Microscopic identification of Compo
Organic
TCF Ammonia Departmen
'emours &Z Company, Inc., Wilmington 98, Del.
bromide esters for carboxylic acids, picrates a n d benzamides for amines, 2,4-dinitropbenylhydrazones for aldehydes a n d ketones, dimedone eondensation products for aldehydes, phthalirnides for amides, a n d 2,4-dinitroehlorothio ethers for mereaptans. A convenient m e t h o d of identifying those monohydric primary a n d secondary alcohols wbioh form bydroryl derivatives w i t h difficulty is based on dehydrogenation to t h o corresponding aldehydes and ketones. Glyools with vicinal hydroxyl groups are identified conveniently after periodate oxidation to t h e aldehydes.
organic compounds requires the use of t h e petrographical microscope. Properties such as optic axial angle, refractive indexes, dispersion, a n d optical cbaracter normally are m o s t desirable in positive identifications. Dispersion of t h e optic axes often has proved to be an extremely useful property a n d has permitted the quantitative analysis of m a n y binary homologous mixtures. Derivatives suitable for the identification of organic compounds include: 3,5-dinitrohenmates a n d p-phenylazobenzoates for aloohols, p-bromoanilides and p-bromophenacyl
T
.. .
.
...
acetate salt. The monohalogen substituted aoetic acids are .. . , .. . crystaume . uistinguisnea reaauy wnen precipitatoa as tnev barium salts (16, SZ). In polarized white light between crossed Niools, many crystals show a high order of interference colors. The crystalline aggregates have caused certain wave lengths of polarized light passing through them to he destroyed by interference, thus producing a sequence of brilliant colors and causing the external structure of the crystal t o stand out in sharp detail. Thus, fused methyl ethyl aoetic p-bromoanilide appears as irregular radial plates, orange t o purple in color. Fused propyl 3,5-dinitrobenzoate appears as radial spherulites, in which the blue, green, and pink colors predominate. Fused see-butyl 3,6dinitrahenza-
HE chemical and petrographical microscopes are valuable tools M the analytical laboratory for the identification of
....
"
organic and inorganic compounds, The chemical microscope is well suited for the observation of crystal habit, a means of identification of elements and compounds that is valuable when crystah of truly characteristic shape can be formed. The potential applications of this rapid, qualitative method of analysis are discussed ably by Chamot and Mason (S), who present tests for the detection of most of the elements, many anions, and a few organic acids. In common with other physical procedures, these tests often are limited to particular mixtures. In some cases the desired crystals aTe obscured by other oompounds. A great value of this technique, however, lies in its direct application t o small areas where the location of oonstitueuts is important and only minute quantities of sample are available. During the past few years numerous papers have described new types of crystals suitable for microscopic identification. Typical of these are the large octahedra formed when beryllium salts react with sulfuric acid (IO), the cbaracteristic salts of nickel, colialt, manganese, and copper from reaction with saccharin ( I 8 ) , the pink hexahydrous crystals of cobalt salts after reaction with nitric acid followed by sulfuric acid ( I I ) , the distinctive salts of lead, tin, copper, zinc, iron, and arsenic (2), and the picrolonates of sodium, potassium, strontium, ammonium, copper, and sinc (14). Additional metallic reagents have been reported for the identification of organic acids. Formic acid forms characteristic crystals with cerom ion; acetic, propionic, and mbntyric acids form distinctive crystals with mercurous ion (b6, 67). Mercnrous nitrate has proved to he a satisfactory reagent for the positive identification of propionic acid in the presence of acetic and butyric acids (96). Probably the mast useful reagents for the optical identification of formate and acetate. however, are the well-known silver salt of the former and sodium uranyl salt of the latter. Silver formate crystallizes in thin elongated plates of low order gray and white interference colors which extinguish oblique to the principal boundary plane. These crystals tend to darken on standing. Sodium uranyl acetate crystallizes as yellow isometric tetrahedra which optically are isotropic. No other fatty acids give products that could he mistaken for the
,..
.
Figure 1. Fused see-Butyl 3,5-Dinitrobenzoate
448
V O L U M E 21, NO. 4, A P R I L 1 9 4 9
449 materid is compared with known prepare, tions between crossed Nicols. No more t h m 5 minutes are required for an analysis. Pure adipic acid recrystallizes in flat radial plates with sharp boundaries,,as shown in Figure 2, A . When 0.1% succinic acid is present in the adipic acid a noticeable feathering of the crystal boundahes is evident (Figure 2. B ) . , W i t h 1% succinic acid a complete ;hering of the crystals is observed (Figure 3, while a t 3% concentration the crystal tit has changed almost completely and 111 spherulites together with irregular tes result (Figure 2, D). Above 3% suc.o acid only small spherulites of low order intexference colors are observed (Figure 3).
C
1'hese characteristics are illustrated in Figure 4, which shows the regular degradation of adipic acid crystals with increasing succinic ack1 contents. Nearly equally reliable analyse!3 can be made on adipic acid containing glutaric and lauric acids. This technique is most sensitive in the range 98 to 100% adipic acid. For the determination of higher concentrations of other acids in adipic acid, optical crystallographic measurements are necessary. D R a t e of crystallieation from the molten sample is applicable to tho estimation of impurities in some materials. McCrone and his co-workers (24) in succesFfully applying this technique to the determination of p,p'-DDT in technical DDT, pointed out that the procedure of analyzing materials by their rates of crystallization is limited to mixtures which Figure 2. Fused Adipic Acid neither decompose nor sublime on melting, A . Pure c. Containing 1 % anosinic acid which can he supercooled under controlled B. Containing 0.1% succinic asid D. Containing 3% succinic a d d conditions before complete solidification, and which grow crystals at a measurable rate a t a convenient temperature. Numerous investigators have continued to employ the polarizing microscope for the identification of crystalline materials, basing their conclusions on observations of the crystal profile, the type of extinction, and the order of interference colors. Such measurements were reported for the microscopic identification of s u g m which were carefully crystallized from water solution by the addition of suitable water-miscible organic compounds (86). These Observations also were applied for the identification of the nitrobarbiturate derivatives of primary aromatic amines (1%). A particular type of polarizing instrument, the petrographical microscope, permits a more widely applicable system of identificatiou. With this type .of microscope specific numerical constants may be measured which w e independent of external a p pearance and minor preparative details. The optical properties Figure 3. Fused Adipic Aoid Conuseful as criteria. in the identification of crystals under the taining 5% Succinic Acid microscope are:
ate appears as clusters of radial spherulites (Figure 1) of varying shades of green. In a limited number of oases crystal habit observations can he employed on a quantitative hasis. For example, W. M. D. Bryant noticed the marked effect of small quantities of other dibasic acids on the appearance of adipic acid, when the fused preparation was observed between crossed Nicols (IS). By maintaining carefully controlled conditions, the author has found this fusion to be reproducible, permitting the estimation of as little as 0.05% of succinic acid in adipic acid. I n making an analysis, 6 mg. of the acid are placed on a microscope slide. A cover glass is placed on them and the slide is h a o + d n.*nf..ll..-i+h "I.~."..J " . l .
