The Particle Size of Insecticidal Dusts A New Differential Manometer-Type Sedimentation Apparatus LYLE D. GOODHUE AND CHARLES M. SMITH Bureau of Entomology and Plant Quarantine, U. S. Department of Agriculture, Beltsville, Md.
T
H E determination of the The sedimentation apparatus A new type of sedimentation apparatus particle size of finely divided to be described here employs is described which employs the principle of materials by s e d i m e n t a t ion the principle of the differenthe differential manometer. A movement analysis has been carefully intial manometer, using two imof the meniscus in the manometer correvestigated by Od6n (S, 6, ?). m i s c i b l e l i q u i d s . By this sponding to a change in concentration of The method is based on Stokes’ means the rise obtained can be law, which gives the relation beincreased to 50 or 100 times that 50 mg. per liter in the suspension can easily tween the velocity of a particle obtainable in the manometer be detected without optical magnification. and its radius when falling in a of Wiegner’s a p p R r a t u s , and The density of the dispersed substance fluid medium under the influence the decrease in density of the can be determined directly in the appaof a c o n s t a n t force, usually suspension can b e m e a s u r e d ratus with sufficient accuracy to be used in gravity. He obtained a curve accurately. A movement of the by plotting the weight of acmeniscus in the m a n o m e t e r the calculation of particle size, at least for cumulated sediment as ordinate corresponding to a change in dusts that do not vary greatly in particle against time as abscissa and density of 0.00002 gram per cc. size. from this the particle size and can easily be detected without Dilute ethyl alcohol near the concentraparticle-size distribution were the aid of optical magnification. tion having maximum viscosity is recomcalculated. Although not the This change in t h e d e n s i t y first to use Stokes’ law in this of the suspension corresponds mended as the most satisfactory sediconnection, O d h contributed to a change in concentration mentation medium for insecticides in this the graphical method for the of about 50 mg. per liter when apparatus. determination of particle-size the dust has a density of 3.0 distribution from the accumugrams per cc From the density of the suspension, d,, the suspending lation curve. A condensed report of the extensive researches medium d,,, and the top liquid, dt, together with the settling by O d h and others on this subject is given in Alexander’s height, H , the following equation can be derived to calculate the “Colloid Chemistry” ( 7 ) . rise, R, in the manometer produced by a suspension of dust: Instead of measuring the weight of sediment accumulated, Wiegner (9) measured the decrease in the density of the suspension with an apparatus that would be very similar to the one shown in Figure 1 if all parts above the center stopcock, F , were removed. The large tube contained the From this equation it is evident that R suspension and the small manometer tube the suspending can be increased eit,her by increasing the liquid. Since the density of the suspension is greater than concentration of the suspension or by that of the pure liquid, the meniscus in the manometer tube decreasing the difference in the densities will stand a t a higher level than that in the sedimentation of the two liquids. Practically there is tube. As the particles settle and pass the opening t o the B limit to both of these variations. S stopcock, D, their effect on the density is lost and the difDescription of Apparatus ference in the level between the two columns gradually deThe apparatus, shown in Figure 1, is creases. made from standard Pyrex tubing. The It is not desirable to use a suspension containing more than large tube, A , is 50 cm. long by 4.5 cm. in 1per cent of solids (10 grams per liter) in any sedimentation diameter. A smaller tube, C, about 2.5 cm. analysis. If this concentration is used, it is evident from a in diameter extends the total length t o 86 cm. The iacketed manometer tube, B, simple calculation that the rise in Wiegner’s manometer tube 4-mm. inside diameter, is joined through a will not exceed 1 cm. even for the most dense materials. 3-mm. bore stopcock, D, 13 cm. from the Kelly (6) has magnified this movement in the manometer bottom of the large tube and again through tube by having it nearly horizontal a t the top, so that a a similar stopcock, E, 8 cm. from the top slight change in level causes a large movement of the meniscus. of the apparatus. A scale, S , is inserted behind the upper half of the manometer Evaporation and the dficulty of adjusting the level of the tube inside the jacket. The jacket rotects suspension, as well as the position of the tube itself, are the delicate manometer tube anx when serious disadvantages. Gessner (4)has magnified the rise e v a c u a t e d reduces the effect of rapid in Wiegner’s apparatus photographically and recorded the changes in temperature. The 2-mm. stopcock, F, is used t o set the level of the change on a photographic paper attached to a revolving drum. suspension. Even with this arrangement it is necessary for him t o use a The. construction around stopcocks D 5 per cent suspension to obtain a suitable change. Moreover, and E is important. They should be not the apparatus is complicated and must be used in a dark more than 2 cm. from the large tubes, and the connectingtube must be blown out large room. A differential manometer has been used by Crowther and cone-shaped. This prevents the entrap(2) and by Puri (8) in studying the sedimentation of soils, ment of bubbles at D and decreases the but their type of manometer and the construction of their amount of sediment that settles there. instruments are different from the one developed in this The tube connecting stopcock Eshould drain FIGURE1. APand fill easily t o the plug in the stopcock. PARATUS laboratory c
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A slightly oblique angle in the UP er tube between the stopcock and the end of the manometer ut!e is also important.
