Application of"the Grating Microspectrograph To the Problem of Identifying Organic Compounds EDWIN E. JELLEY Kodah Kr.search Laboratories. Rochester, N. Y .
T
HE study of the optical properties of crystals has rela-
K i t h all three classes of molecular crystal, na is usually low enough to be determined by the immersion method, using the light from a sodium vapor lamp for the final adjustment of the refractive index of the immersion liquid. The temperature variation method is not often applicable to organic compounds on account of their solubility in liquids having a suitably high thermal variation of refractive index. The author prefers to use the method of mixing two liquids of very low volatility and widely different refractive index on the microscope slide until the lowest refractive index of the crystal under test is matched. The immersion liquid is then transferred t o a microrefractometer or an Abbe refractometer. Mixtures of tri-N-butyl citrate and n-butyl phthalate cover the range 1.447 to 1.492, and of Nujol and a-iodonaphthalene, cover the range 1.484 to 1.701 (2). Kunz and Spulnik have suggested the use of mixtures of heptylic acid and a-bromonaphthalene to cover the range 1.423 to 1.658 (6).
tively greater importance in microscopic methods of identifying organic compounds than in the identification of inorganic ones. This is partly because with inorganic compounds chemical reactions of both cation and anion may usually be carried out under the microscope, whereas with organic compounds chemical tests are restricted to detecting the presence of certain reactive groups. However, in general, it is not possible to use optical crystallographic methods as the sole means of identification of organic compounds, because the optical constants have been determined for relatively few of the vast number of such known compounds. When, however, the optical data of organic compounds are used in conjunction with other physical and chemical data, they may be of considerable m e t o the microchemist, particularly where the problem may be reduced t o that of finding out if an unknown substance is identical with one of a group of known substances. The relative usefulness of various optical properties of organic compounds for such comparison purposes may be judged from the following considerations.
Birefringence The birefringence of a doubly refracting crystal may have any value between the limits of zero for rays traveling along ne in the case of uniaxial an optic axis, and n, - n, (or Q crystals) for rays traveling normal to the plane of the optic axis or axes. Consequently it is usual to restrict the term to the three constants ny - np, ng - n,, and ny - na of biaxial crystals, and nw ne of uniaxial ones. The birefringence of organic compounds is due to two factors, molecular anisotropy and structural anisotropy, and in general n, - na is large for rod-shaped or planar molecules which are oriented in the same direction, and is small for roughly spherical molecules and for planar molecules which have several orientations in the crystal. It may also be small for crystals containing solvent of crystallization, particularly when the solvent of crystallization also has planar molecules. Methods of measuring birefringence with the aid of compensators entail measuring the thickness of the crystal, and as such thickness measurements can be made only with a low order of accuracy, the birefringences so determined are not very accurate.
-
Principal Refractive Indices
It is convenient t o follow the classification given by Wooster (8) and to divide molecular crystals into three main classes :
-
1. Almost spherical molecules, such as pentaerythritol and some of its esters. Crystals of this class have refractive indices well within the range of organic immersion media. Mixtures of
Nujol with a-bromonaphthalene or a-iodonaphthalene have a sufficiently low solvent action to be used as immersion media. 2. Rod-shaped molecules. This classification includes the majority of long-chained aliphatic compounds. Usually the refractive indices of crystals of this class are low, unless the molecule contains substituents of high refractivity, such as bromine, iodine, sulfur, etc. The crystals may have an appreciable solubility in immersion media, so that refractive index measurements are not as a rule easy to make. Increasing the length of carbon chain by one or two carbon atoms does not greatly affect the refractive indices, and as the limit of accuracy of determining these refractive indices is about +0.002, it follows that the measurements may have but little analytical value. 3. Planar molecules. This classification includes the majority of aromatic compounds. Unless the molecules are grouped in nonparallel directions, n-,and often np are beyond the range of known immersion media. Solubility of the crystal in immersion media may render it impossible t o determine its refractive indices. The position and nature of weakly polar or nonpolar substituents usually have little effect on the refractive indices.