. . 1 _ " " "
o 6y" l-:n-nh..-,.,.-&n . . d lllllyuYLYc. ,,",mu: ~~~
-.."aL---?&:-..
-L ="""e
Y l l r
"lnhrug
point of the sample. Then the slide is placed on a brass plate and allowed to cool to room temperature. The recrystallized
A. Numerical Optical Constants of Anisotropic Substances 1. Principal refractive indexes (a,,%and y , or and w ) 2. Birefringence 3. Optic axial angle 4. Dispersion a. Of refractive indexes b. Of optic axes (add,crossed ads,l plane, and axis1 die persion with change of aign) c. Of biseotrices (inclined, crossed, and horizontal dispersion) 5. Absorption (cry&,l speotrophotometry) B. Numerical constants (partly geometric) 1. Optical orientation 2. Extinction angle C. Quualitative and semispecific properties
.
~.~~
n ~ ~ . : ~~~, ~ , I. upml(iBL onsraorer 2. Elongation 3. Profile angles
-
ANALYTICAL CHEMISTRY
450
Figure 4.
Effect of Suoeinio
Habit of Adipic Acid
retractive indexes. Direct measurements may be made with the aid of a oompensator which introduces a known degree of birefringence, and when equivalent t o that of the crystal under study restores the extinction of the crystal. One of the more recent tcohniques was reported by Jelley ( l g ) , who designed grating microspectrographs for the measurement of birefringence and other optical properties. In Figure 6 the birefringence (and refractive indexes) of b e n d a t various wave lengths as determined by refractive index difference is compared with that measured by the method of Jelley (19). OPTIC AXIAL ANGLE
When a seoondsry lens, the Bertrand lens, is inserted into the microscope body tuhe just below the ocular t o intercept the bundle of rays emerging from the rear aperture of the objective, Figure 5.
Birefringence and Refractive Indexes of Benzil Birefringent. data, Bryenf e Jelley 0 REFRACTIVE INDEX
Isotropic crystals have only one refractive index, which is independent of the direction in which i t is measured. I n uniaxial and biaxial crystals, however, the refractive index varies in different directions. In the past the indexes of these anisotropic crystals were determined lasgely by tho trial and error immersion method in which the lowest index found mas assumed to be ol and the highest, y. In recent years increased use has been made of the more reliable and fundamental assignment of the principal index directions, as determined by interference figure observations. Once these orientations are established, the immersion method, in conjunctioil with the Becke line test, is convenient. Using this technique, Keenan ($0) measured the principal rcfractive indexes of many of the crystalline substances listed in the United States Pharmacopoeia. Eisenberg and Keenan ( 2 4 ) determined these constants on the several metal pierolonates which they prepared. BIREFRINGENCE
The birefringence, or double refrsotion of a crystal, numerically is equd to the difference between the maximum and minimum
Figure 6. Interference Figure of Muscovite At 3650 i.: 2Ho. 41 5'
V O L U M E 21, N O . 4, A P R I L 1 9 4 9
Figure 7.
Abbe Apertometer
an optical pattern or interference figure is observed. Figure 6, showing the biaxial interference figure of muscovite (S), is a good
example. The points of emergence of the optic axes "or directions of single refrsetion" are indicated by the centers of the two dark hyperbolio brushes or isogyres. The linear difference between their centers, the melrttopes, can he measured. It is a constant, provided the same lens system and wave length of light are used, and is proportional to the sine of the optic axial angle. With a. suitably calibrated optical system, this value can be converted to appment optic axial angle, one of the most useful optical crystallographic measurements (2E or 2H, depending an whether an air or oil immersion objective is employed). 2E or 2H in turn can he converted to the true angle, 2 V , if the intermediate refractive index, 8, is known. Precise optic axial angle measurements may be made d h the aid of the eyepiece miorometer scale, oonveniently calibrated directly by means of an Abbe apertometer (Figure 7 ) .
451
This instrument, originally designed by Abbe to measure the numerical aperture of an objective, contains two scales. The inner one is graduated in angular units up to 2E = I8O"and the outer one, in units of numerical aperture. The application of the apertometer for precise micrometer scale cdibration does not appear to he very wellknown. (Hartshorne and Stuart, 17,mention the use of au apertometer for angular measurements.) When used as a calibration instrument, the aperbometer is placed on the stage of the microscope (Figure 8). The cylindrical surface is illuminated with diffuse light, such as that furnished by a n incandescent desk lamp, and with the aid oE a low power objective, the apertometer is carefully centered by focusing the optical system upon the slit of the silvered disk. In this position the apertometer is fastened with small C clamps securely to the stage of the microscope. Each combination of ocular, with linear micrometer scale, and objective must he calibrated a t a fixed draw tube position. After the desired pair have been inserted into the body tube, the apertometer slit is brought into sharp focus and the Bertrand lens is inserted. In this position the black cross lines of the apertometer cursor are seen in the plane of the ocular micrometer and are brought into sharp focus by adjustment of the draw tube. The apertometer should be so fixed that on moving the cursor from one extreme position to the other, the intersection of the cross lines follows a straight line in the &&-west direction, making contact with all divisions of the micrometer disk. Then the cursor is set successively on each gradation of the arc and the position of the crms lines is read on the micrometer scale. Readings to the left and right of the control index are noted. Angular readings are made directly in calibrating air objectives, while the numerical aperture scale is used for oil immersion objectives. Typical data, shown in Table I, were taken from the cdihration of a 7X ocular with 1.25 N.A. oil immersion obiective. When the data, scale reading os. 2H, are plotted graphically, a curve is obtained of the type shown in Figure 9. The interference figure (Figure 9) presents a direct means of determining optic axial angle, principal refractive index directions, optical charaoter, and dispersion. With this information a rather complete optical crystallographic classification can he accumulated. Thus thiourea. gives the data. shown in Table 11. Winchell's texts (52,SS) on the optical identification of inorganic and organic compounds, Rogers and Kerr'n book (939) on minerah, and Groth's comprehensive series on chemical crystallography (16) contain a wealth of information on the optical constants of crystalline materials. I n some cases, however,
Table I.
