AGRICULTURAL DUSTS Determination of Particle Size Distribution DELOR.4 K . GULLSTROM AND H. P. BURCHFIELD ‘Vaugatuck Chemical Dicision, U . S . Rubber Company, Nuugutuck, Conn. Particle size distribution of organic fungicides and insecticides alone or in admixture with diluents can be determined at solids concentrations as low as 0.5 gram per liter by the sedimentation pipet method supplemented by optical or polarographic methods. The nature of the diluent used during grinding in part determines the distribution of the organic component of a mixture.
ASY agricultural formulations consist of mixtures of two or more components in which a solid organic fungicide or insecticide is ground in intimate mixture with inorganic diluents and wetting agents. Occasionally mixtures of fungicides and insecticides may be used to control several types of infestation with a single application. As biological activity has been shov 11 to depend on the particle size of the active ingredient ( 7 , 1 4 ) , a method is desirable by n-hich it is possible to determine the particle size distribution of the active component of a mixture in the presence of diluents and other auxiliary materials. Particle size distribution in the range of from 1 to 2 5 ~ radius is most conveniently determined by gravitational sedimentation in an aqueous medium. Methods that measure the average properties of a dispersion such as the balance method (IO) or the determination of the change in hydrostatic pressure during Sedimentation (8) are not applicable. However, the sedimentation pipet method (2) has proved to be useful if convenient and sensitive analytical methods are available which permit the estimation of the active component in the presence of other constituents. The pipet method is based on Stokes’ law ( I ) , which states that a spherical particle falling through a fluid medium reaches a limiting velocity a t which it will traverse a distance s in time t , where s is giveii by
The total solids content of the suspension should not exceed 3 to 5%; hence in the examination of dilute dusts containing only a few per cent of the active ingredient, it is necessary to estimate concentrations ranging from 0.05 to 0.5 gram per liter. To accomplish these analyses spectrophotometric, colorimetric, and polarographic methods are preferred, because they are sufficiently rapid to allow for the analysis of a large number of samples, and a t the same time are sensitive enough so that small aliquots from the sedimentation cylinder can be used even when the concentration is low. Most organic fungicides and insecticides contain groups that absorb light in the near ultraviolet, and many are electro-oxidizable or electroreducible; hence, the development of a suitable analytical procedure is usually not difficult unless other organic materials are present. Several specific examples are described below. EXPERIMENTAL
A number of methods for withdranjng samples from the sedimentation cylinder were investigated, but none appeared simpler in operation or more accurate than the pipet procedure described by Robinson (9). The following modification w-as used. A 0.5- to 50-gram sample of the compound, depending on the concentration of active ingredient, is made into a smooth paste
I
and d is the density of the material, do the density of the medium, the viscosity of the medium, and g the acceleration due to gravity. If a finely ground powder is dispersed in an aqueous medium a t a low solids concentration and permitted to sediment in a cylinder, the various particles settle independently of one another a t rates determined by their densities and radii. For any class of particles of radius r1 and density dl it is possible to calculate froin Equation 1 the time required for the particles initially a t the uppermost boundary of the suspension to sediment to a point z c n. from the top of the column. A sample taken at this: point will contain a concentration of particles less than radius T I equal to their initial concentration. rlll particles of radius greater than r1 will have settled past the sampling point. The difference between the initial Concentration of material of density d1 and the concentration a t z after time t gives directly the amount of material of radius greater than T I . By taking a series of samples a t calculated time intervals, it is possible to construct a cumulative distribution curve for the material. Sedimentation measurements are made a t low concentrations to eliminate interference between particles during settling. Inasmuch as any size class can be estimated accurately in the presence of smaller and larger particles, it appears reasonable to assume that materials of different composition and density will settle independently, providing deflocculation is complete.