Dispersion of Birefringence The dispersion of refraction, dn/dX, of a crystal is usually different for each of its principal refractive indices, and as birefringence is the numerical difference between two such 196
March 15, 1941
ANALYTICAL EDITION
indices, it follows that birefringence varies considerably with wave length. So-called iianomalous" interference colors are caused by extreme dispersion of birefringence, but in no known case does the scale of interference colors given by a crystal exactly correspond to Newton's scale of colors given by an air film. Crystals which are pleochroic in the violet or near ultraviolet show marked differences between the dispersions of the principal refractive indices, and consequently have a strong dispersion of the birefringence, particularly along the acute bisectrix when the optic axial angle is small. By way of an example, the values of ny - np of ammonium picrate and o-dihydroxybenzene are compared for various wave lengths. The measurement of dispersion of the principal birefringences is complicated in the case of monoclinic and triclinic crystals which possess an appreciable degree of dispersion of the axes of the optical ellipsoid. I n such cases the author prefers to measure the dispersion of birefringence presented by a fixed orientation of the crystal, and to take care that an identical orientation is used in comparisons with other crystals. Whenever possible, dispersion of the maximum birefringence, ny - na, is chosen for comparison purposes, as errors of up to 10' in orientation of the crystal do not seriously affect the accuracy of the results. The measurements have considerable practical interest, as it is not necessary to know the thickness of a crystal in order to determine its dispersion of birefringence. TABLE I.
(ny
x, mLI 513 541 589 656 a
-
np)
O F .4 COLORLESS AND A YELLOW CRYSTAL o-Dihydroxybenzene Ammonium Picrate
0.0106 0.0104 0,0099
0.0093
0.044 0.000 -0,037a - 0.071a
Vibration directions of n y and nb interchange a t 541 mp.
Optic Axial Angle Observations of the interference figure given by a crystal in convergent polarized light have a well-recognized value in determining whether a crystal is uniaxial or biaxial, and, in the latter case, it is sometimes possible to measure the optic axial angle. Observation of the type of dispersion of the optic axes, when it can be made, renders possible differentiation among rhombic, monoclinic, and triclinic crystal symmetry. Principal Absorptions Observations of the principal absorptions of strongly colored crystals are usually qualitative and are confined to the study of dichroism or trichroism. This is understandable, as a great many strongly absorbing organic compounds form monoclinic or triclinic crystals which often have extreme dispersion of the axes of the optical ellipsoid, so that the optical properties of the crystals are not easy to interpret. Severtheless, it is this complication of optical properties of absorbing crystals which makes them all the more specific and useful for comparison purposes. The above considerations show the importance, from a determinative standpoint, 0 . studying the optical properties of organic crystals for as wide a region of the spectrum as possible. It is obviously not practicable to repeat a complicated series of observations for many wave lengths, particularly as the eye is neither very sensitive nor very well focused for the extreme blue end of the spectrum. Grating Microspectrograph It was in order to simplify the study of crystal optics throughout the visible region of the spectrum that the author turned to the aid of photography and developed a transmis-
197
sion grating microspectrograph for use with the petrographic microscope. The first model of the apparatus used 8.125 X 10.625 cm. (3.25 X 4.25 inch) panchromatic cut film (6); the second model was much more compact and used 35-mm. Panatomic iilm (3). In neither of these instruments was the light collimated before its passage through the grating. The achromatic lens which projected the image of the slit onto the film had a very small diameter relative to the projection distance; the astigmatism thereby introduced into the spectrum was not a t first considered serious. As experience in applying the apparatus to specific problems was accumulated, it became evident that it would be advantageous to have a spectrum completely free from astigmatism. The generous gift of a plane metal grating from H. D. Babcock of Mt. Wilson Observatory made it possible for the author to construct a reflection-grating microspectrograph working on a somewhat different principle, which incorporated the improvements suggested by the earlier work. The general principle of the new apparatus is shown in Figure 1. A Steinheil lens is used to focus the microscope image on the slit. The microscope, which is used with an eyepiece not fitted with cross webs, is focused for an image distance at infinity, so that the Steinheil lens is used at its principal focus from the slit. I t is provided with centering screws, in order to bring the center of rotation of the image on the center of the slit. The collimatin lens is a cemented achromat which collects light from the slit and gives a collimated beam which is then reflected by a silvered right-angled prism on the plane grating. A Wollaston prism, which is provided with a fine adjustment to its rotation, may be swung into the beam for studies of dichroism. Over half the light incident on the gratin is diffracted in one of the first-order spectra, which is collected%ya cemented achromatic camera lens and is brought to a focus in the image plane of a reflex camera which uses 35-mm. Panatomic film. I t was found that much less coma was obtained when the plane side of the camera lens faced the grating and that all the spectrum was sharply in focus at the same time. The grating is mounted in a heavily built holder which rotates on spindles fitted with Hoffmann ball bearings. The grating is rotated by a micrometer fitted with an accurately worked face plate of sapphire which presses against a steel ball fixed in the grating housing. In the center of the reflex ground glass, there is a clear patch fitted with adjustable cross webs. The s ectrum can be focused on these cross webs which are viewefwith a X 12 Ramsden ocular. Wave lengths of emission lines may thus be determined to 0.2 mp. S ectra of very low intensity are observed through a telescope fftted with a reflecting prism which intercepts the light from the grating before it reaches the camera lens. SCREEN WITH CROSS WEBS
SILVERED CAMERA LENS
PRISM
' l
MICROMETER
MIRROR
~5illl"&FILM OR PLATE
I
Compared with the transmission grating apparatus described by Chamot and Mason ( I ) , the new apparatus is considerably easier to manipulate, as it is merely necessary to place the microscope under the microspectrograph. Accurate centration is not necessary; the microscope is moved on the baseboard until the spectrum is evenly illuminated, a matter of a few seconds' adjustment.