Calibration of Optical System of Miorosoope
N.A. (Apertometer) 0 0.1 0.2 n
Sin X N.A. na
0.0 0.06601 0,13201 0.5 0.33003 1.2 0.79208 = relraetive index of immersion oil
H 0.00 30 49' i' 35' ISD 6' 52" 23' = 1.515
Linear Miorometer Soak Reading Right Left 258 258 267 250 275 243 299 219 356 163
Table 11. Optical Crystallographic Properties of Thiourea (4) Melting point. 182.4° C. (eon.) Refraetiye indexes. a = 1.636 * 0.003,B = 1.790 * 0.005, (X = 5461A..t=25*3~C.),r=18l(oalod.) Birefringence. 6 - a = 0.154. 7 - a-0.17 (h = 5461 A,) Optical axial +e. 2Ha = 44.0'; 2V = 37O (from 2Ha and 8 )
(X = 5461 A.) Dispersion. Axial, r>u:
crossed a x i d plane; uniaxial st about
3780 A.
Figure 8. Apertometer Mounted on Bausah & Lomb LD Microsoope
Optiod character. Negative Orientation. Axid plane = (I (001) for visible range, axial plane = c (100) forultritviolet. Bz. = 6 (010) Elongation. 11.0 in BE, sections from water solution: I/Y in melt: 1) Bin Bz. wetions from water solution Crystal System Orthorhombic bipwamidal (18)
A N A L Y T I C A L CHEMISTRY
452
Table 111. Optical Crystallographic Data on Organic Compounds M p O.C.
Riffr;lctive indexes" (5461 A.)
Dihydroxyacetone (8) 81.9
Glucose (8) 159.1 (dec.)
Sucrose (8) 190
1.450 1.575 1.581 0.131
1.530 1.555 1.567 0.037
1.540 1.567 1,573 0.033
a
8 Y
Biiefringence Optic axial angle) (5641 A,) 2Ha 28
,
...
...
2Ho Dispersion Optical character Crystal habit
...
-
Flat plates of rhomboidal outline!
Compact plates and tablets!
A 4'
.
y
M p., C Rlfractive indexes (5461 A.)
1.471 ( 0 ) 1.483 (e)
0.041
0.012 Uniaxial
Prisms and tablets from HQO
Elongation Bxo Bxo Probable crystal system
Tetragona1g;h
/IOptic
0.088
68' 54=c 52' a
87.5' 86' C 84' e
270 21°Cs6
r>u
crossed
M.p.,.O,C. Refractive indexes (5461 A,)
Mercaptobenzothiazole
198-200 (dec.)
181.4
1.700 1.715 1.752 (calcd.) 0.52
a
B Y
Birefiingence Optic axial angle (5461 A.) 2Ha 2 v r
31.OoC
> u, &&sed axial plane, uniaxial a t 4740 A,
+ ...
0.005
* 0.005
-0.4
C
...
r>v inclined
-
It 8
I/ P
23.5'
(4916
h,39 .OD (5780 A,), 43.5O (6234 A.)
Monoclinic I/ 8 2Ha = 61.5" (4358 A.), 67.5' (5780A.), 69.5' (6234 A.)
89.50C
88' e
87.5'
- 4150fi.
+ Radial aggregates of plates and needles from melt
...
Monoclinic 2Ha = 83.5O (4358A.), 89' (6243A.), 89.5' (6908A.)
Slightly (nolined
+
Flat plates 'and compact prisms from Hg0 (twinning common)
hlonoclinic
II Y
Pyridinium Sulfamate (8) 117 (dec.)
Glutamic Acid Hydrochloride (8) 209-210 (dec.)
Glycolic Acid (I) (8) 78.9
1.487 1.630 1.665 0.178
1.582 1.567 1,590 (calcd.) -0.038
1.434 1.511 1.518 0.084
530 0 490C..
81.5' 78' C
...u
33.0' 330 c 32' a
-115' r > u
r>u
r>
-
Prisms from CsHs. Radial aggregates of needles from melt
... ... E
A
67' 50'
35.50
2Ha
1.667 1.965
2 .06 (calcd.)
85.5'
II a
II 8 /I 0
Monoclinic See Figure 21
2 4-Dinitrophenyliydrazine Hydrochloride
1.370 * 0.005 1.460 1.563 0.193
r>a Horizontal-6908 = 20
r>'v
crossed
Pri ms an 2 acicular crystals fr m 1 : 1 HIO-CnHaOH
Y
1.497 1,534
96'
Lamellar and columnar crystals from 50% Ha0 in CiHrOH
II
A 28 ' k 3 ' a 2Ha = 19.5O (4358 A.) = 13O (6908A.)
Sodium Formate (8) 253
...
-
A 2 5 *3O 3 Monoclinic See Figure 21
Elongated plates from CrHaOH
t-Caprolactam (8) 68.0 1.585
-
BXO
BXO Probable crystal system Remarks
Elongation BXO BXO Probable crystal system Remarks
.
...
Elongation
Monoclinic prismatic
'
1.599 1.850 1.930 0.331
A.)
Optical character Crystal habit
... II,a
II e
axis
1.462 1.842 1.861 0.399
2Ho Dispersion
Optical character Crystal habit
II Optic
Tetragonalb'
-
+
2-.4minomethylcyclopentylamine Picrate 257-9 (dec.)
