elaborate equipment, although they do not usually attain the sensitivity, versatility, and fundamental quality of measure
m ent possible with a spectrophotometer. The abridged spec
trophotom eter is used as an analytical instrum ent 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 instrum ent 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 (88), variable depth of solution type (4, 32), and photoelectric with a series of filters (3, 10, 24, SO).]
The use of a neutral wedge in photometric apparatus is not new (5 ,1 1 ,1 9 , 29). The neutral wedge visual abridged spec
trophotom eter discussed in the present paper is an improved form of th a t previously described by Clifford and Wichmann (7). The present authors’ use of the neutral wedge principle had its inception in an instrum ent devised by H ardy and Pfund (15) for the Food and Drug Administration for grading rosin, consisting of two photographic film prints of neutral
\ B
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 instrum ent, and its satisfying performance are con
sidered worthy of description.
D e s ig n o f I n s t r u m e n t
The complete instrument comprises a prism assembly, an eye
piece, two glass neutral wedges, 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 eye
piece, is that used in the Bausch & Lomb Optical Company’s nemoglobinometer. The photometric field is the simple compari
son 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 neat from the light source.
Illumination is provided by a 115-volt, 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 dia- phragmed windows, 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 absorption 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 adopted was made of Jena NG 5 “light neu
tral” glass. This was the most satisfactory wedge material in
APRIL 15, 1940 ANALYTICAL EDITION 219
Fi g u r e 2 . Sp e c t r a l Tr a n s m i s s i o n Cu r v e s
end, and less than 0.5 mm. at the other, with 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 wedge. 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.
~To compensate for the density gradient of the wedge in the field of view, a short second wedge, identical with the first 25 mm. of the large wedge, 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 without incurring the loss in brightness which would result if the compensating wedge were placed in the same beam as the long wedge. The small wedge is movable laterally to provide a con
venient scale zero adjustment.
M o n o c h r o m a t ic F ilte r s
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 eliminate any hue difference in the two halves of the photometric field when a colored solution is placed in one beam; they transm it enough light to give adequate field brightness for routine laboratory work w ithout resorting to a lamp of inconveniently high w att
age; and the series covers the spectrum in twelve roughly equal steps w ithout serious overlapping of transmission ranges.
Considerable care in the selection of the glasses and the ad
justm ent of their thicknesses was necessary in constructing the filters. Wherever possible, use was made of glasses having sharp spectral cutoff or sharp absorption bands. The glasses composing the filters are specified in Table I. M elt numbers were not available on most of the glasses used.
In attem pting to duplicate these filters it m ust be borne in mind th a t color variations exist between melts for all types of glass, and even within a given melt for some types.
All components were cemented together w ith Canada balsam. Certain components, such as Jena VG 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 perm anent to a high degree.
Spectral transmission curves, plotted on a logarithmic scale, are shown in Figure 2. The measurements on the as
sembled filters were made on a photoelectric spectropho
tometer, using slit widths of from 1 to 4 m /i. Filters 42 and 46 have a very slight transmission for red light, and No. 51 has a slight transmission near 560 mu, 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 transm it 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 II. All data refer to the 19311. C. I. standard observer (20) and to an illuminant of color tem perature 3000° K. (estimated for the source used). Spectral centroids and luminous transmission w'ere calculated from summations a t 5 m u intervals in the usual way (IS).
As a measure of monochromaticity the authors have chosen to use an expression
r\< + AX / / • »
M = / EVTdX / / EV'I'dX
J \ c- AX / JO
where \ is the spectral centroid of the filter, AX an arbi
trarily chosen wave-length interval, E the spectral
distribu-220 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 12, NO. 4 the I. C. I. standard observer, and T the spectral transmission of the filter. This expression is similar to th a t used by Staats (31) for expressing the purity of mercury lamp monochro- 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. W hen using this filter, the observer should work in a dark room.
Although the spectral centroids given in Table I I refer to the standard observer, calculations show th a t 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 to filter 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. In 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 instrum ent for each monochromatic filter in terms of the rotating sectors. As the wedge is not strictly neu
tral, calibration with each of the twelve filters was necessary.
A linear relationship between scale readings, S, and den
sity, D, can be expressed:
D = a(S — So) (1)
where S 0 is the scale reading with the sample beam unob
structed, or if So is made equal to zero:
D = aS (2)
The latter condition (Equation 2) can be realized by ad
justing the compensating wedge.
The wedge scale was found to be accurately linear w ith each of the twelve filters. The conversion factors, a, for this wedge for each filter are given in Table II.
U s e o f I n s t r u m e n t
In ordinary “colorimetric” chemical analysis conversion of scale units to densities m ay not be necessary, and the opera
tor 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 trans
mission in the spectral region where the substance under ex
amination exhibits maximum absorption. If a spectrophoto- metrie 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. Consider
able opportunity is offered for proper selection among the 12 filters listed. Choice of standard volume and cell length con
trols 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 m ay be adjusted by means of the com
pensating wedge to allow for a slight residual color or turbid
ity, and a standard test glass may be used from time to time to check the wedge adjustm ent.
In abridged spectrophotometric work, where it is desired to express results in terms of fundam ental 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
APRIL 15, 1940 ANALYTICAL EDITION 221
Fi g u r e 3 . Tr a n s m i s s i o n Cu r v e s
zero, w ith nothing in the specimen beam, by means of the compensating wedge. The absorbency ( —log transm ittancy) of a solution is derived in a similar way 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 rela
tion:
E = a(S — So) /be (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 Equa
tion 3 : c — a(S — S 0)/bE, and if b is held constant: c = F (S — So).] T hus a simple factor can be derived to express concentrations in terms of scale reading.