Vol. 13, No. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
198
through the ascending orders. Spectrograms are then made on Panatomic-X film, exposures of 1 t o 20 seconds being given, according to the width of the slit. A Philips tungsten arc lamp is used as a light sonrce, as it gives, in addition to the continuous spectrum, neon lines which 8ewe to locate the wave-length oalihratian. A ribbon-filament I ~ can D also he used. but in this case an additiodal expomre is mide with the microscow scale& 10 mm. on the bromide enlarrement. -It
FIG-
enlargements. If the wedgecrystal spectrogram shows several interference bands, it is best to proceed according to Figure 4. A line is ruled through the spectrum in a position corresponding to the thick end of the wedge crystal. Irregularities
2. REFLECTION-GRATING MICP.OSPECTROGRAPE
The reflection-grating microspectrograph is shown in Fig-
ure 2, and the complete setup, with a Leita universal polarizing microscope constructed to the author’s specification, is shown in Figure 3. A silvered right-angled prism is used over the eyepiece to deflect an image of the crystal onto a screen. When the spectra are to he recorded photographically, the prism is thrown out of the optical axis of the system. There is, however, a disadvantage attendant on the use of a metal reflection grating; the violet end of the spectmm is relatively weaker than that ohtaiied with a transmission grating. This is because the speculum metal on which the grating is ruled has a lower reflectivity in the violet, whereas the violet end of the spectrum is accentuated with a transmission grating by virtue of the higher refractive index in this region of the cellulose nitrate from which it is made.
disto&on introduced by the camera lens. -A wave-lendh scale is then attached to-the enlar ement, and the wave lengths %, &+,, Am+*. .. ... at which the TL& (n 1)th, (n 2)th. . .. , . interference bands cross the line are recorded. The value of n 1s readilv counted if the extreme t l D of the wedee amears In the
+
+
Applications of Grating Microspectrograph In the author’s first publication on the microspectrograph (6),its use in the determination of optic axial angles of orthorhombic crystals for the visible spectrum was discussed. It was proposed to use a calcite plate cut perpendicular to the optic axis for calibration purposes. This plate was calibrated with sodium light (X = 589.3) by means of a universal stage. In the second publication (9), a spectrogram was given which showed the dispersion of the optic axes of o-nitroacetanilide. The use of the microspectrograph in conjunction with a universal stage in determining the dispersion of a single axis of praseodymium sulfate octahydrate was also discussed. These publications also dealt with the “wedgecrystal” method of determining dispersion of birefringence. Three microspectrographic methods of measuring b i r e fringenee and its dispersion as a function of wave length have ‘heen worked out.
DISPERSION OF BIREFRINGENCE OF WEDGE CRYSTALS. This is applicable to substances having a melting point (without decomposition!) below 300’ C. On an optically worked microscope slide of fused quartz is placed a square of No. 2 or No. 3 cover glass. Another square is placed so that one edge rests on the first square, with the opposite edge resting on the miomsoape slide. Some crystals of the substance t o he studied are placed near the second cover slip, and the preparation is heated over a microhurner until the substance melts and is drawn under the sloping cover glass to form a .wedge of liquid. On cooling, the substance crystalliaeg,toform wedge-sha d crystals. Refusion may be necessary to et~satisfactoryres$s, and the preparation may need to be seesed by touching it with a needle which has heen’charged with crystds of the suhstmce. Such a wedge pre aration shows ascending orders of birefringence when examine$ witha low-power objective between crossed nieols. The image ofi the wedges h m d crvstah. which have a slom of about 2’. is thrown on the slitbf th~micr&pectmgraphandAisso oriented’that the slit euts
F I Q 3.~ COMPLETE MICBOSPECTROQRAPH SETUP
March lS, 1941
ANALYTICAL EDITION
199
of the birefringence for any other wave length is obtained by multiplying the value for sodium light by the dispersion factor as obtained above.