B
2 Ho Dispersion
Elongated plates from CHIOH or C6Ha. Dendritic growths of needles from melt
...
v > r
Hexamet hylenediamine Picrate 228 (dec.)
a
BiTefringence Optic axial angle-(5461 2Ha 2V
1.403 * 0.005 1.616 1.624 0.221 17' 1 G 0 C , 18'
+
'
1.418 1.468 1.559 0.141
Elongated plates and needles from Hr0. Radial and parallel growths of needles from melt
I1 o
axis
Monoclinic Biuret (8) 205 * 2 (dec.)
r > v
No;r;lal
-
Radial spherulites of needles from melt. Needles from hexane
Dimethyl Oxalate ( 8 ) 54.3
... ... ...
...
M P., O,C Refractive indexes (5461 A.)
Monoclinic apderoidalh
1.519 (t) 1.560 ( 0 ) [Jniaxial
99.50 r > v
It B
/I a
Pentaerythritol 192 * li
-
Compact prisms from Ha0
Variable
Orthorhombic#
BirYefringence Optic axial angle (5461 A.) 2Ha -2v . 2Ho Dispersion Optical character Crystal habit
... ...
89Od
C
-
Pentaerythritol Tetraacetate 80.6
a B
f
Weak, indetkiminate
u > r
Mono- or triclinic ?' .@
1,532 1.62 * 0 . 0 1 1.68 0.01 -0.15
50' e
v > r
-
Elongation Bxo BXO Probable crystal system
50' 48'
60.5' 50" C
220 2106 220 4
hfesitol (8) 71.6
Elongated plates and compact prisms from CHsOH
II
+
...
-
Rectangular plates from HrO containing HCl
II a
Y
A 28 * 3' a Orthombic or monoclinic
Rhombic bispheroidalh
...
...
Plates and tablets.! Radial aggregates of needles from melt Nearly ll B A about 35' Monoclinic prismatich 2Ha = 29O (3650
A,)
V O L U M E 21, NO. 4, A P R I L 1949
453 Table 111. (Continued)
Glycolic Acid (11) (8)
h1.p 0.c. Refrictive indexes (5461 A.) a
B BiTefringence Optic axial angle (5461 A.) 2Ha 2v
...
D-Tartaric Acid (8) 172.1
... ..* st;&
1.500 1.540 1.6052 0.105
85 4320
...
2Ho Dispersion
r->
Weak, indeterminate
Elongation Bsa
+
Radial growths of plates from melt. Readily reverts to stable modification
Radial aggregates of plates from HnO. Elongated plates from melt
...
...
BXO Probable crystal system Remarks
u
moderate horizontal
+
Optical character Crystal habit
Monoclinic
...
1.488 1.505 1.593 0.105
74.50 74,",c 73 ; 111 r > u weak crossed
740 74O,C 72Oi
Elongated prisms and tablets from H10. Radial and parallel growths of needles from melt
Radial spherulites of prismatic needles from melt
1050 u > r
~~
-
+
/I 13
A23 i l ' y
Monoclinic spheroidalh
...
Adipic Acid (8) 152.1
1.458 1,525 1 . 5 7 (calcd.) 0.11
78' 75.6'C 76O. i 7,8" e 107
...
Diglycolic Acid (8) 143.1
...
II a Monoclinic 2Ha = 71° (3650 A.) 72.5' (4358 A,)
1l.Y
Monoclinic prismatich
...
1 As received from Eastman Kodak Co. 0 Winchell (88). Groth, (16). Transition temperature. i Calcd. from 2Ha and 2Ho.
i 0 . 0 0 3 unless otherwise specified. + l o ,unless otherwise specified. Calcd. from 2 H a and 8. d Calcd. from 2Ho and B. Calcd. from a, p , and y. a
b
C
A measurement of the optic axial angle seemed to be sufficient for the identification of melamine and cyanuric acid. On recrystallization from water, melamine consistently gave a single crystalline modification and cyanuric acid presumably gave only the dihydrate, which on short exposure to the atmosphere dehydrated to the opaque anhydrous acid. I n one instance anhydrous cyanuric acid recrystallized from hot water as transparent crystals whose optic axial angle was the same as that of melamine. The 8 index. however. was markedlv lower. Data sufficient for the identification of each of these "compounds are included in Table IV. Table IV.
Identification of Melamine, Cyanuric Acid, and Cyanuric Acid Dihydrate Cyanuric
Melamine Acid hI.q., 0 c . a 361-2 360 Optic axial angle, X = 5461 A,, 2 Ha 37 .O (a) Opaqiit 0.5 (b)37 1 Refractive index, X = 5461 A., B 1.858 i 1.700 * 0.003 0.003 Optical character Negative Negative Dispersion u>r u > T 150
e00 e50 300 MICROMETER SCALE READING
Figure 9.
350
Optic Axial Angle Diagram
Calibrated by N.A. scale of Abbe apertometer Tube analyzer in 45' position Tube length, draw tube, 165 m m . , Amici-Bertrand tube, 0 mm. 1.25 N.A. (97 X ) oil immersion objective, 7 X Zeiss Muygenian ocular
the recorded data were obtained from unreliable sources, and, therefore, occasional data are in error. Optical crystallographic properties of a few compounds not previously reported are given in Table 111. lJ7inchell (32) has included some data on sucrose, glucose, pentaerythritol, pentaerythritol tetraacetate, and tartaric acid. Groth (16),in addition to mentioning these compounds, discussed glutamic acid hydrochloride, and glycolic and adipic acids. RlcCrone and coworkers (25) in their valuable monthly reports of crystaLlographic data have studied the properties of adipic acid a t 5893 A. Roth. DeJTjtt. and Smith (SO)described x-ray and microscopic constants of the polymorphs of red phosphorus. Often only one or two constants are required for the positive identification of crystalline compounds. In one case, however, this procedure led to a mistaken identity ( I S ) .
Cyanuric Acid Dihydrate
17.'; l o
... .*
Negative Weak, indeterminate
...
Elongation, BXG II B 1I 7 (1 Melting points were determined by spot-melting on a Dennis-Parr bar Model MPB6. When heated slowly, melamine starts to sublime at abou6 214O C.
