A Neutral Wedge Abridged Spectrophotometer PAUL A. CLIFFORD AR’D BROOKS .A. BRICE Food and Drug Administration, U. S. Department of Agriculture, Washington, D. C.
The apparatus described consists of a photometer with a permanent wedge of Jena neutral glass, a tungsten source, and a set of twelve permanent glass filters of relatively high monochromaticity. The glasses composing the filters are listed, and the assembled filters are described by means of spectral transmission curves, and values for luminous transmission, spectral centroid, and monochromaticity. Used with a selected filter and absorption cell of appropriate length, the instrument pro-
vides a convenient means of determining the concentration of most colored solutions by reference to simple standard curves. Wedge readings are proportional to density for all filters and hence proportional to concentrations for a solute obeying Beer’s law. If desired, abridged absorption curves for a solution can be determined by using all the filters and converting scale readings to density values. Yarious applications are mentioned.
S
PECTROPHOTORIETRE’ is finding increasingly wide application in the field of chemical analysis (25). Abridged methods are widely used because they require less elaborate equipment, although they do not usually attain the sensitivity, Tersatility, and fundamental quality of measurement possible with a spectrophotometer. The abridged spectrophotometer is used as an analytical instrument by referring absorption values of unknown colored solutions to the values obtained under standard conditions with known amounts of the pure substance to be determined. The light is usually selected by monochromatic filters, and if the instrument is equipped with a sufficient number of these filters spaced throughout the spectrum, a great variety of “colorimetric” chemical analyses can be handled, and, in addition, useful abridged absorption curves can be obtained. [Various types of abridged spectrophotometers are: variable aperture type (18, 36), polarization type (28), variable depth of solution type (4, 32), and photoelectric w t h a series of filters (3, IO, 24,30).I The use of a neutral wedge in photometric apparatus is not new (6, 11, 19, 29). The neutral wedge visual abridged spectrophotometer discussed in the present paper is an improved form of that previously described by Clifford and Wichmann ( 7 ) . The present authors’ use of the neutral wedge principle had its inception in an instrument devised by Hardy and Pfund (15) for the Food and Drug Administration for grading rosin, consiqting of two photographic film printq of neutral
77
A
wedges, three monochromatic filters, and a broken plate-glass photometer head (27). While no fundamentally new principles are presented, the carefully selected monochromatic filters, the improved design of the instrument, and its satisfying performance are considered n-ort’hy of description.
Design of Instrument The complete instrument comprises a prism assembly, an eyepiece, two glass neutral \?edges, an incandescent light source, twelve monochromatic filters, and four absorption cells for holding solutions. The optical system is shown diagrammatically in Figure 1. The photometer head containing the rhombs, biprism, and eyepiece, is that used in the Bausch & Lomb Optical Company’s hemoglobinometer. The photometricofield is the simple comparison type with a diameter of about 10 . The filters are mounted in readily interchangeable slotted metal plugs which slip in fixed position into the exit end of the eyepiece, each plug having its own peephole. They thus receive negligible heat from the light source. Illumination is provided by a 115volt, 100-watt projection lamp at the center of curvature of a spherical reflector and at the focus of a condensing lens system. Ground surfaces on the diaphragmed Kindows, DD‘, effect a uniform photometric field but with some sacrifice in intensity. However, with this arrangement the position of the lamp filament is not highly critical. The two beams are practically parallel light. Pyrex absorpt’ion cells having plane-parallel fused-on windows, inside diameters of 15 mm., and lengths of 12, 25, 50, and 100 mm., fit in a metal V-trough in the sample beam. The wedge finally ado ted vas made of Jena KG 5 “light neutral” glass. This was t i e most satisfactory wedge material investigated, as it possesses the desired qualities of permanence. satisfactorv neutralitv throughout the Gisible spectrum, negligible temperature coefficient, and linearity with d e n s i t y . Eastman gelatin xvedges showed too much .change in t r a n s m i s s i o n characteristics with age. Kedges of Bausch & Lomb I “Smoke C” glass showed serious departure from neutrality at the violet and red ends of the spectrum, giving FIGURE1. DIAGRAX OF OPTICAL SYSTEM rise to hue differences in the d. 100-watt 110-volt projectiiin type lamp two halves of t h e photoB. Spherical reflector metric field and nonlinear CC‘. Lens, lS-cm. focal l e n g t h DD’. Frosted nindows standard curve with certain E. Cell yellow and blue solutions. F . Pliotometdc wedge The wedge used is 150 mm. G. Compensating u e d g e H . Filter m o u n t long, 8.5 mm. thick at one
-v --I-+--
718
APRIL 15, 1940
ANALYTICAL EDITION
219 IO0
0
0.4
0.8
gw
IO
0
a
+ 1.2
W
u
a
s I
>
6
1.6
2
w n 2 .o
2.4
2.8 0 .I 400
I
440
I
400
I
I 520
FIL;TER
560
NO.