BIREFRINGENCE AND DISPERSION OF BIREFRINGENCE OF LENSCRYSTAX An elegant way of determining both bjrefringence and dispersion of birefringence may often be a p plied to readily fusible organic compounds.
A small plano-convex lens, preferably of fused silica, and an oDtiertUv worked microscore slide of fused silica are reauired.
tion has’ been successful, a negative lens of crystal is fofmed, which has zero thickness at the point of contact of the silica lens and silica slide, and which shows oircles of increasing birefringence between crossed nicols. FIGURE4. COMFWTATION OF BIREFRINGENCE
The appearance with sodium light between crossed nicols of a lens-crystal of a-nitronaphthalene is shown in Figure 6 (left). When crystals of more than one orientation are pres-
corre3pond:, M a retardation of 589 tup X 7 = 4.125,’. Similarly, the retardiltion at As is 647 rnp X 6 = 3.922, and at A. is 5011mp X 9 = 4 . 5 ~irctnrdxtiou = birefrinccnec x thickness of vrvstal). . The values of the relative birefringence are, therefore, 0195 for 647 mp, 1.00 for 589 mp, 1.09 for 500 m+, and so on. These values are plotted against wave length. Some typical curves are given in Figure 5, in which Newton’s color scale for an air film is represented by a horizontal line. This method of computing the dispersion of birefringence utilizes the light from a single point on the wedge crystal, and no assumptions are made as to the uniformity of slope of the wedge. When, however, only a few bands are prese sary to measure the average distance apart gence bauds on the photographic enlargem wave lengths. Let the average distance between bands at wave lengths XI, .. , be dt. da. dx.. . . .. The birefringence at these wave l>hs is then represented by A,/&;&$/&;-e%/&. . .... where e is a constant depending on the slope of the crystal wedge and the ma. ification of its image on the bromide enlargement. If X, is 5 8 m u . the relative birefringence at Xz is dA/dl.h, and so ob. These kalues are plotted agzust wave length, as %hove. ~~~
measu>he distance ddng &e wedge between the 2nd aKd 12th (or higher) interference bands when the microscope is illuminated with a sodium vapor lamp, If the distance between the 2nd and 12th bands, measured with a screw micrometer eyepiece, is dmm. m d the slope 9, the birefringence is 0.00589/d tan 9. The value
FIGWRE 5. TYPICAL CURVES
ent, birefringence rings corresponding to the different orientations are produced, as in the case of piperonal (Figure 6, right) where the rings of small diameter correspond to ng ne, and the larger ones to n1 - ns.
-
In actual practice, it is usually necessary to recrystauize the substance by fusion B few times in order to obtain satisfactory lens-crystals. I n order t o obtain suitable interference figures, it is desirable to have a lens with a relatively large radius of Dumature for use with substances of high birefringence, and one of
RAMS
_____I
smaller radius of curvature for those of smaller birefringence. Those at present in use have radii of 83 and 31 mm., respectively. Birefringence spectrograms of lens-crystals are ohtained at low magnifications, a No. 1 objective and a X 5 eyepiece being used. The diaphragm of the substage condenser is restricted t o a numerical aperture of 0.05 in order that the direction of rays of light within the crystal shall not deviate too far from the axis of the lens, particularly when measurements of birefringence are
oth'er side, gut, as it i s h easy to mark the ex& center of the rings, both first-order interferenee hands should always he included Birefringence spectrograms obtained in this way are shown in Figure 7 (left), which is that of p-nitrophenol, and Figure 7 (right), which is that of benail.
di&. -In order t o make the hngs clearly visible by transmitted ~~~~
~
audvine a droD of a 1 per cent solition of l.l'-diethyl-*-cvanine ... ... ~~~,~, ~~~~~~
~~~
~~~~~~~~~~
~~
sodium light. The Newton's ri& pro"duczd by such treated surfaces are quite as good as those given by half-silvered surfaces, and the dye films are very easily applied and removed, operations which take but a second or so. The Newton's rings are phatoermhed on the same leneth of 35-mm. Panatomic-X film as is ;sea t o record the birefringence spectrograms of the lens-crystals. ~~~~~~
0~
~~
~~~~
~
~
~
500
5
400
~~~~~~~~
Vol. 13, No. 3
INDUSTRIAL A N D ENGINEERING CHEMISTRY
200
~
the ground-glass screen. By using the same microscope setup for both the Newton's rings and the lens-orystals, the magnifica tion is unchanged, so that the thickness of crystal necessary t o produce any interference hand is directly measured in terms of the wave length of sodium light.