An extension of the microscopic technique has been reported by Bailly ( I ) , who adapted a fluorescing image tube to a polarizing micros2ope that permitted observations in the near infrared (9000 A.). The crystallographic properties of various minerals, which are opaque in the visible range of the spectrum, were observed readily a t this wave length. DISPERSION
One of the recent outstanding developments in connection with the microscopic identification of organic materials concerns the increased emphasis on dispersion, formerly considered a rare phenomenon. Derivatives are being chosen which exhibit one or more types of dispersion, because this function offers an additional powerful means of identification. This property often can be determined qualitatively by examination of the colored margins surrounding the isogyres of an interference figure when observed in white light. Dispersion of the optic axes is the most common type (Figure 10, a ) . If the dispersion is greater for red light than for blue (expressed as r > v ) , both isogyres will be
ANALYTICAL CHEMISTRY
454 margined with red or orange on the convexside, with blue or green on the concave side. When the colors a n reversed, the dispersion is greater for blue than for red light (expressed as v > r). Crossed axial plane dispersion is evidenced by a continuous decrease in optic axial angle with change in wave length until the angle becomes uniaxial, after which it increases again in a
Figure 10.
Types of Dispersion in Biaxial Crystals
plane a t right angles to the first. This is due to the fact that the refractive indexes change a t such different rates with change in wave length that the identities of two of the principal indexes hecome interchanged. Dispersion with change of sign is characterized by strong dispersion of the optic axes where, with change in wave length, 2V passes through go", resulting in a change in character. This rare type was first noted in organic crystals by Bryant (7) and later was found hy Mitchell and Bryant in some amine picrates (25). These three forms of dispersion are the only ones which occur in orthorhombic cryst&. Dispersion of the bisectrices occws in monoclinic crystals. In addition to the types of dispersion just disoussed, this group includes inclined, horizontal, and crossed dispersions. Inclined dispersion (Figure 10, b) is observed when with change in wave length of light, one optic axis changes more rapidly than the other. When strong, this type of dispersion results in a color reversal around one optic axis and heavier color margins around one isogyre than around the other. Horiiontal dispersion is evidenced hy a displacement of the optic axial plane horizontally along 0, when the wave length of the light is changed (Figure 10, e ) . In white light, this type shows a color reversal around one isogyre, when the Nicols are changed from the 45" t o the 0" position. Crossed dispersion occurs when the trace of the optic axial plane is rotated about the acute biseetrix as the wave length of the light is changed (Figure 10, d). In white light the isogyns are margined with mixed colors in the 45' Nicol position. Dispersion in triclinic crystals is unsymmetrical with respect to all crystallographic and optical directions. Axial dispemion with change of sign (Figure 11) is illustrated in the interference figures of dimethylamine piorate (25). The optic axial angle of the a-8section decreases with inorease in wave length of the light, while that of the 8-7 section increases. mPropylamine picrate (Figure 12) represents an excellent example of Strong crossed dispersion (24). Here with decrease in wave length from 6234 A,, the. optic axial angle rotates around the optic axis, until a t 4750 A. a rotation of approximately 80' has occurred. Some compounds exhibit several types of dispersion.
V O L U M E 21, N O . 4, A P R I L 1949
455 "-
I "
,
t:
1
Figure 12. Crossed Dispersion
. .. .
-. . . ... .. .
?
.
.. le :d
io re m ic
&'
1. ;h e, n'S 3-
:dl id rW
io et [n
al le a1
:d
20-
U N
I
(85). The point of intersection of each curve with the dotted line represents the wave length of uniaxiality. In Figure 15, representing dispersion with change of sign, the apparent and actual optic axial angles of stilbene (2H and 2 V , respectively) are plotted against wave length (7). At the point of interseotion ofthe a-y section8 (4070 * 50 A,) the crystal is optically neutral-i.e., 2V = 90'. Representative amine picrates illustrate inclined dispersion measurements ($5) (Figure 16). The upper curves are a plot of optic axial angle as a function of wave length, and the lower curves, the inclined dispersion from 6908 A. to any given wave length. It is interesting to note the sudden extreme dispersion given
I > 7000
4000
5000
8000
W A V E L E N G T H IN K.UNITS
Figure 14. Crossed Axial Plane Dispersion in Amine Picrates
456
ANALYTICAL CHEMISTRY 120
8 2 w
v)
OPTICALLY NEUTRAL AT e4070t50A. WITH L H - l Z l
2
n
z 60
.
CALC. FROM DATU hi OF BOER IS
3600
4000
Figure 15.
4400 4800 5 2 W 5600 6000 WAVELENGTH IN I(NCSTROM UNITS
6400
6800
Axial Dispersion with Change of Sign in Stilbene
4800
5900
5600
6000
WAVE LENGTH,
6400
6800
A.
