1645
V O L U M E 27, NO. 10, O C T O B E R 1 9 5 5 have been found to be vanishingly low, for reasons discussed above. On the other hand, in gases of higher carbon monoxide content and in freshly prepared air mixtures, when there has not been sufficient time for appreciable decomposition of carbonyl by oxygen, the concentration of the former may be significant. I n any case, as no information is available concerning allowable toxic limits of iron pentacarbonyl when gases containing this contaminant are used for breathing, they should be passed successively through concentrated sulfuric acid and water as suggested above. ACKNOWLEDGMENT
The authors are indebted to the late Martin Shepherd and to Shuford Shuhmann, Gas Section, National Bureau of Standards, for generous portions of their analyzed stock supply of commercial (Du Pont) carbon monoxide. LITERATURE CITED
(1) Ambler, H. R., and Sutton, T. C., Analyst, 59, 809-11 (1934). (2) Bunte, H., and Terres, E., Gus- u. Wasserjach, 65, 145-7 (1922).
(3) Croxton, F. E.. U.S. Atomic Energy Commission Rept. AECU171 (K-365, Part 11) (,March 22, 1949).
(4) Dewar, J., and Jones, H. O., Proc. Roy. SOC.London, 76A, 558-577 (1905); 79A, 66-80 (1907). (5) Engineering Unit, Div. of Industrial Hygiene, National Institutes of Health, IND. ENG.CHEM.,ANAL.ED.,16, 346 (1944). (6) Eyber, G., 2. physik. Chem., 144, 1-21 (1929). (7) Fieldner, A. C., and Jones, G. W., Am. Gue Assoc. Monthly, 6,439-48 (1924). (8) Grassner, F., Mikrochemie, 10, 255-71 (1931-32). (9) Griffith,R. H.. and Holliday, G. C., J . SOC.Chem. Ind., 47, 31112T (1928). (10) Guhin, H., “Trait6 de Manipulation et d’Analyse des Gas,” p. 445. Mason et Cie, Paris, 1952. (11) King, J. G., and Sutcliffe, J. A., J . Sot. Chent. I n d . , 4 7 ,3561 (1928). (12) Marciante, A., Chimica e industria (Milan), 29, 35-6 (1947). (13) Mond, L., and Quincke, F., J . Chem. SOC., T r a n s , 59, 604-7 (1891). (14) Pohland, E., and Harlos, W., 2. anal. Chem., 90, 192-9 (1932). (15) Roscoe, H. E., and Scudder, F., Proc. Chem. Soc. (London), 6, 126-8 (1890). (16) Sendroy,’J., J;., Liu, S. H., and Van Slyke, D. D., A m . J . Phys ~ o Z . , 90, 511-12, 1929. (17) Sheline, R. K., J. Am. Chem. Soc., 73, 1615-18 (1951). (18) Stoffel,A., 2. unorg. Chem., 84,56-76 (1913). (19) Trauta, M., and Badstubner, W., 2. Elektrochem., 35, 799-SO&(1929). RECEIVED for review August 17, 1954. Accepted June 4, 1955.
Simple Photoelectric Polarimeter THOMAS B. CRUMPLER, WiLLIAM H. DYRE’, and ALDENLEE SPELLZ Richardson Chemical Laboratory, Tulane University, N e w Orleans, l a .
A n attachment which can convert a photoelectric colorimeter to a polarimeter employs two Polaroid plates and eliminates the need for an angular scale. From successive readings of transmittance values, the angle of rotation is calculated. Its performance has been evaluated with solutions of known rotation, and by comparison with a visual saccharimeter. The reproducibility of rotation measurements is about 0.03’. Multiplication of the result by a conversion factor is necessary.
PRE
VIOUSLY reported photoelectric polarimeters have been adaptations from traditional visual models. I n these instruments an end point setting was required, followed by the reading of an angular scale. A precise angular scale is expensive. In general, the photoelectric cell (or cells) simply replaced the eye as a recorder of a null point or a minimum transmittance (2-5, 7-9, iz,14-17). In 1937 Landt and Hirschmueller (10) reported a radically different and ingenious design for a photoelectric polarimeter which eliminates the need for an angular scale and which employes inexpensive Herotar polarizing filters. Their design was based on optical principles first employed in 1849 for studies of magnetic rotation of infrared radiation by de la Provostaye and Desains ( I S ) , who used tKo calcite rhombs set 45’ apart with a thermopile to receive the ordinary ray. The angle of rotation could be calculated from readings of the galvanometer first without and then with the magnetic field applied across the transmittance medium which was flint glass. Landt and Hirschmueller gave no reports on the sensitivity and reproducihility of their instrumental design in either their paper (10) or their patent claim (11). The authors of this paper decided, therefore, to construct an instrument of similar design and test its usefulness and precision. +
1
2
Present address. Rohm & Haas Co., Pasadena, TPX. Present address, Brown University, Providence, R. I.