Choice of Liquids The choice of liquids to be used in this apparatus depends largely on the sedimentation medium that is suitable for a particular material. The physical or chemical nature of either the dispersed dust or the dispersing medium must not
TIME I N
MINUTES
FIGURE2. MANOMETER READINGS change when the suspension is prepared. Fortunately many insecticides can be suspended in water, alcohols, or mixtures of these liquids without appreciable solution or reaction. Water alone and some alcohol-water mixtures were tested in this apparatus. The next problem was to find an upper liquid that was immiscible with the sedimentation medium, and had a slightly lower density. It is usually more convenient to use a mixture adjusted to the proper density rather than to search for a pure liquid. A mixture of hydrocarbons adjusted to the proper density by the addition of chlorinated compounds was used with the sedimentation media mentioned above. It is very important that the sedimentation medium and the upper liquid have a very low mutual solubility. A movement of one liquid phase into the other interfererj with the sedimentation by changing the nature and density of the medium. A small change in the density of either liquid also introduces a large error in the manometer readings on this sensitive instrument. The mutual solubility of the liquids can be checked by running a blank determination. In this case the manometer reading corresponds to the point of complete settling of the suspension during a regular run. The ideal pair of liquids would give a stable zero reading which would not change, a t least, during the time required for a sedimentation analysis. This is nearly attained with the pair of liquids described below, which are recommended for use in this apparatus in the analysis of many insecticides. Dilute ethyl alcohol made by mixing equal volumes of water and 95 per cent alcohol was found to be a suitable sedimentation medium. As this mixture is approximately 50 per cent by volume, it will be referred to as such. The density is 0.9280 gram per cc. and the viscosity is 0.02346 poise a t 25" C., although these.values vary slightly with different lots of alcohol. This medium has several advantages. Because of its Iow surface tension, no foaming occurs and there is no interference with the movement of the meniscus in the manometer. While the solubility of many inorganic salts is lower in 50 per cent alcohol than in water, thus decreasing the action of electrolytes, this mixture is still capable of dissolving many
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of the common deflocculating agents. Another important factor is its relatively high viscosity. The maximum viscosity for an alcohol-water mixture occurs near the 50 per cent point and is about three times that of either component. A substantial shortening of the sedimentation tube is therefore possible without sacrificing accuracy in the coarse fractions. The top liquid is made by adding 1,2,4-triChhrobenzene to decahydronaphthalene to obtain a density of about 0.9000 gram per cc. A small amount of an intensely colored oil-soluble dye is added, and the mixture is extracted three times with a slightly ammoniacal 50 per cent alcohol solution. This extraction removes soluble substances in the upper liquid and saturates it with dilute alcohol. If an emulsion persists in the oil layer, it may be removed by filtering through a fluted paper. The solution is then adjusted to a density of 0.9000 * 0.0002 gram per cc. by adding more 1,2,4-trichlorobenzene. About 100 cc. are used for each determination. A liter or two of this liquid is sufficient, since it can be recovered and used again after the density has been adjusted.