Figure 17. Horizontal Dispersion in Amine Picrates
TRI-N-BUTYLAMINE
5600
4800
6400
7200
n "I
0
I
4800 4600
5200
5600 6000 $400 6800 5 600 6000 $400 6800 W A V C LENGTH IN A U N I T S
7200 7200
Figure 16. Inclined Dispersion in Amine Picrates
by hexamethylenetetramine picrate below 5200 h. In Figure 17 similar plots represent horizontal dispersion measurements (25). That of diisopropylamine picrate is unusually strong. The curves to the left in Figure 18 illustrate the crossed dispersion of hexamethylenediamine and 2-aminomethylcyclopentylamine picrates ( I S ) . Actual values were obtained by measuring the degrees rotation of the microscope stage required to :eturn the optic axis to the original index position at 6908 A. Corresponding optic axial angle measurements are shown to the right in Figure 18. The sharp minimum displayed by hexamethylenediamine picrate is unique (see 25 for other examples of crossed dispersion in amine picrates). In Figure 19, the three types of dispersion in 2,2'-bipyridine are illustrated (6). This plot
records the actual ocular micrometer scale measurements with change in wave length. Crossed axial plane dispersion, accompanied by inclined dispersion above its wave length of uniaxiality, must exhibit horizontal dispersion below this wave length or vice versa. In recent work in this laboratory dispersion has been made the basis of a technique of analysis. Bryant (4) observed that in mixtures of acetaldehyde and propionaldehyde 2,4-dinitrophenylhydrazones, the apparent optic axial angle, 2E, varied with composition. He obtained angle measurements on mixtures of the acetaldehyde derivative with as much as 407, of the propionaldehyde compound. The author has studied the complete system ( I S ) , employing the more widely applicable apparent optic axial angle, 2Ha. Figure 20 shows a plot of the optic axial angle obtained from mixtures of the two aldehyde derivatives. measured a t four of theomajor mercury lines (isolated by suitable filters) and the 5630 A. line (isolated by a monochromator). It is apparent that compositions containing up to 3570 propionaldehyde can be analyzed by measurement of the optic axial angle. Change in composition from 35 to about 80% of the three carbon aldehyde derivative had little effect on this constant Apparently a phase change occurred between 80 and 82.5%. In Figure 21 (upper) is shown the effect of the percentage of propionaldehyde derivative on the wave length of uniaxialitp This curve demonstrates clearly that with increase in propionaldehyde concentratioii the uniaxial point shifts towards the longer wave lengths. This measurement alone serves as a useful means for estimating the composition of this binary mixture in the range 0 to 3570 propionaldehyde 2,4dinitrophenylhydrazone The melting point diagram of this system is given in Figure 21 (lower). All these data w&e reproducible. Identical values were obtained either from synthetic mixtures of the derivatives or from coprecipitation of the derivatives from mixtures of the aldehydes, followed by a single recrystallization from alcohol. A few other binary systems of the 2,4dinitrophenylhydrazones of carbonyl compounds were studied (19). Although these did not prove so valuable, from an analytical viewpoint, as the acetaldehyde-propionaldehyde system, some offered a means of approximate analysis. The system propionaldehyde-acetone showed crossed axial plane dispersion between about 12.5 and
V O L U M E 21, NO. 4, A P R I L 1949
4sP
with change in composition which were insufficient for p r e cise analytical differentiation. The melting point curve (Figure 25, upper) k compared with that obtained by Brandstatter (S), who employed Kofler's technique ( 2 2 ) . Mixtures of the derivatives of n-butyraldehyde and isobutyraldehyde apparently did not form a mixed crystal system. The optic axial angles of both compounds were observed on examining fused preparations under the microscope. The melting point diagram was similar to ,that reported by Brandstatter (3) (Figure 25, lower). Mixtures of the 2,Pdinitrophenylhydrazones of more than two carbonyl compounds often may be separated by chromatoFigure 18. Crossed Dispersion in Amine Picrates graphic adsorption. Roberts A . Hexamethylenediamine picrate and Green (28) reported sepaB . 2-Aminomethylcyclopentylamine picrate rations of the derivatives of acetaldehyde, p r o p i o n a l d e hyde, acetone, and methyl 70% propionaldehyde. Optic axial angle measurements a t four ethyl ketone. Based on melting point measurements, most wave lengths indicated two-phase changes in this system (Figure of these derivatives separated fairly cleanly. The hydra22). Plots of wave length of uniaxiality against composition zone of acetone, however, could not be separated from that of and melting point data are given in Figure 23. propionaldehyde. A semiquantitative analysis for this binary Mixtures of propionaldehyde and isobutyraldehyde 2,4mixture might be made by the procedure just described. The dinitrophenylhydrazones (Figure 24) indicated some variations chromatographic separation of the p-phenylphenacyl esters of several binary fatty acid combinations also proved practical (df). The ease with which some binary mixtures of the lower aldeANGULAR EQUIVALENT OF MICROMETER SCALE FOR CEDAR OIL (nD 1.51 5) hyde 2,4dinitrophenylhydraaones can be analyzed optically 10 0 10 90 30 40 f 7000 is of value in the identification of several glycols with vicinal hydroxyl groups. Sodium periodate cleaves glycols of this type and oxidizes the fragments to aldehydes. For example: x 6500
formic acid will precipitate the pure p-bromoanilide of the carboxylic acid. p-Bromophenacyl bromide, in addition to 5400 0 20 40 60 80 100 forming a derivative with formic acid, appears to be a better reagent than p-bromoaniline for acids higher than valeric. Picrates are the most widely applicable derivatives for amines. Many picrates are remarkable for their strong dispersion characteristics, but they suffer one disadvantage. Many melt with decomposition and, consequently, often cannot be fused for optical study. Occasionally, the orientations of crystals obtained by recrystallization are not centered with respect t o the acute 1
'
5
By combining this evolution procedure with a gravimetric determination of the derivatives and optical examination of the recrystallized products, the author \vas able to analyze rather easily various combinations of glycols. Strong dispersion is of considerable value in arlalyses of this type, because it effects a marked change in optic axial angle with different wave lengths of light and, therefore, permits several confirming measurements. However, this property is not essential to microscopic semjquantitative or quantitative analyses. Bryant (6) found that the optic axial of mixtures of acetjc and angle, measured at 5461 i., propionic p-bromoanilides varied with composjtion. Similar changes occur in mixtures of adipic acid with smaller quantities of succjnic and glutaric acids (IS). Optic axialangle measurements of fused preparations, combined with melting point data,
b
I
IO
I
20
1
I
50 PROPIONALDEHYDE
30
40
1 60
(WT. %)
1 70
80
90
I
too
Figure 22. Optic Axial Angle ( 2 H a ) of Mixtures of Propionaldehyde and Acetone 2,4-Dinitrophenylhydrazones Wave length in Angstriim units
V O L U M E 2 1 , NO. 4, A P R I L 1 9 4 9
459
5 5600
170
I
1
20
40
Y
J W
>
2 5400
155
os
8 0
20
40
60
80
100
W -1
0
60
80
100
80
ion
u 45
?
4
160
_I
2
;
C 140
4
n
I-
35
z0
u 25
y 120 5 W
= 1000
40 60 P R O P I O NA L D E H Y D E
20
80 (W T.
o4)
100
Figure 23. Propionaldehyde-Acetone 2,4-Dinitrophenylhydrazone Mixtures Upper. Lower.
I 0
20 40 60 80 I00 I S O B U T Y R A L D E H Y D E (WT.%)
Figure 24. Optic Axial Angle (2Ha) of Mixtures of Propionaldehyde and Isobut yraldehyde 2,4-Dini trophenylhydrazones Wave length i n Angstram units
I101
o
I I I 20 40 60 I SOBUTY R A L D E H Y D E
l
(WT.%)
Figure 25. Melting Point Curves
Wave length of uniaxiality Melting point
System propionaldehyde-iaobutyraldehyde 2,4-dinitrophenylhydrazones System n-butyraldehyde-isobutyraldehyde 2,4-dinitrophenylhydrazones_ Broken lines represent data of Brandatatter Cpper.