1.60-
MILLIMICRONS
Figure 1. Absorption Spectrum of DDT [Bis ( p chlorophenyl) 1,1,1 trichloroethane] in Dioxane
-
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V O L U M E 20, NO. 1 2 , D E C E M B E R 1 9 4 8
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Spergon and Arasan Dusts. Measurements w e r e m a d e o n the organic fungicides Spergon it e t r a c h l o r o - p benzoquinone, f1), and .lrasan (tetramethylthiuram disulfide, 13) in dusts containing from 1 to 90yo of the active material diluted with talc. The methods are essentially equivalent to the one described for DDT. The sample size and the dilution system employed must be adjusted n-ith regard to the amount of active material in the dust and its specific estinction coefficient. Measurements were made on Spergon in acetone solution a t 360 mp (Figure 2), and on Arasan in ethanol solution at 275 mp (Figure 3). The specific cxtinction coefficient of the Spergon sample was 1.70, and that of the -4rasan 23.9. Both materials are electroreducible and can be determined polarographically in cases where other materials that absorb in the ultraviolet are present. The half-wave potential of Spergon is -0.20 volt and that of Arasan -0.90 volt against a mercurous sulfate electrode. Currcnt-voltage curves were obtained in a 50T0 dioxane solution buffered a t a p H of 4.8. Cumulative particle size data obtained on a sample of ground Spergon by the spectrophotometric method are shown in Table I MILLIMICRONS
MILLIMICRONS
Figure 2. Absorption Spectrum of SpeFgon (Tetrachloro-p-benzoquinone) in Acetone
300
after the insoluble diluent had been removed by filtration. The abs o r p t i o n spectrum of DDT (Figure 1) shows a more intense maximum further in the ultraviolet. However, measurements made at that point require too high a dilution for many purposes. The specific extinction coefficient determined on a D D T sample with a set point of 102" C. was 2.04 a t 265 mp. The specific g r a v i t y used in the Stokes's law calculations was 1.47. The dioxane was purified by refluxing over sodium, followed by distillation.
Figure 3. Absorption Spectrum of Arasan (Tetramethylthiuram Disulfide) in Ethanol
by the gradual addition of a 0.25% solution of Sacconol KR in water. The paste is worked with a glass rod until lumps are broken up, and it is smooth and free flowing. Additional wetting agent solution is gradually added with stirring, and the suspension is transferred to a sedimentation cylinder and made up to volume. The cylinder is placed in a constant temperature bath regulated to 25" C. and stirred vigorously wit,h a mechanical stirrer with a shaft long enough to reach the bottom of the cylinder. When the temperature of the suspension reaches 25" C., a drop of the material is withdrawn and examined with a microscope a t 1200x to determine whether aggregates of particles are still present. When dispersion appears complete, 2- to 10-ml. aliquots of the material are withdrawn for analysis to determine the initial concentration of the active material in the suspension. The stirrer is then withdrawn, and the initial time noted. During sedimentation, samples are pifietted out of the cylinders a t calculated time intervals and reserved for analysis. The samples are withdrawn a t levels approximately 10 and 20 em. from the top of the column of liquid. The equivalent radii corresponding to the time intervals are calculated from Stokes' law (Equation 1). The samples are then analyzed for the constituent of interest, and the percentage of material that has sedimented is calculated for each equivalent radius value. When analytical results are obtained in the form of a function which is linear with concentration, such as optical density on the spectrophotometer or colorimeter, or as microamperes or millimeters displacement on the polarograph, the fraction of material sedimented is given directly by the ratio of the change in reading during sedimentation to the initial reading, or z = - Ro - R (2 I
RO
where Ro i? the reading obtained on the initial suspension and R is the reading obtained on the sample after sedimenting t seconds. From these data a cumulative distribution curve is constructed from which the frequency curve or histogram can be obtained by standard methods (1). This method of preparing the suspensions and withdran-ing samples was essentially followed in all the experimental work described in this paper; the only differences were with regard to the dimensions of the columns, concentrations, sizes of the aliquots, and heights and times a t which the samples were withdrawn. Concentrations of the individual materials were determined by the methods dwcribed in the following section. ANALYTICAL PROCEDURES
DDT Dusts. Particle size measurements were made on 5070 DDT dusts by suspending a 4-gram sample in wetting agent solution and diluting to 1liter. Five-milliliter aliquots were taken for analysis and diluted to 25 or 50 ml. with dioxane, depending on the extent of sedimentation. Optical measurements were made a t 265 mp with a Beckman Model DU spectrophotometer
.kpproximately 4.5 grams of the material were treated with a 0.25% solution of Sacconol NR, made up to 1 liter, and dispersed as described in the experimental section; 5-ml. aliquots were withdrawn a t a point 20 em. from the top of the column of liquid and made up to 50 ml. with acetone. Optical densities were determined a t 360 mp with a Beckman spectrophotometer. The initial readings were obtained on replicate samples withdrawn from the column prior to sedimentation. Equivalent radii were calculated from sedimentation times using the Stokes' law equation. All dilutions were made to 50 ml. to illustrate the progressive decrease in optical density during sedimentation. More accurate results a t low concentrations can be obtained by adjusting the volumes so that all readings are obtained in the optimum range of the spectrophotometer. Owing to the high sensitivity of the spectrophotometric method it is possible to obtain particle size distribution curves on very
Table I.