Control of Temperature A differential manometer with arms of unequal diameter is very sensitive to changes in temperature. The apparatus must therefore be operated at some constant temperature, or under conditions where the change in temperature is very slow. A room varying not more than 0.5" per hour is satisfactory, but a water bath regulated at a definite temperature is better. A controlled water bath simplifies the calculations, but a slightly different technic must be used. The upper part of the water bath must be provided with a glass window, and the suspension must be stirred by a plunger without removing the apparatus from the bath. This eliminates the cooling effect caused by evaporation from the wet apparatus when it is shaken outside the bath. A long-handled brush saturated with dilute alcohol served as an excellent stirrer. Not less than 50 rapid strokes are neceRsary to obtain a uniform suspension. A correction can easily be applied for the amount of sample removed on the brush. Procedure The following detailed procedure for the determination of the particle-size distribution of a Georgia clay may be used with slight variations for many other materials. A 7-gram sam le was mixed to a thick aste with 50 per cent alcohol to which Rad been added 5 drops oPammonium hydroxide (sp. gr. 0.90). After the lumps had been broken up and a smooth paste obtained, more 50 per cent alcohol (50 cc.) was added and the sample shaken overnight. The apparatus, especially the manometer tube, was carefully cleaned and rinsed with 50 per cent alcohol. It was then filled t o the stopcock, F, with the sedimentation medium (about 600 cc.), and from 3 to 6 drops of concentrated ammonium hydroxide were added. After a thorough mixing, the manometer tube was filled by tilting the apparatus and the stopcocks were closed. Some of the dilute alcohol was poured out and the previously dispersed sample introduced. The alcohol was used to wash in remaining portions of the sample, and finally the level was adjusted by adding sufficient liquid to overflow at F. The suspension was then thoroughly mixed and placed in the water bath until it reached 25' C., the temperature of the bath. After about 20 minutes the suspension was again thoroughly stirred with a long-handled brush or plunger, and the top liquid was added through a long-stemmed funnel having several fine holes in the side of the stem near the bottom instead of one large hole in the end. With this device the liquid was sprayed against the sides of the apparatus, so that it flowed down gently wlthout disturbing the up er layer of the suspension. The to liquid must also be at t i e same temperature as the bath. Tfe s t o p
NOVEMBER 15, 1936
ANALYTICAL EDITION
watch was started immediately after the last stirring, but it was returned to zero after 30 seconds because settling does not start until the eddy currents die out. About 45 seconds are required to introduce the top liquid and open the two stopcocks on the manometer. The upper liquid rapidly replaces the dilute alcohol in a part of the manometer tube and equilibrium is reached in about 2 minutes. A reading can be obtained at 1 minute, but this point, is usually above the curve. The zero point, or the point denoting complete settling, was found to be 20 on the manometer scale. It was obtained by running a blank determination. It is neither practical nor necessary to continue the readings to this point.
Results The readings obtained for the Georgia clay are presented in Table I, together with the readings for a run on samples of dusting sulfur and calcium arsenate. In Figure 2 these data are plotted to show the type and regularity of the sedimentation curve. The particle-size distribution for some fractions is presented in Table 11. The amount falling in each fraction was determined by the graphical method of OdBn. The results of a sedimentation analysis may be expressed in several ways. Borchers and May ( I ) suggest the use of the sedimentation curve with a table giving the particle-size distribution, as has been done above. The distribution may be expressed graphically as an accumulative frequency curve or as a frequency-distribution curve. Since this paper is concerned primarily with the use of a new apparatus, it is not considered necessary to present the distribution graphically. After a little experience the sedimentation curve itself will give considerable information. A curve with a rapidly changing slope indicates a polydispersed sample, while a curve approaching a straight line indicates monodispersion. Unless two samples have approximately the same particle-size distribution, it is possible to pick the finer material without further calculation. The curves also give a graphical comparison of the densities. The material whose extrapolated curve intersects the zero ordinate at the highest point has the highest density. This point is of practical importance, as shown below. Determination of Density Since the rise in the manometer can be calculated from the density of the suspension (Equation l),then conversely the density of the suspension, and hence the density of the dust, can be calculated from the rise. The rise can be obtained by extrapolation of the sedimentation curve and from the determination of the zero point. Therefore R j=g (d, - dt) = d, - d, = Ad
(2)
After Ad is obtained in this manner, the density of the dust can be calculated from the following formula: dmC
C-
Ad
-i
D
where C is the concentration of the dispersed phase in grams per cubic centimeter of suspension and D is the density of the dispersed phase. The precision obtainable by this method is about 0.1 unit in density. For dusts that settle slowly, or that do not vary greatly in particle size, this error in density will shift about 1 per cent or less from one fraction to another. For dusts that vary greatly from some mean particle size, as in the case of the sulfur sample reported here, a shift of about 4 per cent is obtained on some of the larger fractions. For the first type of dust this method of density determination is
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considered sufficiently accurate for the calculation of particle-size distribution, and it may be used in the second case if the error introduced falls within the limits of accuracy desired. Table I11 shows the densities of the three insecticides under discussion when obtained with the sedimentation apparatus, as compared with the values obtained in a pycnometer. TABLEI. MANOMETER READINGS AT DIFFERENT TIMESFOR THREE TYPICAL INSECTICIDAL MATERIALS Manometer Scale Readings Georgia Calcium Dusting clay arsenate sulfur
Time Min. 1 2 4 6
67.0 62.8 61.7 60.8 59.9 59.0
S
10 12 14 16 18 20 22 24 26 28 30 32 36 40 42 48 50 60 70 72
...
