V O L U M E 25, NO. 3, M A R C H 1 9 5 3
457
One other isomeric series was investigated for the change in the interplanar spacings with ring position, the dimethylpyridine picrate series. No conclusion could be drawn from this series, since there is a lack of similarity in the powder patterns, indicating that there is a change in crystal structure within the series. It is interesting to note, however, that the 2,3-dimethyl- and 2,6dimethylpyridine picrates appear to have one structure, while the 2,4-dimethyl- and 2,5-dimethylpyridine picrates appear to have a different structure. The change in the interplanar spacings was also investigated for a homologous series in the 2-position. In this series the side chain increases from methyl to n-butyl:
monoalkylpyridines and the picrate salts (Table I ) for this investigation. LITERATURE CITED
Brown, H. C., and Slurphy, W.8., J . Am. Chem. Soc., 73, 3308 (1951).
Bunn, C. W., “Chemical Crystallography,” Oxford, Clarendon Press, 1946. Clarke, G. L., Kaye, W.I., and Parks, T. D., IND.ENG.CHEM., AN.ir,. ED.,15, 310 (1946).
Hackmann, J., and Kibaut, J. P., Rec. trav. chim., 62, 229 (1943).
Jam, G. J., Rensselaer Polytech. Inst. Bull. (unpublished). Krishnamurti, P., Indian J . Phys., 2, 355 (1928). Loffler, K., and Rocker, P., Ber., 40, 1318 (1907). McKinley, J. B., Nickels, J. E., and Sidhu, S. S., IND. EKG. CHEM.,ANAL.ED., 16, 303 (1944). (9) hfalkin, T., and Tranter, T. C., J . Chem. SOC.,1178 (1951).
CH2CH2CH2CHa (10) Matthew, F. W., and Michell, J. H., IND.ENG.CHEM., Figure 2 shows a graph of the change in interplanar spacing (first strong line) as the number of carbon atoms in the side chain increases. It is seen that the interplanar spacing increases in a regular fashion as the number of carbon atoms in the side chain increases. ACKNOW LEDG3IEYT
wish to thank H. c. Bromn of Purdue LTniversitJ, The for making available the information on convenient synthesis of
ASAL. ED., 18, 1362 (1946). (11) Matthews, F. W., Warren, G. G., and hfichell, J. H , ANAL. CHEM.,22, 514 (1950). (12) Soldate, A. S I ,and iioyes, R. bI., Ibid., 19,442 (1947). (13) stewart, G. n-,, phys. 33, 889 (1929) (14) Triebs, A., Dornberger, F., Schroder, C. G., iilbrecht, A , Reinecke, H., and Emmerich, H., Ann., 524, 285 (1936). (15) Ziegler, K., and Zeiser, H., Ber., 63, 1847 (1930). RECEIVED for review August 18, 1952. Accepted November 17, 1952. Abstracted in part from t h e thesis submitted by R. Solomon in partial fulfillment of the requirements for the degree of bachelor of science in chemistry a t Rensselaer Polytechnic Institute, ne 1952.
Variable-length Cell Compartment for Beckman DU Spectrophotometer LOUIS P. CECCIIINI Division of Chemistry. Sacul Medical Research Institute, Rhtional . Y a d .Medical Center, Bethesda, M d . A variable-length cell compartment was constructed to permit spectrophotometric analysis of solutions in which the concentration of the absorbing component is too low for detection or identification by ordinary spectrophotometric techniques. Such a cell compartment for the Beckman spectrophotometer and a modification of the 10-cm. Hilger variablelength cell for variable lengths up to 20 cm. are described. The necessity for using matched cells of long cell path is obviated by standardization against an air path. Spectral transmittancy at fixed sensitivity with reproducible slit widths is obtained by
T
HE concentration of an absorbing component in a solution may sometimes be too low to permit measurement by ordinary spectrophotometric techniques in which conventional cells of 1-em. path length are used. Kevertheless, spectrophotcmetric analysis may be possible under such conditions, provided the volume of sample available is not limited to a few milliliters. The analysis may then be performed by extending the length of the cell path by means of the variable-length cell and cell compartment described. Such an extension of path length may also be helpful in studying the kinetics of reactions in very dilute solutions wherein association phenomena are less prone to occur. APPARATUS
For the purpose of investigating the parameter of cell path length, a Compartment of variable length to accommodate a