Lower.
bisectrix (Bz,) or the obtuse bisectrix (Bz,). In these cases the number of crystallographic constants which may be determined are limited. Often the benzamide derivatives are better suited for these amines. The 2,4-dinitrophenylhydrazones appear to be the most widely useful for the identification of carbonyl compounds. Aldehydes often can be separated from ketones by their selective reaction with dimedone (dimethyldihydroresorcinol). Preliminary studies indicate that the dimedone condensation products are crystalline derivatives which display satisfactory optical behavior. Most amides, although crystalline, are not very satisfactory for microscopic examination. In general they exhibit a very low order of birefringence and poor interference figures. On several occasions the phthalimide derivatives have shown much better optical behavior. Although studies on the identification of mercaptans (thiols) have been limited, the 2,4-dinitrochlorothio ether derivatives appear suitable. Both methyl and ethyl mercaptans have been identified by means of this derivative. Semiquantitative analyses have been made on mixtures of these two mercaptans. Hydrocarbons and ethers do not form derivatives easily. D. M. Smith and the author have designed and made a low temperature stage which they hope will permit direct optical examination of frozen films of these compounds ( I S ) .
slide screws, the nitrogen inlet tube is shown. The spring clip for handling the cover glass slide holder is seen to the right of the microscope base. Preliminary observations of a frozen benzene film indicate that this hydrocarbon at least gives a clear interference figure.
This unit, shown in Figure 28 in cross section, consists of a brass movable stage insulated with Lucite, 1, which contacts directly a hollowed brass cooling block, 4. The screw and spring mechanism, 5, 6 (one of the two is shown here) permits fine horizontal adjustment of the movable stage in two directions. The center adapter, through which the microscope objective is inserted, is removable to allow free movement in placing or removing the circular slide holder, 3, with a spring clip. Circular slides, 22 and 18 mm., are used as slide and cover glass, respectively. The inner stage is kept free of condensed moisture by circulating dry nitrogen through grooves in the section and over the cover glass slides. The adapter is drilled with a 1.5-mm. hole, 2, to accommodate a thermocouple. The microscope substage condenser, 8, is fitted with a brass sleeve to facilitate heat exchange. Figure 29 shows the low temperature unit mounted on the stage of a Bausch & Lomb LC microscope.
It is fastened with screws; two dowel pins are used to locate center positions. On the left can be seen the inlet and outlet for the cooling solution. On the right, between the two movable
Figure 26.
:i
Apparatus for Analysis of Glycols
A . Reaction tube B . Aqueous scrubber bulb C, D . Standard 29/42 grinds E. Scrubber tube for 2,4-dinitrophenylhydrazine solution
460
A N A L Y T I C A L CHEMISTRY SUCCINIC ACID
Chsmot, E. M., and Mason, C. W.. "Handbook of Chemical Microscopy," 2nd ed., Vol. 11. New York, John Wiley & Eons. 1940. Denighs. G., CompLrend..225, 474-6 (1947). 16idd,225.841-3 (1947). Dewey, B. T..and Plein, E. M., IND.ENG.Ca~aa.,ANALED.. 18,515-20 (1946). du Pont de Nemours & Co., E. I., Ammonia Dept., unpublished results. Eisenberp. W. V., and Keenan, G. L.. J . A8.m. Offic.Agr. Chemists, 27. 177-9,458-62 (1944). Eisenberg. W.V.. and Wilson, J. B.,Ibid.. 30, 563-7 (1947). Groth. P., "Chernische Krystallographie," Vols. I-IV, Vedag, Leipsig, "on Wilhelm Engelmann, 1906. Hartshorne, N. H.. and Stuart, A., "Crystals and the Polarising Mioroseope,"~.211.London,Edward Arnold Co., 1934. Hernandez. F., and Font, M., Mo~on.farm.I/ t e m p . (Madrid), 53. m - R. n.119471 ~~. ....,. (19) Jelley, E.E.,IND.ENo_C~EM.,ANBI. ED., 13.196-203 (1941). (20) Keenan, G. L., J . Assoc. Ofic.Agr. Chemists, 27, 153-61, 177-9 ~
, , O M~ , ~ " ~ ~ , .
Kirohner. J. G., Prater, A. N.. and He*agen-Smit, A. J., Im. ENG.CREW.,ANAL.Ea., 18.31-2 (1946). Kof ler, A., Z.physik. Chem., A187,201-10(1940). GLUTARIC ACID
Figure 27.
ADIPIC ACID
Ternary System Adipic-Glutaric-Succinic Acids
Optio arid angle (2Kcz-74 to 76" Melting point (' C. corrected). n 148.5, 6 149.0, e 150.5,f 151.0, g 151.5
E 149.5,
d 150.0.
.
Me1h n e , W. C., ANAL.CHEM.,20 (March-December 1948); 2, wsnurtry-n' pril 1949). McCmne. W. C . . Srnedal, A., and Gilpin, V., IND.ENG.C a m . , ~~~~, ANAL.Eo.. 18,578-82(1946). J., Jr.. and Bryant, W. M. D., J . Am. Chem. Soe.. 37 (1943). L.,and Kasuuba. F.J.,Ibid., 61,2974-6(1939) I_
' ~
2
Figure 28.
Low Temperature Stage CONCLUSION
Two of the more significant trends in the field of optical crystallography are the increased use of dispersion BS a valuable mema of identification of organic compounds and the application of the microscope in the quantitative analysis of homologous mixtures. The microscope is an important complemontary tool in the instrumental laboratory. ACKNOWLEDGMENT
The author is grateful to W. M. D. Bryant for his many contributions, t o D. M. Smith for his advice and help in the design of the low temperature stage, and to C. W. Hammond, A. N. Oemler, and L. P. Wishowsky for their aid in obtaining many of the new contributions reported in this paper. LITERATURE CITED
(1) Bailly, R., Science. 108,143 (1948). (2) Berisso. B.. Reu.fwm. (Buenos A i r s ) , 89,197-206(1947).