The precision of any such instrum ent is adversely af
fected 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. In the present case low field brightnesses are encountered with speci
mens 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 th a t w ith a properly focused eyepiece, the dividing fine, a t balance, is nearly invisible.
The reproducibility of measurements was tested by making 30 wedge settings on each of two neutral glasses having den
sities 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 trans
mission values. In 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 th a t attainable with the instrument.
Transmission values lying roughly within the limits 30 to 3 per cent (densities of about 0.5 to 1.5) can be measured on the instrum ent with best accuracy. These values correspond to 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 bright
ness w ith certain filters.
To illustrate the performance of the instrum ent in furnish
ing abridged spectral transmission data, scale readings were made on a pair of colored glasses standardized by the National Bureau of Standards for spectral transmission. In 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 trans
mission curves greater than the experimental error of wedge setting and wedge calibration occur for two reasons: (1) The filters transm it a relatively wide spectral range (analogous to a spectrophotometric slit w idth error), and (2) the filter cen
troid is not always the same with and w ithout 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. W ith neutral specimens or w ith a filter which subtends an essentially flat part of a spectral transmis
sion curve, very close agreement w ith spectrophotometric data should be expected.
In the authors’ work the instrum ent 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 solu
tions are involved. Though the precision is not so high as with a good photoelectric photometer, it is believed th a t the instrum ent 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 De
partm ent 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 determ ination of fluorine (9), carotene (25), mercury (34), indole (6), lactic acid (16), arsenic (21), phosphate (12), nicotine (22), pheno- thiazene (26), methanol (1), color of whisky (2), coumarin (33), and cyanide (35). A number of other applications have been brought to the authors’ attention through personal com
munication and will probably be described later. I t would appear th a t the instrument, with its wide choice of filters and cell lengths, is readily adaptable to m ost standard colori
metric or turbidimetric chemical determinations.
A c k n o w le d g m e n t
Credit is due A. G. Sterling for machine work in connection with the development of the instrum ent.
L ite r a tu r e C ite d
(1) B eyer, G. F., J . Assoc. Official Aqr. Chem., 22, 151 (1939).
(2) Ibid., 22, 156 (1939).
(3) Bolton, E . R ., and W illiam s, K . A., A nalyst, 60, 447 (1935);
62, 3 (1937).
(4) Brew ster, J . F., J . Research N all. B ur. Standards, 16, 349 (1930).
(5) C apstaff, J . G., and P u rd y , R . A., Trans. Soc. M otion Picture Engrs., 11, 607 (1927).
(6) C lark, J . O., et al., J . Assoc. Official Arjr. Chem., 20, 459 (1937).
222 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 12, NO. 4 (7) Clifford, P . A., and W ichm ann, H . J., Ibid., 19, 130 (1936).
(8) C om ing G lass W orks C atalog, ‘‘G lass Color F ilte rs” . (9) D ahle, D ., J . Assoc. Official Agr. Chem., 20, 505 (1937).
(10) E velyn, K . A., J . Biol. Chem., 115, 63 (1936); 117, 365 (1937).
(11) E x to n , W . G „ Proc. Soc. E xptl. Biol. M ed., 21, 181 (1924).
(12) G erritz, H . W ., J . Assoc. Official Agr. Chem., 22, 131 (1939).
(13) Gibson, K . S., J . Optical Soc. A m ., 25, 131 (1935).
(14) Gibson, K . S., and T y ndall, E . P . T ., B ur. S tan d ard s, Sei. Paper S475, 1923.
(15) H ard y , J. D ., and P fund, A. H „ unpublished work, 1930.
(16) H illig, F., J . Assoc. Official Agr. Chem., 20, 130 (1937).
(17) H u b b ard , D . M ., Ind. Eng. Ch em., A nal. E d., 9, 493 (1937).
(18) Iv es T in t P hotom eter, Palo Co., New Y ork, N . Y.
(19) Jones, L. A., J . Optical Soc. A m ., 4, 420 (1920).
(20) Ju d d , D. B., Ibid., 23, 359 (1933).
(21) K lein, A. K ., and Vorhes, F . A., Jr., J . Assoc. Official Agr. Chem., 22, 121 (1939).
(22) M arkw ood, L. N „ Ibid., 22, 427 (1939).
(23) M ellon, M . G., “ R ole of S pectro p h o to m etry in C olorim etry”
a n d c ite d re fe re n c e s , I n d . E n g . Chem., A n a l. E d ., 9, 51 (1937).
(24) M üller, R . H ., Ibid., 7, 223 (1935).
(25) M unsey, V. E ., J . Assoc. Official Agr. Chem., 21, 331 (1938).
(26) M u rray , C. W ., and R yall, A . L.* I n d . E n g . Ch em., N ew s E d., 17, 407 (1939).
(27) P fu n d , A. H ., J . Optical Soc. A m ., 18,167 (1928); 19,387(1929).
(28) P riest, I. G., J . Research N atl. B ur. Standards, 15, 529 (1935).
(29) Shook, G. A., and Scrivener, B . J., Rev. Sei. Instrum enta, 3, 553 (1932).
(30) Singh, B. N ., and R ao, N . K . A., P la n t Physiol., 13, 419 (1938).
(31) S ta ats, E. M ., J . Optirxd Soc. A m ., 28, 112 (1938).
(32) Thiel, A., and T hiel, W ., Chem. Fabrik, 5, 44 (1932).
(33) W ilson, J. B., J . Assoc. Official Agr. Chem., 22, 393 (1939).
(34) W inkler, W. 0 „ Ibid., 21, 220 (1938).
(35) Ibid., 22, 349 (1939).
(36) Zeiss Pulfrich P h o to m eter, M ess 431, C arl Zeiss, Inc., N ew Y ork, N . Y.