1
600
I 640
I
600
720
I
760
FIGERE2 . SPECTRAL T R A S ~ M I ~C~LI- O R ~SE S end, and less than 0.5 mm. at the other, 11-ith a density range of approximately 0 to 2. Deviation of light rays is eliminated by an equal wedge of clear glass cemented to the gray 11-edge. The wedge is movable laterally in its beam by means of a rack and pinion, and settings are read with estimation to 0.1 mm. on a millimeter scale with index and magnifier. P o compensate for the density gradient of the wedge in the field of view, a short second n-edge, identical with the first 25 mm. of the large ivedge, is mounted in the sample beam with its gradient in the same direction as the long wedge. This effectively equalizes the appearance of the two halves of the photometric field xithout incurring the loss in brightness which \Todd result if the compensating wedge were placed in the same beam as the long wedge. The small Iv-edge is movable laterally t o provide a convenient scale zero adjustment.
JIonochromatic Filters The filters described here embody the following desirable characteristics: They are composed of stock colored glasses to ensure permanence and reasonable reproducibility; they are sufficiently monochromatic practically to eliminat'e any hue difference in the two halves of the photometric field when 3 colored solution is placed in one beam; they transmit enough light to g i w adequate field brightness for routine laboratory n-ork without resorting to a lamp of inconveniently high n-attage; and the series covers the spectrum in twelve roughly equal steps without serious overlapping of transmission ranges. Considerable care in the selection of the glasses and the adjustment of their thicknesses was necessary in const'ructing the filters. TS'herel-er possible, use n-as made of glasses having sharp spectral &off or sharp absorption bands. The glasses composing the filters are specified in Table I. Melt numbers were not available on most of the glasses used. In attempting t'o duplicate these filters it must be borne in mind that color variations exist between melts for all types of glass, and even within a given melt for some types.
All components lvere cemented together with Canada balsam. Certain components, such as Jena YG 3, Corning 503, and Jena BG 18, subject to surface deterioration by moisture, were cemented on the inside of a filter combination or protected by a cemented disk of clear glass. The filters can be depended upon to be permanent to a high degree. Spectral transmission curves, plotted on a logarithmic scale, are shonm in Figure 2. The measurements on the assembled filters were made on a photoelectric spectrophotometer, using slit widths of from 1 to 4 mp. Filters 42 and 46 have a very slight transmission for red light, and S o . 51 has a slight transmission near 560 mp, too small to appear in the curves. Care was taken to reduce such extraneous light transmission to a minimum for all combinations. All the filters transmit some infrared radiation. A monochromatic filter is specified by three quantities: (1) spectral centroid, expressing its effective wave length; ( 2 ) luminous transmission, expressing its brightness; and ( 3 ) monochromaticity, expressing the effectiveness of the filter for isolating wave lengths near the centroid. These quantities for the twelve filters are given in Table 11. All data refer to the 1931 I. C. I. standard observer (20)and to an illuminant of color temperature 3000" K. (estimated for the source used). Spectral centroids and luminous transmission m r e calculated from summations a t 5 mp intervals in the usual way (13). As a measure of monochromaticity the authors have chosen to use an expression .If = Xc f AX E V T d h / J m E'BTdX
1 A,-
AX