A photograph of the Newton's rings is given in Figure 8. As they are produced by transmission, the center is bright, and the air gap corresponding to the nth bright ring bas a thickness of n/2 X 589.3 mp. Measurements of photographs of both Newton's rings and interference bands produced by the lens-crystals have shown that their diameters vary according to the square root of their order within an accuracy of 1 in 2000, so that it is obvious that the microscopic and spectrographic optical systems are not introducing any appreciable distortion.
OF
.
600
p-NITROPHENOL AND BENZIL
In practice, these measurements can he made with adequate accuracy from bromide enlargements. The center of the Newton's rings is found by bisecting the diameter of one of the lowerorder rings, and the radius of a high-order hri ht ring is measured. Let the order of the ring be M , and b e radius he X. Measurements are then made from the center of the birefringence spectrogram (obtained by bisecting the distance between the two first-order hands) to the Nth order interference band, for some particular wave length, A. Let this distance be y. The birefringence of the crystal for this particular wave lengthis then given by theformulan, n2 = 589Mya/2NhzP, where nt and npare the refractive indices of the crystal for this wave length. Measurements of y are made for various values of A in order t o obtain birefringences for the whole spectrum, and these may he plotted in terms of the value for sodium light in order t o obtain dispersion curves corresponding t o those obtained by the first method. DISPERSION OF BIREFRINGENCE OF
DROPLETCRYSTALS.Fragments of. the substance, weighing between 0.1 and 10 mg. are fused on a micros c o p e s l i d e , p r e f e r a b l y one n optically worked fused silica. On cooling, the droplets crystallize to yield wedgeshaped crystals. Aswith F~~~~~8, N ~ ~ .the first two methods, it is necessary to check the optical orientation of TON'S RINGS the crystals by conoscopic observation. In order to determine the dispersion of birefringence, a spectrogram is made of the rising orders of interference of one of the wedgeshaped crystals at a suitable magnification. A droplet crystal of o-chloroacetanilide gave the spectrogram shown in Figure 9. The method of computing the birefringence-wavelength graph is identical with that of the first method. One or other of the above methods will work with a great many fusible organic compounds. Failure to obtain satisfactory crystals may be due to one or more of the following causes: 1. The substance is impure. Two or three recrystallizations from a suitable solvent will often put this right. 2. Conditions of cooling the fused substance are not correct. In general, excessive supercooling should be avoided. A defective wedge- or Iem-crystal is cautiously heated so that some of the substance remains unfused and is then slowly cooled. This treatment often yields satisfactory crystals.
N A Y T I A L ED T I O N
March 15, 1941
TABLE11. COLORS OF PLATINOCYANIDE CRYSTALS
201
However, with ahmrbing anions and eations, the influence of electric fields in the crystal is great, so that the nature of the other ion and of the crystal symmetry may greatly modify the pleochroism of the crystal. 3. The color of the orvstal mav be characteristic of the mole-
3. The fused substance on cooling forms a mass of small crystals of random orientation. Modifying the conditions of cooline should he tried. and if tllia I d a the method for nonfusiblesubstances should be tried.
DISPERSION OF BIREFRINGENCE OF NONFUSIBLE CRYSTALS. The method of computing the birefringence-wavelength graph described under the first method is readily applied to crystals which possess either a crystallographically or an artificially produced wedgeshaped edge. Such B crystal usually needs to be mounted in some medium having a refractive index reasonably close to that of the crystal in order that the ascending order of interference colors may be seen in the wedge-shaped edge. The mounting medium should be viscous and have as little solvent action on the crystal as possible.
The &t-nkme;l'sub8t&e
bar a refractive index. n'%. of 1:514%.
have a wedge-shaped &e and have suo6 stronglyaefined cleavage planes that they will not give a wedge-shaped fracture. rohded part of the crystal Gives the" sime purpose as a wkdge.
The Color of Crystals An extensive study of colored crystals has been made by means of the microspectrograph. This work has shown that the spectral absorptions of a crystal depend on both its chemical composition and its crystallographic structure. As a result of this study, i t is now proposed to classify the various possible causes of color as follows: 1. Rareearth elements (Atomic Nos. 57 to 71) give line absorption spectra. Spedding (7) has shown that the grouping of lines given by crystals of a rare-earth compound is primarily dependent on the nature of the crystal symmetry. The author (4) found that neodymium and praseodymium sulfate oetahydrates O S S ~ S San extraordinary type of pleochroism, charscteriaed gy the fact that certain of the absorption bauds disappear for specifio ray znd vibration diirectian8. These directions are different for each absorption hand and do not coincide with the axes of the optical ellipsoid. 2. Colored inorganic ions contribute to the color of the crystal. Thus, potassium ferricyanide crystals have a color charaeteristic of the ferricyanide ion, cupric sulfate pentahydrate crystals have the blue color of hydrated cupric ions, and so on. When both the anion and cation are colored, their relative contributions may be different for each of the principal absorp tion spectra, so that the pleochroism of the crystal becomes complicated. In the case of complex cations, it is the arrangement of the coordinating groups, rather than the specific metal atom, which governs the color of the salts. Thus, crystals of the two compounds [Co(NHa)eH.01C18and [Cr(NH&H201Clame almost indistinguishable, and both exhibit the same orange and pink dichroism. In the case of rare-earth compounds light is absorbed by electron transitions in the N shell of the rare-earth ion, which is screened by eight electrons of the 0 shell; hence, the influence of the anion on the absorption is not great.
