walled glass test tube I O mm. in inside diameter and 155 mm. long 9. Standard Y-valve for small refrigerant drums 3. Frame lrom 0.5-inch brass pipe with Ion windows cut in opposite d e s 4. Screw plug 5. Gasket 6. Rubber cushion 7. Hydrometer
in the value of 100 __ calculat.ed for the case in nhich 9OCh of t,he contents of the tank is withdrawn. Thus it appears that the graph is sufficiently a,ccurate for all ordinary purposes.
As a n objective confirmatory test, measurements of the increase in concentration were made on 400-gram samples of an approximately 5y0solution of cottonseed oil in Freon-12. CottonTable
I
II.
Change in Density of Freon-19 Due to Sesame
Sesame Oil
%
Figure 2.
0 2.5
Differential Manometer Apparatus to Determine Vapor Pressure Lowering of Liquefied Gas Solutions
1. Heavy-walled glass tube conbinins mercury 9. Brass frame from 0.5-inch pipe having slob cut in each side 3. Rubber gasket 4. Adapter from 0.375- to 0.195-inch pipe thread
5. 6. 7. 8. 9.
Petcock Needle valve on conbinrr Container LiqueRed sa5 solution Liquefied gas
5.0 10
a
Density
G./cc. 1.303O
1 ,298, 1 ,2985, av. 1 ,298 1.290, 1.292, av. 1 . 2 9 1 1 ,274, 1.274, av. 1 . 2 7 4 1.2648, 1.2552, av. 1 . 2 0 0
15 20 1.2286. 1.2295. av. 1.229 From Bichowsky and Gilkey ( 1 ) .
Oil
ANALYTICAL EDITION
June, 1944
Table 111.
Concentrating Effect Caused b y Removal of Liquid from an Aerosol Bomb
(Containing originally 400 grams of a 5 % solution of cottonseed uii in Freon-12) S o l u t i o n Withdrawn Relative Concentration of Oil ' i hy weight Determined Calculated 0 20
80 90
100 9,
51
70
(100 0 )
100.0 102.1 105,2
101.5. a v .
101 2 105.3, 106.5. av. 105 9 108.9, 109.1, av. 109 0
108.0
heed oil W a 6 chosen instead of sesame oil because it did not oxidize when heated for the analyses. The initial concentration wau determined by withdrawing two 5-gram samples into pressurr test tubes, which were w-righed before and aftvr to .determine the exact weight, of the samples. The apparatus was used n-ithout the hydrometer. The volnt,ile solvent was then allowed to evaporate, and the test tube contmning thc residue was removed from the frame and heated for 30 niinutcJs :it 110' C. The weight of the residue wa? then dett!rrnined and the concentration by weight of nonvolatile matter calculated. Duplicate samples n-ere also taken aftrr 50, 80, and 90% of the solution had been allowed to escapc. 1Iechxnird difficulties made the results unreliable after 95% had h e w removed. All operations were ciirried out nt 80" F. Tht. r(wi1ts :we sho~vnin Table 111. The degree of concordance shown is good, considering the experimental difficnltirs involved. The over-a11 effect is compara-
A
357
tively small until the container is alrnost empty and, sincr, i t is in the direct,ion leading to greater assurance of getting the required minimum concentration, not very important in the actual application of the insecticide. I t might be important, however, to a manufacturer filling small containers from a large one. The last containers to be filled will contain more insecticide than the first unless some compensatory measures are taken. I t is also important when samples of solution are being used for test purposes as a standard of comparison. I n precise laboratory tests it would be good practice to use not more than 50% of the original solution. The simplest procedure in the commercial filling of :ierosol containers is to add sufficient pure Freon to the reservoir at, intervals to maintain approximately the original concentration. This procedure is used by some present aerosol manufacturers. Although the foregoing discussion has been based wholly on data pertaining to dichlorodifluoromethane, the formula developed will obviously apply to all liquefied-gas solutions for which the densities of the liquid and gaseous phascs rrmnin reasonably constant during evaporation. LITERATURE CITED
(1) Bichowsky, F. R., and Gilkey, IF'. K.. IND. ENG.CHEM.,23, 366
(1931). (2) Buffington, R. M., and Gilkey, W. K., Ibid., 23, 254 (1931). (3) Goodhue. L. D.. Ihid., 34, 1456 (1942).