the use of a single cell essentially by employing a “memory-slit” technique. Spectra of 2 X 10-6 .kf aqueous solutions of potassium permanganate obtained with 1-, lo-, and 20-cm. lengths of the absorbing path are given. The absorption bands observed are in agreement with those reported by other investigators. The variation of absorbancy at wave lengths 525 and 535 mw with cell lengths up to 20 cm., for a 2 x 10-8 M potassium permanganate solution, is linear. The variable-length cell compartment facilitates use of long cell paths as a parameter in spectrophotometric analyses of very dilute solutions. IIilger variable-length Baly-type cell, for use with the Beckman hIodel DU spectrophotometer, has been constructed as described below. This action was taken in the interest of economy, and as an alternative to maintaining a ready supply of cells and compartments of different fixed lengths. The compartment is made from 1/4 X 2 X 12 inch metal (Dural) strips, and machined as shown in Figure 1 t o form interlocking sections, A . These are suspended from the two top met71 (brass) rods, B, and aligned by these and the two bottom rods, B , The rods are cut a t 5/32 inch in diameter and 18 inches long. thread. These rods replace the four s u p each end for an porting the original cell and phototube compartments. After removal of the cell assembly supplied with the instrument, the long rods are screwed into the tapped holes, H , present in the monochromator block, M , passed through the orifices in the photocell unit, P , and aligned a t the other end by four 6/spinch holes, H’, drilled through a 0.5 X 5.5 X 6.5 inch metal (Dural) plate, E,
ANALYTICAL CHEMISTRY
458
TOP VIEW FULL 9Cu.E
K
I?
Y
C
I II
n
a I
/
/
SIDE VIEW
Figure 1.
Construction Diagram of Variable-Length Cell Compartment for Beckman Spectrophotometer
that roll$ freely on tn-o fiber wheels, TI-, ivhich n w e turned down from X 2 inch diameter fiber-ball-bearing pulleys t o 1/4 X 1 inch diameter. E is tapped for a 3/4-inch KO. 10 screw 4.5 inches long surmounted by a knurled knob, C, 0.5 X 3 inches in diameter. Tenqion for locking the cell assembly is applied by rotating ion forces the screw against the detent in a 0.5 X 5.5 X 0 inch plate, D.which is slightl- recessed t,o prevent any damage t o the photocell unit. At the sanie time, E is forced against the four knurled nuts, S ( 3 1 8 X 1 i y h diameter), th:Lt serve as stops a t the ends of the rods! B and B , To shorten t,he cell compartment from its maximum length, \\-hich is determined by the lengt,h of the cables attached to the phototube housing and optical considerations, the knurled knob is turned sufficiently t o move the phot,ocell compartment and plate, D! inch toward the end plate, E . An interlocking section, A , ma)- then be easily disengaged and removed by gently pushing it ton-xrd the end plate and lifting it above the two supporting rods. The resultant slack of the shortened cell compartment is taken up by rotation of the knurled knob. If more than one section is reinch in outside diameter, moved, four bushings, K (brass), 2 X must, be inserted between the end plate and the knurled nuts for each additional section removed. Light-tight interlocking covers are provided for each section (Figure 2 ) . The handles, H , for the covers are so designed that any nunihcr can be manipulated as a unit, by merely sliding a splitring cylinder, C, of the appropriat,elength over the handles. I,
01
,
,
,
:
1
#
I1
~
2
FULL SCALE
S
m
Figure 2. Construction Diagram of Cover Sections for 1-ariable-LengthCell Compartment
V O L U M E 25, NO. 3, M A R C H 1 9 5 3
459
END VIEW
Cell Compartment in 1 liter of distilled xater. This reagent, studied in the spectral region of TOO to 400 mp, was chosen because it is an important and widely used reagent, it is easily and accurately prepared, its spect,rum is well kn0n.n and eshibits both general and specific absorptions ( 2 , 4), its aqueous solutions are colored in concentrations as lo^ as 1 x 10-8 AI, and the complesity of its spectrum increases with dilution ( 2 , 4 ) . Figure 4 (curve A) diows the spectrum ohtained \vith a Beckman spectrophotometer (Model DC) for a 2 X .1( potassium permanganate solution, whpn the converitioiul 1-(~in.matched cells and standard technique are used.
Figure 4. .iqueous
\-ariation of Absorption Spectruni of 2 X 10-6-M Potassium Permanganate Solution with Cell Path Length A for 1 cm., B for 10 cm., C for 20 c m .