78.0 74.8 74.2 73.9 73.4 73.0 72.7
...
72.0
...
71.4
57.3 55.9 54.6
...
53.8
...
.. .. .. ...
...
70.4
...
69.2 68.8
51.3 50.7 49.5
...
48.6
...
84 100 120 130 144 150 174
47.5 46.5 54.4
200 220 240 320
42.8
... 44.7
43.7
...
IS0
...
... 52.1 ...
53.0
SO
...
... ... ...
... ...
69.8
... ...
67 56.3 53.3 51.0 48.8 46.7 44.8 43.2 41.7 40.5 39.4
... ... 36.0 ...
37.4
34.8 33.6 32.6
...
...
...
30.7 29.1 28.1
67.9
27.3
... ...
67.2 66.4
... ... ...
.
.
I
... 27.0 ... 26.2
65.1 64.7
... ... ... ... 24.8
63.7 61.6
... ...
66.7
...
24.4
TABLE11. PARTICLE-SIZE DISTRIBUTION OB THREETYPICAL DUSTS Material Calcium arsenate Dusting sulfur Georgia clay
Percentage (by Weight) of Material of Indicated Range of Radii Above 20 p 20-10 p 10-7.5 p 7.6-5 p Below 5 L,I 1 2 3 6 88 38 4 23 161 12 10 9 68
TABLE111. COMPARISON OF DENSITIES OBTAINED ON T SEDIMENTATION APPARATUB WITH THOSEDETERMINED IN PYCNOMETER Material Calcium arsenate Dusting sulfur Georgia clay
B y Pyanometer In 95 per In 50 per cent alcohol cent alcohol 3.26 2:i7 2 :52 ..
H ~ A
B y Sedimenta-
tion Apparatua 3.23 1.93 2.43
Discussion The most desirable form of this apparatus would be one that could be used in any laboratory without special precautions against changes in temperature. With this in mind a tube was constructed having the manometer inside the large tube. As far as the manometer was concerned, the temperature effect was almost eliminated, but rapid changes in temperature also cause eddy currents in the suspension which interfere with normal sedimentation. For coarse materials this error is small, and further efforts are being made to develop a satisfactory apparatus according to this principle. An apparatus similar to that shown in Figure 1, but pro-
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vided with a jacket through which water could be circulated, was also tried. This requires some means of circulating water at constant temperature. No advantage is gained over the use of the simple design in a water bath. The accuracy attainable with the apparatus is subject to the same criticism as that of any method of sedimentation analysis. The shape of the particle and the lack of complete dispersion, partial flocculation, etc., have been discussed a t length by other authors. It is felt, however, that the advantages over the microscopic or air-elutriation methods, as pointed out by Borchers and May ( I ) , are suqcient to justify the use of sedimentation analysis, especially for technical purposes. The precision obtainable with this instrument is dependent upon the accuracy with which the tangents to the sedimentation curve can be drawn. Actual
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readings taken on the manometer during carefully conducted check runs do not vary more than 2 per cent.
Literature Cited (1) Borchers, F., and May, E., Mitt. Biol. Reichansf Land u. Forstw., 50, 5-55, especially 17 (1935). (2) Crowther, E. M., J. Soc. Chem. Ind., 46, 105T (1927). (3) Fisher, R. A., and O d h , S.,Proc. Rou. Soc. Edinburgh, 36, 2 1 9 (1916); 44, 98 (1924).
(4) Gessner, H., Kolloid-Z., 38, 115-23 (1926). (5) Kelly, W. J., IXD. ENG.CHEW,16, 928 (1924). (6) OdBn, S., Kolloid-Z., 18, 33-48 (1916). (7) O d h , S., in Alexander’s “Colloid Chemistry,” Vol. 58, New York, Chemical Catalog Co., 1926. (8) Puri, A. N., Soil Science, 34, 115 (1932). (9) Wiegner, G . , Landto. Vers. Sta., 91, 41 (1918).
I, Chapter
RECEIVED May 27, 1936.