ivliere X, is the spectral centroid of the filter, AA an arbitrarily chosen wal-e-length interval, E the spchctral distribu-
INDUSTRIAL AND ENGINEERING CHEMISTRY
220
VOI,. 12, NO. 4
TABLEI. COMPOSITION OF MONOCHRONATIC FILTERS [Filter No. corresponds approximately with spectral centroid of combination. Glasses are Corning Glass Works unless otherwise indicated. Sharp cutoff glasses are specified by wave-length positions of 5 and 40 per cent transmission (8).] Filter Filter No. Manufacturer’s Designation Thickness Note No. Manufacturer’s Designation Thickness Note Mnk. Mm. 42 No. 511 Violet 2.52 Devised b y R. L-.Bon56 No. 512 Didymium 5.84 Adapted from filter de587 Red purple ultra 2,OO nar. this laboratory. Jena VG 3 1.96 vised by Gibson (15) Dim, with rather 430 Dark shade blue-green 3,50 b u t modified t o repoor cutoff in blue. 350 Traffic shade yellow , , . duoe its luminous 44 511 Violet 5.01 1530. 541 mu) .~ transmission and slizht extraneous 2.54 511 Violet 46 tr&mission which 038 Noviol A (430,436mp) Gibson notes i n green and orange. 58 Jena VG 3 3.95 1.00 This glass has a steeper Jena BG 18 511 Violet 2 08 49 348 Red shade yellow 6.50 iong wave-length 430 Dark shade blue-green ,... (572,581 m d slope in this region 338 Noviol C (468.482 mp) .... t h a n other glasses 61 Jena BG 18 6.14 This glass has a steeper examined. 246 Lighthouse red long wave-length Jena VG 3 2.00 This filter uses a sharp (592,600 mp) 51 slope in this region 5.95 transmission band in 503 Dark theatre blue than other glasses 430 Dark nhade blue-green 7.20 t h e VG3 glass. examined. 338 Noviol C (480,492 m p ) .... 65 Jena BG 18 2.14 241 Dark pyrometer red 4.00 A difficult region t o 503 Dark theatre blue 54 (633,645 mu) isolate sharply. The 3.58 430 Dark shade blue-green 612 Didymium didymium glasa as2.00 68 Jena RG 5 3.80 sista in out05 on 351 Yellow shade yellow .... Jena BG 18 0.70 long wave-length (520,530 mp) 2.17 72 Jena RG 9 side.
.
....
....
....
TABLE11.
?VlONOCHROhfATIC FILTERS
Description of monochromatic filters in terms of spectral centroid, L, uminous transmission, T, and monochromatmty, M . Values for a are factori for converting wedge scale readings (mm.) t o density.] Filter No. xc T M a
I
42 44 46 49 51 54 56 58 61 65 68 72
mp
%
425 444 458 488 514 539 559 579 606 646 677 720
0.001 0.035 0,099 0.077 0.052 0.137 0.986 0.229 0.240 0.078 0.091 0.037
0.28 0.26 0.30 0.30 0.50 0.36 0.69 0.61 0.42 0.43 0.35 0.22
0,01842 0.01749 0.01629 0.01537 0.01535 0.01482 0.01463 0.01472 0.01465 0.01433 0,01346 0.01279
tion of the illuminant, V the relative luminosity values for the I. C. I. standard observer, and T the spectral transmission of the filter. This expression is similar to that used by Staats (91) for expressing the purity of mercury lamp monochromats. With Ah taken as 5 , as in the present case, the expression thus represents for a filter the fraction of its total luminosity lying within the wave-length interval h, - 5 to h, 5 mp. For a monochromator with slits 10 mp wide, M would be unity. Values of M for each filter were determined by plotting luminosity curves on uniform cross-section paper and cutting out and weighing the indicated areas. Adequate field brightness for routine work under conditions of subdued general illumination is obtained with all the filters except the violet filter, No. 42. When using this filter, the observer should work in a dark room. Although the spectral centroids given in Table I1 refer t o the standard observer, calculations show that they are valid within sufficiently close limits for any “normal” observer. Applying Gibson and Tyndall’s data (14) for the average and extreme luminosity curves for 52 normal observers t o filter 61, the calculated centroid was 606.2 m p for the average, 605.8 mp for the lowest, and 607.1 mp for the highest luminosity curve in this region.