Such an ixplanation will not accounl. for the extraordinary colors of my&-& of platinocyanides which are colorless in 801~tion. That the Dresence of water of ervstallieation is resnonsible in some way forihe production ifa li&absorbing mech'anism is obvious from a study of Table 11. It will be noted that the two anhydrous platinocyanides, those of silver and mercury, are colorless. The ammonium salt with one molecule of water has a strong absorption in the near ultraviolet and has, in consequence, an zthnormally high refractive index in the violet which, in turn, is responsible for the selective surface reflection of the extreme blue end of the spectrum. It appears to he a general rule that the greater the molecular proportion ai water in a platinocyanide crystal, the lower the vibration frequency of the absorbed light, and it is interesting to note that both the red magnesium and yttrium salts have seven molecules of water to each platinocyanide ion. The effect ai water of crystallization in crystals of hasic dyes is ususlly great, the hydrated orystal being different from the anhydrous crystal in crystal farm, birefringence, and principal absorptions. Curiously enough, the color of the nonhydrated crystals often bears most resemblance to the color of the solution, which is an indication that water of orystalliaatien in basic dyes may become part of a new absorption mechanism.
Fiunm 9. SPECTROGRAM OF O-CHLOROACETANILIDE 6. The degree and nature of the optical anisotropy of a crystal influence both the intensity and position of the absor tion bauds. The author has studied anhydrous crystals of 26 didrent salts of the basic dye 1,l'-diethyl+cyanine and has found that those of high birefringence have strong absorption and plecohroism, whereas those of low birefringence have not only weak pleochroism but also a very weak depth of color. Crystah of the ~ are very deep red in color, whereas a similar iodide, 2 0 thick,
202
INDUSTRIAL AND ENGINEERING CHEMISTRY
thickness of the trithionate is only very faintly yellowish orange. The dithionate and tetrathionate are rather more birefrin ent and have slightly more color. The p-toluenesulfonate and bna hthalenesulfonate come next in the list and are orange in col%. All these crystals give the same magenta-colored solution in methanol. Solvent of crystallization (other than water, which has specific properties discussed above) may increase or decrease the anisotropy and depth of color of the crystal. Thus in the study of the 1,l’-diethyl-+cyanine salts it was found that, whereas crystals of anhydrous chloride had nearly the same depth of color as the iodide, crystals of chloride containing phenol of crystallization had a much lower birefringence and were relatively weak brownish orange in color. I t appears to be a general rule that a decrease in anisotropy is associated with a shift of the absorption band towards the shorter wave lengths. These six factors often operate in combination, particularly with crystals of salts in which both anion and cation are dyes. For example, the chrysophenine salt of 1,l’-diethyl-+-cyanine forms yellowish-orange crystals having a relatively weak birefringence. The predominant color is, therefore, that of chrysophenine. A similar effect has been observed with crystals of picrates of basic dyes; the strong yellow of the picrate ion is not much affected by the cation, whereas the light-absorption of the dye cation may be greatly changed in wave length and intensity. It is evident that considerable care must be exercised in examining unknown dyes and comparing their optical characteristics with those of known dyes, but the above considerations do a t least indicate some general rules. Thus both known and unknown should be converted to the same salt, and both should be recrystallized three or four times from the same solvent. So far as basic dyes are concerned, the perchlorates are the most satisfactory salts on account of their low solubility, lack of tendency to take up solvent of crystallization, and freedom from color of the perchlorate ion. As an example, methylene blue perchlorate is precipitated when an excess of sodium perchlorate is added to an aqueous solution of methylene blue chloride. The precipitate is washed and then recrystallized from methanol. Very few technical acid dyes can be induced to form crystals suitable for microscopic examination, but some degree of purification can usually be obtained by precipitation as the free acid and re-solution in ammonia, repeating the cycle several times. Crystals of dyes must be extremely thin for microscopic examination. The appearance of a colored crystal in ordinary light, with one nicol, and with crossed nicols, may vary very considerably with change of thickness, so it is necessary to study a range of thicknesses. Evaporation of thin ~s of very concentrated dye solution is one of the easiest ways of preparing crystals for study, suitable solvents for this method being pyridine and benzyl alcohol. a-Bromonaphthalene and a-chloronaphthalene have useful solvent properties. At room temperature they are poor solvents, but a t temperatures of 100” to 300” C. they become powerful ones. Consequently, it is possible to perform recrystallizations on microscope slide preparations by heating some finely ground dye under a cover slip with a-bromonaphthalene or a-chloronaphthalene and then allowing the preparation to cool very slowly. The high index of refraction of these solvents is of particular advantage, as most dyes have very high values of n g and ny. It has been found that there is only one certain way of studying the pleochroism of strongly absorbing crystals. This consists in confining the area of illumination to within the boundaries of the crystal. If the illuminated area is greater than the size of the crystal, the color is degraded in two ways; intersurface reflections in objective and eyepiece dilute the color of the crystal with white light, and light reflected backwards by the objective is reflected by the surface of the crystal to such an extent that the surface color is often the only one seen in the microscope. A striking example of this effect