Particle Size Distribution Data on Spergon (Tetrachloro-p-benzoquinone)Obtained by Spectrophotometric Method
Equivalent Radius. Microns
...
17.6 10.4 6.2 4.6 3.4 2.1 1.0
Initial reading = Ro.
Optical Density 0.760" 0.740 0.618 0.476 0.379 0.310 0.208 0.092
% Sedimented
...
2.6 18.7 37.5 50.1 59.2 72.6 87.9
ANALYTICAL CHEMISTRY
1176
Table 11. Particle Size Distribution Data on a 39'0 Phygon (2,3,-Dichloro-1,4-naphthoquinone)Dust Obtained by Polarographic Analysis Equivalent Radius, 3licrons
Displacement, XIm 181" 100 75 20 21
...
9 9 5.7 3.9
2,s 1.25
cc
Sedimented ,..
11.8 28 6 (2.4 88.4 93.9
11
Initial reading = Ro.
was investigated. a supporting medium a 50% by volume mixture of dioxane xyith a buffer solution 0.1 Y in sodium tartrate and containing O.O5y0 gelatin as a maximum suppressor was chosen. The pH of the final solution is 5.60. I n this medium Phygon gives a Tvell-defined reduction n-ave. The half-n-ave potential is -0.51 volt us. a mercury-mercurous sulfate electrode, and the diffusion current is 3.29 microamperes per millimole with a drop time of 5 seconds and a flow of 0.00194 gram of mercury, per second. Sone of the other materials in the formulations interfered or gave reduction n'aves in this region. The procedure finally adopted \vas to slurry 50 grams of the 37, Phygon dust x-ith a 0.25% solution of Sacconol S R and
2I-
dilute to 1 liter. Five-milliliter aliquots were taken a t stated intervals during sedimentation and made up to the composition described above Iyit,h buffer solution and diosane purified b y distillation over sodium. h portion of the diluted solution n-as placed in a polarographic cell equipped n-ith an external mercurous sulfate electrode, and after deaeration with nitrogen the diffusion current determined with a Sargent Model XX polarograph. Sedimentation radii were calculated using a value of 1.645 for the density of Phygon. Data obtained by the polarographic method on a 3% Phygon dust containing talc, DDT, sulfur, and rotenone are shown in Table 11. The dust n-as suspended in the usual manner in a. wetting agent solution, and samples were wit,hdrawn a t the 22cm. level. Measurements ivere recorded directly in terms of millimeters displacement at a fixed sensitivity on the polarograph. I n the absence of interfering ingredients, the simpler colorimetric method is preferred.
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PRECISION AND ACCURACY
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20
MICRONS
Figure 4. Cumulative Distribution Curves 1. 2.