(31 Brandsf6tter. M.. Mikrochim.. 31.3348 (19441. (4j ~r~~ it, W. M. D., J . Am. Ch&. &e., 55,'3201:7 (1933). (5) Ibid., 60,1394-9 (1938). (6) Ibid., 63,511-16(1941). (71 Bid.. 65,96-102 (1943). It, W. M. D.,unpublishedresults.
Figure 29.
Low Temperature Stage with Microscope Objective Lowered into Place
V O L U M E 21, NO. 4, A P R I L 1 9 4 9 (27) Ramsey, L. L., and Patterson, W. I., J . Assoc. Ofic. Agr. Chemists, 28, 644-56 (1945). (28) Roberts, J. D., and Green, C., IND. ENG.CHEW,ANAL.ED., 18, 335 (1946). (29) Rogers, A, F., and Kerr, P. F., “Thin-Section Mineralogy,” New York, McGraw-Hill Book Co., 1933. (30) Roth, W. L., DeWitt, T. W., and Smith, A. J., J. A m . Chem. SOC.,69, 2881-5 (1947).
46 1 (31) Wilson, J. B., and Keenan, G. L., J. Assoc. Ofic. Agr. Chemists, 27,446-8 (1944). (32) Winchell, A. N., “Microscopic Characters of Artificial Inorganic Solid Substances or Artificial Minerals,” New York, John Wiley & Sons, 1931. (33) Winchell, A. N., “Optical Properties of Organic Compounds,” Madison, University of Wisconsin Press, 1943. RECEIVED November 15, 1948.
chem2cd microecoply sgmposium
Resinography of Some Consolidated Separate Resins T. G. ROCHOW AND F. G . ROWE Stamford Research Laboratories, American Cyanamid C o m p a n y , Stamford, Conn. An unstressed single physical-chemical phase of any resin is characteristically without structure under a light microscope of even the highest resolving power. Generally, only polyphased resin systems manifest light-microscopical structure. Confronted with light-microscopically homogeneous resins, a ttention was paid to electron microscopical techniques. Replicas of either polished or molded surfaces were more characteristic of the method of preparation than of the specific resin. Therefore, the cold-embrittled resin was fractured under standardized conditions and electron micrographs were taken of replicas of the fracture surfaces. The resins examined
0
NE common characteristic of a separate resin is its optical homogeneity. In its consolidated state a resin appears to be structureless to the unaided eye. Even under the highest resolving power of the light microscope (approximately 0.2.4 a resin possesses inadequate variations in refraction, reflection, or absorption of visible light for the perception of structure within the boundaries of the resin. Therefore, it is classified as a single physical phase. Under the usual classification, a resin does not manifest the geometric configuration of its natural surfaces and internal graininess which are characteristic of a polycrystalline phase. Only “incompatible” resins in admixture (polyphased resinous systems) usually manifest structure under the light microscope. Possibly because of their very optical heterogeneity, polyphased resins are not used commercially as much as single-phased resins. Occasionally, however, heterogeneous resins are encountered by the resinographer-for example, phenol-formaldehyde resin plus butadiene-styrene elastomer (GR-S). Such a mixture was submitted (by D. JT. Toung, Esso Laboratories, Standard 011Development Company, Bayonne, N. J ) as a test bar 6 inches long and 0.5 inch by 0.5 inch in cross section. The bar 7%-assmoothed in cross section, flattened on abrasive papers, and polished on a cloth lap with water and magnesium oxide (“free of sulfate,” Merch). The two-phased system appeared by vertically reflected light as shown in Figure 1 at 1OOX. The phenolic resin is identified as the discontinuous phase because this IS lighter in shades of gray, corresponding with the higher refractive index A few of these areas are marked P in Figure 1. Thus the continuous phase (shown black in the photomicrograph) is the GII-S elastomer. Because the elastomer is the continuous phase, it is free to be distended and the mixture is therefore resilient. A resinographic examination of this type of plastic reveals the number of phases, their mode of association, particle sizes and
were RIelmac, polyacrylonitrile, a 3 to 1 mixture of the two, copolymer, Lucite, polystyrene, and Polythene. All the resins manifested different structures. It is tentatively concluded that these structures are typical and characteristic. Most of the fundameFta1 units are round particles only hundreds of Angstroms in diameter, approaching molecular dimensions. More empirical data are probably needed for a general theoretical explanation but, in the meantime, this resinographic method should be of practical value in the study of both accidental and experimental cracks and breaks among commercial resins and their plastics.
shapes, and relative reflectivity (refractivity). Separate chemical tests and some physical tests (such as impression and scratch hardness) may often be made on the separate phases. Another example of light-microscopical heterogeneity shows how differences in refractive index are perceptible pictorially by reflected light. Figure 2 is one photograph of the molded surface of two laminates placed side by side, taken by unidirectional, oblique, reflected light a t lox. The two laminates are a pair which was produced under identical experimental conditions. R e are concerned with two layers: a surface (protective) layer composed of a thin sheet of paper impregnated with a Melmac resin and, i n the place usually occupied by the layer carrying the design or decoration, a sheet of RIelmac-resin impregnated black paper for the experimental purpose of showing scattered light t o the best advantage. On the left the protective layer is satisfactorilj transparent, for it displays the “design” layer in almost its original degree of blackness. On the right, the protective layer scatter? so much light that it almost entirely obscures the black design layer. The only difference between the two samples was in the. fiber composition of the paper in the protective layer: viscose fibers (satisfactory) versus cellulose acetate fibers (unsatisfactory). Figure 3 shows each entire protective layer in polished cros:: sect’ion, at 500X by vertically reflected light. Figure 3 (left) corresponds to Figure 1 (left) and shows the viscose fibers (medium gray) as a discontinuous phase in close contact with the continuous phase of Melmac resin, The measured average refractive index of the viscose fibers was 1.545 and that of the hlelmac resin, was 1.652. These refractivities were sufficiently close to transmit light satisfactorily to the design layer (Figure 2, left) but sufficiently different for microscopical differentiation in polished section (Figure 3, left). In Figure 3 (right), the cross section of the unsatisfactory layer, not only are the cellulose acetate fibers shown in darker gray (average measured refractive index 1.470) but they are surrounded by black bands, assumed to be spaces of air (refractive index 1.000). Probably a t the airfiber and air-resin interfaces so much light is scattered that the