+
Calibration of Wedge Scale The relation between scale reading and density is derived by determining wedge settings for a series of known densities placed in the sample beam. Known density values of high accuracy between 0 and 1.0 were furnished by a set of fixed sector disks rotated successively in the sample beam. I n the
density range 1.0 to 2.0 the sectors were supplemented by a plate of NG 5 glass having a density near 1.0 and accurately measured in the instrument for each monochromatic filter in terms of the rotating sectors. As the wedge is not strictly neutral, calibration with each of the twelve filters was necessary. A linear relationship between scale readings, S,and density, D(can be expressed:
D
= a(S
- SO)
(1)
where SOis the scale reading with the sample beam unobstructed, or if Sois made equal to zero‘
D
= aS
(2)
The latter condition (Equation 2 ) can be realized by adjusting the compensating wedge. The wedge scale was found to be accurately linear with each of the twelve filters. The conversion factors, a, for this wedge for each filter are given in Table 11.
Use of Instrument I n ordinary “colorimetric” chemical analysis conversion of scale units to densities may not be necessary, and the operator works from a standard curve prepared under standard conditions with the pure substance to be determined. For maximum sensitivity the filter chosen has its highest transmission in the spectral region where the substance under examination exhibits maximum absorption. If a spectrophotometric curve is not available, this selection may be made by noting the filter which gives the greatest difference in reading between two concentrations of the test solution. Considerable opportunity is offered for proper selection among the 12 filters listed. Choice of standard volume and cell length controls the concentration range to be covered. Standard curves are permanent, and for a fixed chemical system the making up and reading of standard solutions need be carried out only once. Scale reading may be adjusted by means of the compensating wedge to allow for a slight residual color or turbidity, and a standard test glass may be used from time to time to check the wedge adjustment. I n abridged spectrophotometric work, where it is desired to express results in terms of fundamental quantities, density (-log transmission) of a specimen is derived from Equation 1 or 2, using the appropriate conversion factor for the filter used. The main wedge is first made to balance a t or near
ANALYTlCAL EDITION
APRIL 15, 1940
7uu FIGURE 3. TRANSMISSION CURVES aero, with nothing in the specimen beam, by means of the compensating wedge. The absorbency (-log transmittancy) of a solution is derived in a similar n-ay with the main wedge balanced a t or near zero, with cell and solvent in place in the beam. If desired, the approximate specific absorptive index, E (extinction coefficient), may be calculated from the relation: E
u(S - So)/bc
(3)
where b = solution depth in cm., and c = concentration of solute in grams per 100 ml. of solution. Since the solute normally obeys Beer’s law and the filters are monochromatic, curves correlating concentrations and scale readings are, in practice, usually linear. [From Equation 3: c = a(S - So)/bE, and if b is held constant: c = F ( S - So).] Thus a simple factor can be derived to express concentrations in terms of scale reading. The precision of any such instrument is adversely affected by low field brightness, the presence of a hue difference in the two halves of the photometric field, and incomplete disappearance of the photometric field dividing line. I n the present case low field brightnesses are encountered with specimens of high density with the violet filter and some of the other filters. Hue differences have not been found serious in using the filters described in routine chemical analysis, and the optical design is such that with a properly focused e>rep’iece, the dividing line, a t balance, is nearly invisible. The reproducibility of measurements was tested by making 30 wedge settings on each of two neutral glasses having densities of near 0.6 and 1.8, using filter 51 (one of the dimmer filters). Standard deviations were 0.0032 and 0.0074 density unit, respectively, or about 0.8 and 2.0 per cent of the transmission values. I n most cases the average of five settings will give a sufficiently precise figure. Very few chemical systems have been investigated which demand a higher precision than that attainable with the instrument.