Vol. 13, No. 3
is shown by crystals of l,l’-diethyl-+-cyanine perchlorate. Examined in the ordinary way, with the microscope slide illuminated with polarized light filling the entire field of view of the microscope, the crystals appear to be colorless and pale brown dichroic, but when the illumination is restricted to the crystal, the dichroism is colorless and very deep crimson. The swing-out type of condenser, which is usually fitted to petrographic microscopes, is not well adapted for work on very small colored crystals, as it is difficult to illuminate a small enough area of the slide. The author now uses either an achromatic condenser or a Pmm. achromatic objective (N. A. = 0.65) as a condenser, by means of which the image of a distant tungsten arc lamp or ribbon-fllament lamp is focused on the crystal under observation. Double reflections from the microscope mirror have been overcome by having i t aluminized. For visual work the dichroism of the crystal is studied by inserting the analyzer in the path of light, no polarizing prism being used. Neither the polarizer nor the analyzer is used for microspectrographic work; instead, the image of the crystal or its conoscopic image is projected on the slit of the microspectrograph, and the Wollaston prism is used to produce two adjacent spectra corresponding to the vibration planes of the crystal. In order to set the crystal so that its vibration planes agree with the vibration planes of the Wollaston prism, the microscope polarizer is temporarily inserted and rotated until it extinguishes one of the images given by the Wollaston prism, and the crystal is set a t extinction. The polarizer is then withdrawn and the spectra are photographed. It was mentioned above that strongly absorbing monoclinic and triclinic crystals may have strong dispersion of the axes of the optical ellipsoid. When this is the case it becomes impossible to make dichroism spectrograms which represent principal absorption spectra. The use of a sodium vapor lamp for the preliminary study of interference figures of colored crystals is strongly recommended, as this greatly simplifies the task of interpreting the usually highly complicated interference figures obtained with white light. Sodium illumination is also of considerable service in studying figures presented by the obtuse bisectrix and normal to the optic axial plane, as even thick crystals give clearly defined “hyperbolic” figures under these conditions. SURFACE COLOR. Colored crystals which possess a strong absorption in some region of the spectrum often have a strong surface color in consequence. If the strong absorption is confined to one vibration direction of the crystal, the selectively reflected light is polarized, and consequently, trichroic crystals may reflect different colored light from different faces and exhibit a “reflection dichroism”. The refractive index of an absorbing crystal is abnormally high on the red side of the absorption band and abnormally low on the blue side. In dichroic crystals these abnormalities in refractive index occur only for the ray which is absorbed, and the other ray does not show them. Consequently, the sign of the birefringence may be reversed twice in the visible spectrum. The color of the surface reflection is on the red side of the absorption band for crystals mounted in air, but on the blue side for crystals mounted in a medium of high refractive index, such as a-iodonaphthalene; consequently, it is necessary to mount crystals for comparison in the same mounting medium. The surface colors of many basic cyanine dyes have been examined with the microspectrograph. Well-formed faces of the crystals were illuminated with a cover-glass illuminator between the objective and crystal. Reflected light from the crystal was analyzed with the Wollaston prism. No sharp maxima or minima were observed. The surface reflection of very small crystals is best studied by means of a prism vertical illuminator equipped with a 2-
March 15, 1941
203
ANALYTICAL EDITION
mm. oil-immersion fluorite objective in a short (metallographic) mount. The objective must be specially selected for freedom from birefringence. An analyzer is used in the microscope for visual examination, and the stage is rotated in order to observe reflection dichroism. For microspectrographic work, the Wollaston prism serves as an analyzer. The vertical illuminator is used without a polarizer for this work. This particular aspect of the optics of strongly absorbing crystals is worthy of further study by the chemical microscopist, as the surface color of such crystals is independent of their thickness.