Phygon Phygon ground with talc
small amounts of material. For instance, a modification of the procedure has been used to determine distribution curves on Spergon adhering to treated seed. Phygon in Dust Sprays. An extended study was made of the particle size distribution of the organic fungicide, Phygon (2,3dichloro-1,4-naphthoquinone, 12), a t concentrations of 370 in general-purpose garden dusts which contained varying amounts of DDT, rotenone, cub6 root and resin, sulfur, w-etting agents, and inorganic diluents. Because many of these materials absorb strongly in the ultraviolet, modified methods of analysis were required. Phygon is readily hydrolyzed by hot 1 S sodium hydroxidc to the sodium salt of 2-hydroxy-3-chloro-l,4-naphthoquinone ( 3 ) which has an intense orange-red color. The extinction coefficient measured with a 465 mp monochromatic filter on a Lumetron Model 402E colorimeter was found to be 10.1 based on the original weight of the Phygon. In the presence of sulfur, however, a greenish color is produced which is not suitable for photometric estimation. This difficulty can be overcome by permitting the reaction to take place in the cold over a 24-hour interval. A further complication is introduced by the presence of cub6 root and resin in the mixture. Interaction between Phygon and the constituents of the root in sodium hydroxide solution inhibits the development of the red coloration which is used as the basis of the analysis. Because Phygon is electroreducible to 2,3-dich1oro-lj4-dihydroxynaphthalene, the possibility of a polarographic method
Errors introduced into the sedimentation procedure by convection currents, turbulent settling, incomplete disaggregation, hindered settling, and nonsphericity of the particles have been adequately discussed (4). It is, however, necessary to establish whether a material will sediment a t the same rate in the presence of a large escess of diluent as i t mill alone. The assumption that it will is contained implicitly in the general theory of the sedimentation procedure, for if interference does not occur between the various size fractions of a chemically homogeneous dispersion, there is no a priori reason t o expect interference in the case of a chemically different species. To test t,his assumption a 3% dust was prepared by shaking pulverized Phygon with talc in a closed container. Sedimentation analyses were made on the blend and on t'he original material, using the colorimetric method previously described. Sis samples were taken during each sedimentation at time intervals corresponding to the range of from 1 to 20 microns. I n order to facilitate the interpretation of the results a sample of Phygon was chosen for these experiments which had been previously demonstrated t,o have a particle size distribution closely approximating a logarithmic probability relationship. This function can be expressed in linear form as
p =a
+ blnr
(3)
where r is the particle radius and p is proportional to the amount of material sedimented and is expressed in probit units (5). The parameters a and b thus serve to define the characteristics of the distribution. Calculations of a and b and their variances are shown in Table 111. The results indicate that the rate of sedimentation of Phygon is not altered significantly by the presence of talc. The average value of x 2 for both sets of data is 0.10, which indicates that the probability that the distribution follows this relationship is greater than 0.99 (6). The largest discrepancies n-ere observed in the region below 2 microns where deviation from the logarithmic probability relationship is suspected. I n cases where interfering factors do not exist, the accuracy of optical and electrochemical methods of sedimentation analysis can be predicted from the knoiyn variances of the instrumental methods and the sedimentation procedure. .la thcsc will vary
1177
V O L U M E 20, NO. 1 2 , D E C E M B E R 1 9 4 8 widely depending on the methods and conditions used, a general treatment of the subject would be of little value.
Table IV. Effect of Diluents on Grinding of 2,3-Dichloro1,4-naphthoquinone at Concentrations of 39’0 Median Radius, Microns
PARTICLE SIZE DISTRIBUTION OF MIXED DUSTS