To permit the use of cells of any combination of varying cliameters with variable lengths, some means of aligning the cell in the compartment is essential. This is accomplished by the use of t n o removable V blocks, V , adjustable for height by sciews, as shown in Figures 1 and 3 . The V blocks may be placed a t various positions in the compartment to acrommodate cells of different lengths. .ill reflecting surfaces are coated \+ith Kodas brushing lacquer No. 4, dull blark, t o eliminate the effects of stray light. As the cell can be convenientlv and rapidly prealigned exteinally prior t o placement in the cell compartment, a jig has been devised. It consists of two cell sections, identical Fyith those. -4. indicated in Figure 1, made t o slide on a support of two plates 0.5 X 3.5 X 12 inches (Figure 3) for accommodation to various cell lengths. Fastened t o one end of this support, S , and to the distal end of the second cell section are plates, P , n i t h apertures corresponding accurately to the po)sitions of the openings in the exit window and photocell compartment. By sighting through the apertures, A , in the plates, the optical cell, 0, can be raised or lonered on the F' blocks until the plane of its horizontal axis iq in line n ith the centers of the plate apertures, *$, E.XPERI.MEhTA L RESULTS
-Ifresh, aqueous stock solution of 0.01 M potassium pernianganate (Baker's analyzed) waq prepared hy dissolving 1.5803 grams
The variable-length cell compartment ivas attached t o the Beckman instrument and a, 10-cm. variable-length Hilger cell WBS filled with distilled water anti aligned as described above. The transmitt,ance values for the distilled water blank ?IJOiv (!) were obt,ained by standardizing the instrument against an air path. The Beckman spectrophotonieter used must be equipped with a quartz lens for reducing the angular spread of the exit lieam from the monochromator. Instruments with serial numl-iers heloxS o . 446 have not been so equipped ( I ) . The values for the sample solution Ta0lnwere ohtnined 13y placing the 2 x 10-6 .I1potassium permanganate solution in the identical 10-em. celJ used t o obtain the values for the l'eoivr and also standardizing agaiwt an air blank. The percentage transmittancy, 100 T 8 ,plotted in Figure 4 (curves B and f ) was obtained by calculating the ratio
Td"
7 X
100 a t each w v e Iriigth.
fsdv
This procedure eliminated the necessitJ- of using matc,lied cell+. For greater convenience and speed in analysis, and to minimize handling of the variable-length cell, a "memory-slit" tech,nique \\-as successfully employed. Essentially, the technique consists of determining the slit schedule necessary t o obtain T,,i, a t fixed inst,rument sensitivity for all the wave lengths of interest. This slit, schedule is then reproduced t o obtain T,,!, at the dame \\-ave lengths.
The ratio
TBd" X 100 is calculated at each wave
I
fsolv
length. This procedure also eliminates the neceseitJ- of using matched cells. However, the technique is not recommended for general use, as its success depends upon the assurance that by allowing an adequate warm-up period for the lamp and instrument, the direct-current drift is minimized. Although variations of this technique have also been successfully employed elsewhere, usually in conjunction with automatic control and recording. nevertheless: in the absence of the necessary statistical data on the precision so obtainable, it is best to avoid possibledifficultiesb?. employing the more conventional air path standardization described above. The spectrum sho\\-n in Figure 4 (curve (') was ohtained hy placing the 2 x 10-6 J f potassium permanganate solution in a cell extentled t o 20 cni. and folloning the procedure described
460
ANALYTICAL CHEMISTRY
above. The 10-em. variable-length Hilger cell was modified by cutting the shoulder off the plunger, reversing the position of plunger in the barrel, and calibrating the cell for 20 em. A rubber sleeve was placed over the barrel and plunger interface t o prevent seepage and to ensure constancy of path length. The calibration was accomplished by carefully inserting a depth gage into the hollow plunger cylinder, noting the distance between the squared cut end and the window surface. One hundred millimeters were subtracted from this value and the resulting distance measured from the squared edge toward the window represented the point at which the 100-mm. graduation was scribed on the exterior surface of the plunger. When the plunger was placed in the barrel and the 100-mm. graduation lines on both parts coincided, the path length of the cell was 200 mm. The curves of Figure 4 show the four familiar points of inflection characteristic of absorption spectra of potassium permanganate solutions. They appear as a shoulder at.568 mp, a well resolved absorption a t 545 mg, the absorption maximum a t 523 mp, and another shoulder a t 508 mp. In addition to these specific absorptions, curve B shows three others superimposed on the general absorption curve. They appear as shoulders a t 490, 473, and 457 mp. Curve C has a band shoulder a t 439 mp. The absorption characteristics noted above agree with those reported by Hagenbach and Percy (2). In accordance with Mellon’s (3) terminology, the absorbancy, A,, is proportional to the length of the absorbing path, b, according to the expression
I
000
Figure 5 .