A Variable Vapor-Volume Barometric Type of Vapor Pressure Apparatus EVERETT M. BARBER
AND
A. V. RITCHIE, The Texas Company, New York, N. Y.
IS
MANY problems involving the use of gasolines, naphthas, and similar volatile products it is desirable to know the vapor pressure of these products over the initial range from zero to some intermediate per cent evaporated. In connection with several problems requiring this type of information, a variable vapor-volume barometric type of vapor pressure apparatus, which gives somewhat more detailed and accurate information than the usual Reid vapor pressure test and yet is sufficiently rapid in operation to allow its use for semi-routine testing, has been used by the writers with considerable success. It is the purpose of this paper to describe briefly the design and operation of this apparatus and to indicate the manner in which the results obtained from it may be applied to several typical problems, particularly to the measurement of evaporation losses and the vapor-locking characteristics of gasolines. Design of Apparatus The apparatus, which consists essentially of two glass tubes joined a t the bottom to a leveling bottle, is illustrated in Figures 1 and 2. The long glass tube, A , is 220 cm. long, has a scale graduated in millimeters over 200 cm. of its length, and is open
to the atmosphere at the top. The tube, B, is sealed off at the top with a large carefully ground three-way stopcock and has alternately long narrow sections 30 cm. long and 1 cm. in diameter to obtain accurate readings of the liquid volume, and bulbous sections 15 cm. long and 3 cm. in diameter to accommodate a relatively large vapor volume. The open tube, the buret, and the leveling bottle, which constitute the essential parts of the vapor pressure measuring system, are filled with mercury, and in order to obtain temperature control are immersed in water bath a t 37.8’ C. (100’ F.). The bath is thermostatically controlled and has a temperature gradient of 0 . 0 5 O C. (0.1’ F.) from top to bottom and a variation in the average of about 0.1” C. (0.2” F.). Readings are made by means of a cathetometer which clamps to the tie rod at the front of the apparatus and is adjusted between the meniscus level and fixed stations on the tube and buret by means of a 40-thread micrometer screw. Successive trial measurements between two fixed points indicate that the settings of the cathetometer hair lines can be reliably reproduced to about 10.004 ml. in sections 1, 3, and 5. A bright light behind the apparatus silhouettes the meniscus levels and greatly facilitates the setting of the cathetometer cross hairs.
Operation of Apparatus Preliminary to running each test, the mercury level is dropped below the air injector, D, and with the stopcock opened to the
TABLEI. RESULTS OF REPRESEXTATWE TEST iSamule 131.8) , .
Mercury Levelb Corrected
Vapord Vapor Volume Liquid0 Mercurya Level AP Pressure Barometer Observede Correctedl Volume Evaporatedh V/Ll Mm. Nm. Mm. 1Mm. Mm. Mm. cc cc. MI. % 5.6 10 I008 0.31 351 157 756 7.1 0.56 599 358 508 10.000 0.39 162 756 9.1 9.1 594 390 0.72 397 552 9 2 . 5 1 800 4 6 . 2 294 7 6 . 0 462 501 756 4.73 502 795 3.72 9.665 68.7 120,7 320 704 756 7.10 436 711 1024 3 . 9 4 9 . 6 4 3 7 2 . 8 325 1 2 8 . 2 756 7 .56 806 431 813 1131 5.24 9.500 96.8 344 178.5 756 10.20 412 917 918 1261 6 . 6 4 9 . 3 7 3 2 4 0 . 3 1 2 2 . 4 369 756 1 3 .00 1134 387 1141 1503 6.72 9.364 241.6 124.0 13.29 1152 371 756 385 1159 1523 a Values given as mm. of mercury, are obtained by direct observation of mercury levels in tubes. When approximately 10-cc. sample is used, subtract 7 mm. in sections I, 3, and 5; 1 mm. i n b Columh 2 corrected for height of column of gasoline. sections 2 and 4. 3; or column 1 - 2 corrected. c Difference in mercury levels: column 1 d Vapor pressure = barometric pressure (column 6 ) - A P (column 4 ) . e Volume of vapor above gasoline. VP f Corrected to 760 mm. = observed vapor volume X 76037.75’ , C. (100’ F,). g Volume of liquid gasoline =, volume between meniscus levels volume included in meniscus (1,8; Figure 3). Original volume, 10,045 ml. initial volume - liquld volume x 100. h Per cent evaporated = initial volume i Vapor-to-liquid volume ratio vapor volume (oolumn 8 ) + liquid volume (column 9). At 37.75O C . (loo0 F.),760 mm.
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I
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