Acknowledgments I n conclusion the author wishes to thank H. D. Babcock for the gift of a speculum grating, J. L. Houghton and Max Wiedling for valuable assistance in the constmotion of the microspectrograph, Fred Lee for the gift of a specially worked
sapphire disk, and L. G . S. Brooker and Frances M. Hamer for many splendid specimens of strongly absorbing crystals.
Literature Cited
(1) Chamot,
e. M., and Mason, C. W., “Handbook of Chemioal
Microscopy”, 2nd ed.. Vol. I. pp. 183-6, N e w York, John Wiley & Sons, 1938. (2) Jelley, E. E., J . Roy. Mieroscop. Soc., 54, 234 (1934).
(3) Ibid.. 56, 101 (1936). (4) Jelley, E. E., Nature, 136, 335 (1935). (5) Jelley. E. E., Phot. J., 74, 514 (1934). (6) Kuns, A. H., and Spulnik, J., IND. ENQ.Cmaa., Anal. Ed., 8. 485 (1936). (7) Spedding, F. H., J . Chem. Phys., 5, 160 (1937); Phys. Ra., 50. 574 (1936).
(8) Wooster, W. A., “Crystal Physics”, Cambridge, England. Cambridge University Press, 1938. P n s s s ~ before ~ ~ o the Division of Mieraohemistry at the 100th Meeting of the American Chemical Society, Detroit. Mich. Communication 781 from the Kodak Researoh Laboratories.
An Electric Heating Mortar For Use in Carbon and Hydrogen Microcombustions G. FREDERICK SMITH AND WM. H. TAYWR
University of Illinois, Urbana, Ill.
T
H E heating mortar usually employed in carbon and hydrogen microcombustion analyses has not proved entirely satisfactory, as this glass heating device containing boiling cymene with an air-cooled reflux condenser and gas microburner is both fragile and cumbersome. Another disadvantage is insufficient variability of temperature adjustment. This type of heating mortar has been improved upon by Schneider and Van Mater (2), who use electrical heating with thermostatic control. A second improved electrically heated and thermostatically controlled heating mortar, known as the “micro thermostatic sleeve”, is now commercially availahle (f). The present discussion bas for its object the description of an electric heating mortar, thermostatically controlled, which is compact, simple in construction and operation, and
FIQUEE1. HEATING MORTAB
at the same time provides constant temperature control over a comparatively wide range of temperatures. For the most part standard units of laboratory equipment are used. The construction of the remaining parts involves simple machine
tool manipulations. InFimre 1theheatinemortarwith thermometerwell.thermam-
bridae. Mass.). shown at D, may be replaced by
a suitable lamp
The mart&-sectioned diagram (Firmre 2) shows details of the
The b&d of the h e a t h mortar is gem. long with the screw w p .&own at The It,fr full\ extended nnd is 48 mm. in outside cli3mrter. The 1,oIe nt e k l i end nnd through thr griphire disk* nre II mm. in hruerrr. Tlie rleetrienl rontdctd for ntruciinreiit of the thermostatic control switch are 4.5 mm. in diameter and extend approximately 15 nun.from the body of the heating mortar, with a 1-cm. space between. The casing of the heating mortar serves as one electrical contact, and the brass shoulder at the left of the Transite disk insulating ring at the exit end serves a8 the other electric contact. The side N& of the barrel of the heating mortar are insulated by use of a Transite tube &s shown. The electrical circuit is thus through the easing (using the right-hand lug which is insulated from the casing) into the resistors and out through the right-hand “Anyheat control” lug. (According to the assembly as described, if the electrical contacts of B are not checked a short circuit may n m l t between the operator and a ground connection. This point should be checked, using a voltmeter, and reversing the contacts either at the power supply line or the thermostatic control switch from the transformer, so that no short circuit will he possible.) The carbon disks (Figure 2) are 37.5 mm. in diameter and 1.5 nun. thick (Allen-Bradley Co., Milwaukee, Wis., Type E2910 rheostat uaphite disks). A sufficient number are slotted as shown, to-provide for an opening below the thermometer tube. The Nichrome resi8tors we 37.5 mm. in diameter and are made from 3-nun. wire cut from asuitable wound helix.