To determine the effect of the nature of the diluent on the particle size distribution of an organic material ground in admixture with it, a series of formulations containing 37, Phygon v a s ground in a Raymond laboratory pulverizer equipped with an 0.25-inni. (0.01-inch) herringbone screen. The samples were prepared using recrystallized Phygon and given two passes through the mill. The materials nere dispersed in wetting agent solutions and particle qize determinations were made by the colorimetric method previously described. Under these conditions the cumulative distribution curve for pure Phygon is approximately linear on a logarithmic-probability grid, while the incorporation of diluents such as talc and calcium carbonate leads to skewness in the direction of small particle size (Figure 4’. The use of materials such as Celite and sulfur yields producf s n-ith relatively high median and modal radii (Table IT). Inasmuch as attrition during grinding is caused by collisions between particles moving a t high velocities, it is evident that the physical properties of the diluent will affect the distribution of a chemical ground with it, and that this effect will become more pronounccd at lower concentrations of the chemical, where the
Modal Radius, hiicrons
% Less T h a n Ir
% Greater
T h a n 20,~
probability of heterogeneous collisions involving particles of the organic material becomes high. The data in Table IF’ give a specific rather than a general picture of the process, as the net effect d l depend on the nature of the chemical and the diluent as \vel1 as on relative concentration, crystalline form, and initial particle size. The values are apparent and do not take into account the formation of permanent aggregates formed during grinding by collisions between particles of unlike materials. Although significant changes in distribution can be produced by the incorporation of diluents, the effects are much less profound than those obtainable by altering the method of grinding. This is illustrated by Figure 5 , which shows the frequency distribution curves of a sample of Phygon ground in a micropulverizer, and the same material ground in a microatomizer. DISCUSSION
Table 111. Effect of Diluents during Sedimentation on Apparent Particle Size Phygon 1
2 3 4 5
50 Radius 3.38 3.71)
Variance Variance in b in u
3 61 3.5’9 3.8!1 3.680 *0.121
b -2.54 -2.22 -2.43 -2.30 -2.24 2.346 10.121
6.40 6.28 6.36 6.28 6.32 6.328 10.047
0,088 0,071 0,081 0,074 0,056
0.063 0,053 0.057
0.042
0.12 0.11 0.11 0.11 0.08
3.73 3.64 3.79 3.48 3.66 3.664 10.107
-2.41 -2.26 -2.46 -2.41 -2.57 -2.420 *0.099
6.38 6.25 6.42 6.31 6.46 6.362 10.076
0.087 0.083 0,031 0.115 0.038
0.062 0.060 0,023 0.081 0.027
0.11 0.12 0.04 0.18 0.04
U
0.053
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Phygon and Talc 1 2 3
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The sedimentation pipet method supplemented by optical and polarographic analytical procedures is useful for particle size distribution measurements on small quantities of organic materials and for determination of the apparent distribution of the components of heterogeneous mixtures. Distribution curves can be obtained rvith as little as 0.05 gram of material and on dusts in which the concentration of active ingredient is as low as 1%. The total solids content of the suspension can readily be adjusted to provide for unhindered settling. Particle size distribution measurements on agricultural dusts are useful for evaluating such characteristics as flowability, relative biological activity, and selective retention of size groups to treated surfaces. The study of dust mixtures may provide valuable information on the role of the diluent in attrition and agglomeration of unlike materials during grinding. Many of these effects may be worth further study. ACKNOWLEDGMENT
The authors wish to thank Gladys E. Kiely for assistance with some of the experimental work described in this paper. LITERATURE CITED
0.30
(1)
dp dr
illexander, “Colloid Chemistry,” Vol. I, Chap. 58, New York, Chemical Catalog Co.. 1926. Andreasen, A. H. M.,and Lundberg, J. J. V., Ber. deut. keram. Ges., 11, 249-62 (1930).
Beilstein, “Handbuch der organischen Chemie,” Vierte Auflage, Vol. 7, p. 702, Berlin, 1925. Berg, Soren, KoZZoidchem.-Beihefte, 53, 149-376 (1941).
0.20
Bliss, C. I.. Ann. Amlied Biol.. 22, 134-67 (1935). (6) Croxton, F. E., andcowden, D. J., “Applied General Statistics,” p. 875, Kew York, Prentice-Hall, 1942. ( 7 ) Heuberger, J. W., and Horsfall, J. G., Phytopathology, 29, 303-21 (1939).
0.IO
(S) Kelley, (9) (10) (11) (12) (13)
0
(14)
Figure 5. Frequency Distribution Curves 1. 2.
Microatomized Phygon Micropulverized Phygon
W.J., Ind. Eng. Chem., 16, 928-30
(1924).
Robinson, G. W., J . Agr. Sci., 12, 306-21 (1922). Schurecht. H. G., J . Am. Ceram. Soc., 4 , 812-22 (1921). Ter Horst, W.P., U. S.Patent 2,349,771 (1944).
Ibid., 2,349,772 (1944).
Tisdale, W.H., and Williams, Ira, Ibid., Reissue 22,750 (1946). Wilcoxon, F., and McCallan, S. E. A., Contrib. Boyce Thompson Inst., 3,509-28 (1931).
RECEIVED M a y 12, 1948 Presented before the Division of Agricultural a n d Food Chemistry a t t h e 113th Meeting of t h e AXERICANC H m r 1 c . m SoC I E T Y , Chicago, Ill.