IO CELL PPW-LE‘IGTH l b l IN CM
20
J
Plot of Absorbancy versus Cell Path Length
2 X 10-8 M aqueous K;\InO4 solution a t wave lengths 525 a n d 535 mp
(us, the slope of the lines in Figure 5 ) is constant to within 1% for the plotted points a t path lengths ( b ) between 1 and 20 cm. ACKNOWLEDGMENT
where c is the concentration and a, is the absorbancy index. Therefore, a t a given concentration and wave length, change of A , with b should be linear. When variable-length cells are employed, a plot of absorbancy versus cell length must be determined if the data subsequently gathered are to be used with confidence. Any departure from linearity may be an indication of unsatisfactory optical alignment, stray or scattered light, and losses of light resulting from beam divergence and/or reflection. Figure 5 shows such a relationship at the wave lengths 525 and 535 mp, corresponding to the points of inflection, for ( A ) maximum absorption, and for ( B ) maximum transmittance between the peaks 525 and 545 mp. The value of the absorbancy index
The author is greatly indebted to John F. Bronson, head of the Metal Model Shop. for his many valuable suggestions, and to Walter Paris, toolmaker, for his excellent workmanship. LITERATURE CITED
(1) Beckman Instruments, Inc., South Pasadena, Calif., il’atl. Tech Bull. 91c, 8. ( 2 ) Hagenbach, A., and Percy, R., Helv. Chim. Acta, 5 , 454 (1922). (3) Mellon, M. G., editor, “Analytical Absorption Spectroscopy,” New York, John Wiley Br Sons, 1950. (4) Sundara Rao, 8 . L., Current Sci., 6 , 154 (1937). RECEIVED for review Rlarch 26, 1952. Accepted December 12, 1932. T h e opinions expressed are the author’s and do not necessarily reflect those of t h e N a v y Department.
Modification of Atlas Twin-Arc Weather-Ometer J. W. ThAIBLYN AND G. ht. ARMSTRONG Research Laburatories, Tennessee Eastman Co., Division of Eastman Kodak Co., Icingsport, Tenn. IT11 the development of more durahle p1,istic materids for outdoor applications, the need for reluble accelerated weathering tests becomes more urgent. The shortcomings of the tests in use a t present are generally recognized by those working in this field. Hendricks and White (3),for example, stated recently that in the case of polyvinyl chloride-type plastics the modern carbon-arc machines show “a dangerously low degree of correlation with actual sunshine esposure.” Various attempts to improve this situation are under way in a number of laboratories. This paper is an account of the preliminary stages of one such attempt. In the present state of our knowledge about accelerated exposure testing, it must be generdly agreed that to predict an outdoor lifetime of 1.50 )-ears for a plastic product on the basis of the results of several hundred hours of exposure in a Fade-Ometer requires a remarkable degree of boldness. The Type DL-T.S Atlas Triin-.irc Weather-Ometer, which was available for this work, is a carbon-arc machine in which the arcs are enclosed by borosilicate glass globes. This is a service-
able machine, giving a highly reliable mechanical performance and operating on a convenient 24-hour cycle. The spectral distribution of its emitted radiation has been measured by Lozier, Xull, and Bowditch ( 5 ) and is shown in Figure 1 in comparison with noontime summer sunlight on a clear day. According t o these measurements the radiation in the shorter ultraviolet is lower than in sunlight. This is shown very strikingly by measurements made a t the sample position on the Weather-Ometer drum and under the midday summer sun by means of a Westinghouse ultraviolet meter 531-200, whose sensitivity begins a t 370 mp and increases fairly uniformly to a maximum a t 250 mp. The ultraviolet count, determined with this instrument, !+-as only one fifth as great in the Weather-Ometer as outdoors a t noon on a clear day in midsummer. To increase the output of shorter ultraviolet, eight Westinghouse 20-watt fluorescent sun lamps were installed in the WeatherOnieter. The distribution of radiant energy emitted by these lamps, measured by Stair ( 6 ) , is also shown in Figure 1. T h e energy scale was adjusted for this curve to make it approximately the same as the scale used by Lozier, Null, and Born-ditch (6). With this arrangement, the ultraviolet count at the test specimens was raised above that of noon June sunlight a t the point of closest