Modified Bailey Buret
L O R E N H A M M A C K AND CHESTER L. N A E G E L I N ' Chemical Laboratory, San Antonio A S F Depot, Grayson Street Station, San Antonio 8, Texas
T
HE Sail Antonio Army Service Forces Depot Laboratory has, for some time, been analyzing large numbers of samples of mayonnaise and other semiyolid salad dressings. Tests have been made according to the methods of the Association of Official Agricultural Chenikts ( I ) . IVeighing out such samples n i t h the Bailey weighing buret ( 2 ) ctoiy. The only size available to the has never proved sat aut,hors has been the 30-m1. capacity ~i-liichholds an insufficient quantity of salad dressing if duplicate determinations are to he made. Salad dressings, moreover, are of such consistency that there is almost no flon. of material from the tip of the buret due to gravity alone. Forcing the sample out with t,lie plunger is a very .;low process and results in the accumulation of a considerable quantity of material on the adapter and on that portion of the plug which projects above. Such a sitiiation inevitably results in 1 0 ~ s . A fairly *iniple modification of the Bailey buret was decided upon as tlie best solution of this problem. The changes involved mere enlarging t,he buret to a capacity of 100 ml., straightening out t.he top of the buret completely, eliminating the constriction, and adding a plunger to go down inside tlie buret and around the plug. A11 clearances were kept to a minimum. Buret d is constructed from 51-mm. Pyrex glass tubing with joint a t top and inside 15/20 and outside outside 50/12 10/18 joint at. the constricted bottom. Distance between upper joint, and beginning of constriction is 50 mm. Flask B has outside l5j20 -$- joint and over-all diameter of 67 mm. Plug C is a 6-mm. glass rod. 165 mm. in length - .(over-all). and contains inside 10/18-$- joint. Plunger D is of 18-8 8-mm. (l/az-inch) stainless steel, consisting of a tube and disk mot-welded together. Inside diameter of tube and diameter of dihk are of sizg to give snug fitting around plug :tnd invide buret. respectively. Adapter c,r stopper E i+ hollow-ground and has inside j0/12 joint and orifice to fit over tube D. The modified buret is filled while sitting on the fla\k base B, with plug C in place. Plunger D is then fitted around the plug I
1
,
Present address, San Antonio Bir Depot, Kelly Field, Texas.
and allowed to rest on the materitil
The adapter, E , is finally placed in position, and the assembly is weighed. To remove the sample, the plug is raised and held in open position and: Kith the same hand, the plunger is pressed downward; forcing out the material. When enough sample has been taken, the plug is pushed into the joint, pressing out the last drop before the buret is returned to the flask. I t is immediately apparent that the total weight of t,he assembly, filled, is too great for the capacity of an ordinary analytical balance. The assumption is that the larger samples are to be weighed on a more rugged balance, an accuracy of 0.1 or a t most, 0.01 gram being adequate. The modifiedBailey buret should find use in analyzing samples, such as soft grease, paste paints, cert,ain asDhalts, water-reDellent emulsions, and other semisolid materials. The plunger, too, can always be removed and the buret can be used to an atlvantage for any bulky sample. Satisfactory working models ot this buret were obtained from the Scientific Glass Company, Bloomfield, S.J., and have been in rontinuous use with marked success during the past year. Steps are now being taken to remove minor defects in the design. LITERATURE CITED
(1) Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, pp. 475-7 (1940). (2) Bailey, H. S., J. IND. ENG.CREM.,6, 941 (1914).