221
Transmission values lying roughly within the limits 30 t o 3 per cent (densities of about 0.5 to 1.5) can be measured on the instrument with best accuracy. These values correspond t o wedge settings of about 30 to 100 mm. For transmission values above 30 per cent the experimental error of wedge setting becomes an increasingly large proportion of the total wedge displacement. Thus for a transmission value of 90 per cent, the total wedge displacement is only about 3 mm., while the standard deviation of wedge settings is about 0.2 to 0.3 mm. Accuracy of measurement for transmission values below about 3 per cent may fall off because of low field brightness with certain filters. To illustrate the performance of the instrument in furnishing abridged spectral transmission data, scale readings were made on a pair of colored glasses standardized by the National Bureau of Standards for spectral transmission. I n Figure 3 the points shown are measured transmissions, converted from scale readings, plotted against the spectral centroids of the filters. The smooth curves are drawn through the Bureau of Standards data, Deviations from the true spectral transmission curves greater than the experimental error of wedge setting and wedge calibration occur for two reasons: (I) The filters transmit a relatively wide spectral range (analogous to a spectrophotometric slit width error), and (2) the filter centroid is not always the same with and without the specimen in the beam (hue difference error). These deviations will occur where the slope of the transmission curve is not zero or near zero. These samples provide a severe test, and the agreement shown may be considered good for an abridged spectrophotometer. With neutral specimens or with a filter which subtends an essentially flat part of a spectral transmission curve, very close agreement with spectrophotometric data should be expected. I n the authors’ work the instrument has proved superior in sensitivity, accuracy, and convenience to comparators of the Duboscq type in which, as usually employed, chromaticity discrimination and the frequent making up of standard solutions are involved. Though the precision is not so high as with a good photoelectric photometer, it is believed that the instrument described is more sturdy and trouble-free than a photoelectric photometer capable of using the filters listed here. The neutral wedge photometer has been applied in the Department of Agriculture to an increasing number of chemical analytical problems. Since its application to the determination of lead (7, 17) it has been applied to the determination of fluorine (9), carotene (25), mercury (34), indole (6). lactic phenoacid ( I @ , arsenic @ I ) , phosphate ( l a ) , nicotine (W), thiazene (66), methanol ( I ) , color of whisky (b), coumarin (SS), and cyanide (S5). A number of other applications have been brought to the authors’ attention through personal communication and will probably be described later. It would appear that the instrument, with its wide choice of filters and cell lengths, is readily adaptable to most standard colorimetric or turbidimetric chemical determinations.
Acknowledgment Credit is due -4.G. Sterling for machine vork in connection with the development of the instrument.
Literature Cited (1) Beyer, G. F., J . Assoc. 0flczaZ Agr. Chem., 22, 151 (1939). (2) Ibid., 22, 156 (1939). (3) Bolton, E. R., and Williams, K. A.. Analyst, 60, 447 (1935); 62, 3 (1937). (4) Brewster, J. F., J . Research Y a t l . Bur. Standards, 16, 349 (1936). (5) Capstaff, J. G., and Purdy, R. A . , Trans. Soc. Motion Picture Engrs., 11, 607 (1927). (6) Clark, J. O., et al., J . Assoc. Oficial Agr. Chern., 20, 459 (1937).
INDUSTRIAL AND ENGINEERING CHEMISTRY
222
(7) Clifford, P. A , , and TTichmann, H. J., Ibid., 19, 130 (1936). (8) Corning Glass Works Catalog, “Glass Color Filters”. (9) Dahle, D., J . Assoc. Oficial Agr. Chem., 20, 505 (1937). (10) Evelyn, K. A., J . Biol. Chem., 115, 63 (1936); 117, 365 (1937). (11) Exton, IT. G., Proc. SOC.Ezptl. B i d . X e d . , 21, 181 (1924). (12) Gerrite, H. W., J . Assoc. Oficial A g r . Chem., 22, 131 (1939). (13) Gibson, K. S., J . Optical SOC.Bm.. 25, 131 (1935). (14) Gibson, K. S., and Tyndall, E . P. T . , Bur. Standards, Sci. Paper 5475, 1923. (15) Hardy, J. D., and Pfund, A. H., unpublished xork, 1930. (16) Hillig, F., J . Assoc. Oficial AQT.Chem., 20, 130 (1937). (17) Hubhard, D. M., ISD.ENG.CHEJI.,Anal. Ed., 9, 493 (1937). (18) Ives Tint Photometer, Palo Co., Xew York, N. Y. (19) Jones, L. A , J . Optical SOC.Am., 4, 420 (1920). (20) Judd, D. B., Ibid., 23, 359 (1933). (21) Klein, A. K., and Vorhes, F. A., Jr.. J . Assoc. Oficial Agr. Chem , 22, 121 (1939). (22) Markwood, L. K.,Ibid., 22, 427 (1939).
