Specific-Surface Determination of Nitroguanidine by Microscopic and Air-Permeability Methods A. S. BASS and H. M. STERNBERG‘
U. 5. Naval Powder Factory,
Indian Head, Md,
By the introduction of area and perimeter shape factors, the specific surface of acicular nitroguanidine crystals of irregular cross section was determined mieroscopia l l y . This value was oompared with those obtained with two routine air-permeability pieces of equipment. I t was determined that the air-permeability equipment could validly be used for the routine speoifiesurface determination of needlelike nitroguanidine crystals in the range 7000 to 9000 sq. c m . per eo.
Let: =
s u d h
=
a
8 c
A
N INVESTIGATION has been made to determine Tx-hether the specific surface of crystals of nitroguanidine can he measured in a routine manner, with either the Fisher subsievesizer or the draft equipment described in a joint Army-Navy specification ( d ) . The equation, K, on page 9 of this specification contains errors and should read:
The operation of both pieces of equipment is based on the principle that the finer the particle size, the more difficult it is for sir to permeate the sample bed. The precision with both pieces of equipment is very good, and excellent checks are obtained within certain permissible tolerances. The Fisher subsieve-siser, for example, doea not claim an accuracy within +lo%. For a product containing needlelike crystals, such air-permeability equipment could be used to control the size of the crystals, slthough the values obtained might not he true values, provided the properties of the product remain within an acceptable tolerance range. However, the degree of accuracy of such instruments is uncertain, since the values ohtained may he far from the true value, especially for acicular crystals of irregular cross section typified by nitroguanidine. Because nitroguanidine is highly aspherical, the shape factor becomes important. Drinker and Hatch ( 1 )refer t o the effectof shape factor upon the ultimate size of irregular particles and point out “that the frequent assumption of spherical shape far dust particles is not in accordance with facts.” It waa decided to obtain a third measurement of specific surface with which to compere the values found with the air-permeability equipment. An equat,ion for specific surface was derived based on the periphery or shape of the crystals, and a particlesize determination was made microscopically. Although every crystal irregularity cannot. be considered with the microscope, it nevertheless can he used to make direct crystal measurements with either an oculm micrometer or by photographic enlargement. m d subsequent projection onto a Screen where measurements can he made more conveniently and with less eye fatigue. This latter procedure n-asadopted and a nitroguanidine lot cantaining conventional, acicular crystals was used for this determination.
specific surface (ratio of sqi to cubic centimeters of vol crystalsurface (surface area: = crystsl volume = projected diameter of r r p t a l cross section = crystal length = cross-section perimeter shape factor (ratio of perimeter to diameter) = cross-section area sham factor (ratio of area to sauare of diameter) = ratio oi crystal length to diameter
S
Then
+= length,’h 2Od’ = surface, s volume, v
=
”
neglected vithout introducing any appreciable error, and Equation 1 reduoes to
PROCEDURE
An acetone slurry of clear cellulose acetate plastic plus 5 % by weight of suspended nitroguanidine was extruded through a die under a pressure of 600 pounds per square inch, givmg-lrstrand approximately 0.15 inch in diameter. At this pressure the nitroguanidine cry&aIs became oriented lengthwise in the direction of extrusion. Transverse sections (about 10 microns) of these strands were prepared with a. microtome and then were photomicrographed a t 950X. Figure 1 shows a typical photomicrograph. These photomicrographs were then projected onto a screen, and the outlines of the crystal crass sections traced. Measurements were made on 200 crystals t o find the projected diameter of cross section, the square root of the area of
DERIVATION OF SPECIFIC-SURFACE EQUATION
The nitroguanidine crystals were treated as right cylinders of irregular cross sections. 3 Present address, U. S. Naval Ordnance Laboratory, White Oak, Silver &dn& Md.
Figure 1. Photomicrograph showing cross sections of nitroguanidine crystals suspended in cellulose acetate plastic s1sx
809
V O L U M E 27, N O . 5, M A Y 1 9 5 5 cross section, the perimeter of cross section, and the length. The cross-section perimeters and areas were measured with a map measure and a planimeter, respectively. The remaining measurements were made microscopically, the length being measured on unimbedded nitrogusnidine crystals, such as are shown in Figure 2, and the projected diameter being measured on the transveme sections of the strands. Frequency-distribution curves of the dat,a were made, and from these curve8 the shape factors, a and 0,were obtained by comparing the perimeter and square mot of the area, respectively, with the projected diameters. The ratio of crystal length to diameter wa8 similarly obtained.
ate between the microscopic and air-permeability values. The microscopic value is probably on the low side of the true value, as fissures and minor surface irregularities cannot he taken into account in making direct measurements. Furthermore, an accuracy within *IO% is not claimed for the Fisher subsieve siser and the value actually obtained with this instrument could he as much 8s 10% lower than that noted ahove. These consid-
RESULTS
Frequency distributions of the data. on the nitroguanidine crystals are shown in Figure 3. The values on the abscissa of Figure 3 represent (in per cent) the numher of particles less in diameter, square root of area, length, or cross-section perimeter than the corresponding measurement in microns on the ordinates. These graphs give the following values far the constants: u =
3.41
0 = 0.i3 c
=
20.8
Figure 3. Frequency distribution of crystal lengths, cross-section perimeters, cross-section diameters, and
the two types of instruments. All the measurements, therefore, may be considered as within a practical range of the true value, and the air-permeability equipment may he used for the routine determination of the specific surface of nitroguanidine within the range of 7000 to 9000 sq. cm. per cc. ACKNOWLEDGMENl
Figure 2. Photomicrograph of nitroguanidine crystals showing relative dimension of longest axis (length of orj-stal) 22ox
Instrument
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The above values for the constants and the data from the cro86section-diameter distribution shorn in Figure 3 were substituted in Equation 1 t o obtain a value for the specific surface. The average specific surface as determined with the three instruments and the per cent deviation from the microscopic value me a8 follows: MiorOsW'e. Fisher subsieve size1 Draft equioment Average of Fisher subsieve sizer and draft equipment
The authors wish to acknowledge the assistance of C. N. Bernstein, Leon Whitman, and Bryan Hancock, who furnished
Speoifie Surface. Sq. Cm./Ce. 6702
LITERATURE CITED
(1)
Drinker. P., 81id Hatoh, T., "Industrial Dust." McGmw-Hill. New York,
1 nlC IIYY.
(2) Joint Army-Navy Specification, "Nitroguanidine (Picrite)," JAN-N-494 (Sept. 10.1947). RECEWED
ior review Ootober 25. 1954
Aocepted January 21, 1855.
% Deviation from
M ~ O I O S DValue OP~
...
8460
8910
26.2 32.9
8685
29.6
The values obtained with the Fisher subsieve sker and the draft equipment are in reasonshly close agreement, aa would he expected, inasmuch as they both depend on the air-permeability principle, hut the respective values far the two air-permeability instruments do not indicate such close agreement with the value obtained microscopicdly. However, the apparent difference in the values obtained with the two types of instruments (microeeopic and air-permeability) is not as great as the results might indicate. The true specific-surface value is prohahly intermedi-
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Determined by Light Scattering-Correction
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trmgelo, S.V. R., Clay, Barhara, Fishman, M. M., Hagan, A. G., Lazrus, Allan, and Zagar, Walter, BNAL.CHEM.,27, 262 (1955)l the expression in Equation 1and the paragraph ahove Equation 1 should read: H'/r, Equations 9 and 10 should read 0/2, not P. On page 264, second column, below- Table 11, the expression in line 3 should read: M,e0.'5Ba.