VOL. 12, NO. 4
(23) Mellon, M. G., “Role of Spectrophotometry in Colorimetry” and cited references, IND.EXG.CHEW,Anal. Ed., 9, 51 (1937). (24) Miiller, R . H., Ibid., 7, 223 (1935). (25) Munsey, 1’.E., J . Assoc. OficWl Agr. Chem., 21, 331 (1938). (26) Murray, C. IT., and Ryall, A. L., ISD.EBG. CHEM.,Kews Ed.. 17, 407 (1939). (27) Pfund, A. H . , J . Optical SOC..4m., 18, 167 (1928) : 19,387 (1929). (28) Priest, I. G., J . Research h’atl. Bur. StandaTds, 15, 529 (1935). (29) Shook, G. A , and Scrivener, B. J., Rev. Sci. Instruments, 3, 553 (1932). (30) Singh, B. K.,and Rao, N. K. A., Plant Physiol., 13,419 (1938). (31) Staats, E. M., J . Optical SOC.Am., 28, 112 (1938). (32) Thiel, A., and Thiel. W., Chem. Fabriic, 5, 44 (1932). (33) TYilson, J . B., J . Assoc. Oficial Agr. Chem., 22, 393 (1939). (34) Finkler, W.O., Ibid., 21, 220 (1938). (35) Ibid., 22, 349 (1939). (36) Zeiss Pulfrich Photometer, Mess 431, Carl Zeiss, Inc., New Tork,
s.Y .
Heat Control Units JOHN A. RIDDICIC Research Department, Commercial Solvents Corporation, Terre Haute, Ind.
T
HE most common laboratory methods for controlling
the temperature of heating elements involve the use of polarized rheostats and lamp banks. The field, or point contact, rheostat gives a stepwise change in temperature. The Forsythe, or sliding contact tube, type gives a more uniform temperature change but has the disadvantages of being a hazard when used on 110- or 220-volt circuits, because of exposed live metal parts, and of “sweating” when watercooled. The lamp bank is usually constructed of porcelain receptacles with exposed terminals, and a close control of the heating element is impossible. All three control units generate heat and the lamp bank emits light that is usually unwanted and badly placed. Of the three methods mentioned, the Forsythe rheostat is the most satisfactory, as it occupies a relatively small space and polarized hookups are easier to make.
I
-
,< \L2
T h r e e - P o l e Conveniance Receptacle
FIGURE1. SCHEMATIC WIRINGDIAGRAJI G . Identified or ground nire L. Xeon glow lamp, 1 watt S. Snitch, P & S, Despard V . Variac, Type 200-C
The introduction of the non-vacuum-jacketed fractionating column, such as the Penn State type, into the analytical laboratory necessitated a closer control of heat than was possible with the field rheostat and a safer control than was possible Ivith the Forsythe instrument. The number of heating ele-
ments in the jacket of a fractionating column depends on its length and the use for which it is designed. Since laboratory fractionating units are made of glass, the number of wires in the heated jacket type should be a t a minimum to allow unobstructed vision and placement of thermocouple leads. The minimum number of leads to the heating elements is one more than the number of elements, An ideal heat control would allow its use with any column on the rack. This necessitates a certain amount of standardization in attachment plugs and the control unit. The variable autotransformer has been found to overcome the disadvantages mentioned, and the panel type is suited for incorporation in a control panel of maximum adaptability. There are several such instruments on the market. The T’ariac, Type 2O0-CUJ manufactured by the General Radio Company, Cambridge, Mass., has been used in these laboratories and has given excellent service. For general laboratory work the 860 volt-ampere instrument has proved satisfactory. The three panels described below have been found to meet any need that has arisen in these laboratories since the control panel was developed two years ago. They are constructed to give maximum safety with judicial use, and mag be used as wall panels or set on the laboratory desk top. The first panel developed mas a two-autotransformer unit, the schematic miring diagram of which (front view) is shown in Figure 1. The symbols used are the common conventions except those 8 for the convenience receptacles. The connections to the three-pole convenience outlets in all diagrams are numbered to call attention to the uniform connection of poles with the controls. These positions are determined from the connections of the A center, or common, B three-pole c o n v e n i e n c e FIGURE2. ALTERNATELOAD The left autoPOSITIOSS transformer connects to
- -
‘P’ T
*