DESIGN 6 P A ONE-PHOTOCELL POLARIMETER
The design schematically represented in Figure 1 is essential1 the same as that described by Landt and Hirschmueller (10, 1131 The alternating current from a Raytheon constant voltage regulator after being stepped down by a variable voltage transformer supplies energy for a tungsten lamp (5O-watt, 6- to &volt Mazda). The light from the lamp’s filament passes through an infrared filter (Corning, ligheshade Aklo), is then collimated by a lens, and finally filtered through a color filter which transmits a band of wave lengths between 575 to 605 mp with maximum transmittance a t 590 mp. This corresponds closely with the D line of sodium. The resulting beam which is almost parallel and nearly monochromatic, then passes through two polaroid plates (factorymatched pair of high-transmittance polaroid plates) and the solution cell, and strikes the faee of a Weston Photronic photocell.
I1
I
!
I
i
I.C.
Figure 1. A. B. C. D. E.
Schematic diagram of single-cell polarimeter
Voltage regulator Transformer Lamp, 6- t o 8-volt Mazda Heat-absorbing filter Lens F. Color filter 6. Aperture
H . Polarizing polamid plate I . Analyzing Polaroid plate (a& jacent position) J. Polarimeter tube K. Alternate position for analyzer polaroid L. Photovoltaic cell M . Galvanometer with v a r i w sensitivity shunt
ANALYTICAL CHEMISTRY
1646 The current from the photocell is measured on a galvanometer with variable sensitivity. The first Polaroid plate functions as polarizer and remains in fixed position. The second Polaroid as analyzer is set a t 45' to the polarizer and can occupy the alternate positions before and after the solution cell in the light path. The analyzer is also mounted to permit its rotation through 90' to give an angle of 135' to the polarizer. THEORY OF THE DESIGN
If the polaroids were adjacent and their axes parallel, the diminution in intensity would be due entirely to light absorption by the media in the optical path. According to the Bouguer-Lambert-Beer law:
I
=
KlIolO-zc
(1)
where K , is the reflection loss factor and Zla gives the sum of all sources of extinction. From Malus' law:
I = KnIa cos28
(2)
where 0 is the angle between the axes of crossed Polaroid plates. If then the axes of the polarizer and analyzer differ by angle 6 but they remain in adjacent positions ahead of the solution cell, it follows that: I = K3I010-z. C O S 2 8 (3)
If now the analyzer be moved to its alternate position so the solution cell is between it and the polarizer, the effective angle becomes the resultant of e and of a,the angle of rotation of the solution in the cell, hence: I, = K31010-2f COS* ( e i a )
&=-cos2 (0 f. a ) cos2 e
From the law of Malus it can be shown by the standard methods of differential calculus that de/dI is minimum when 8 i s 45". Or, dI/de is maximum when 8 is 45". Therefore: R, =
R
ecos2 (* ) 45
= cos*(45
* a)
0.500
Equation 6 is equally valid when 0 is 135' or 225" or 315". Solving for a,the equation may be restated: a =
'/2
cos-1 [ ( R a / R )- I] - 45
(7)
Although R,/R may be less than unity, and result in a negative value for the bracketed factor, such negative values are perfectly admissible, provided a is not greater than 45" because then cos-L [(R,/R) - 11 will always have values between 90" and 180'. A
'rable I.
Soln. No. 1 2 3 4 5
(I
POLARIZED LIGHT EMISSION BY THE LAMP
The initial model had the analyzer in fixed position with the polarizer movable. It proved impossible to obtain reproducible results from settings of the polarizer a t 45' and 135' to the analyzer. It was found that with a single polaroid platein the beam of light there were two maximum and two minimum photocell readings evenly distributed over the 360" arc of rotation of the Polaroid plate. Identical results were obtained with a Kicol prism. Investigation of various lamps revealed as much as 15% difference between maximum and minimum settings. S o lamp was found which did not show this effect. T h e & to 8-volt Mazda automobile lamp showed the least polarization. Willey ( 1 7 ) reported the observation of two maxima and two minima and attributed this effect to the Iight which was not polarized by the Polaroid plates. He estimated the unpolarized light to amount t o only 2% however. From this hypothesis, all lamps should give identical effects, and the maxima and minima should not he observed when a Nicol prism is substituted I t was found that by setting a fixed polarizer to the angle of minimum photocell response and then setting a movable analppr at 45" to the polarizer, reproducible results could then be ohtained for 45" and 135' settings of the analyzer. I t would appear therefore that the only feasible arrangement is fixed polarizer and movable analyzer. OPERATION
(4)
Dividing Equation 4 by 3 and assuming proportionality of current reading R to light intensity:
R
table of values of u corresponding to. various R,/R ratios covering the rotation range of 0" to IO" has been found very usefd.
Following the setting of the polarizer as described above, the analyzer is inserted and rotated to the position giving a maximum photocell response. This corresponds to approximately zero angle between the axes of the two polaroids. From Equation 2 it can be seen that when the analyzer is rotated 4500thephotocell response must drop to one half of the value for 0 , because cos 45 = (1/2)1'*. Following this preliminary adjustment, fine adjustment can be continued until identical readings are obtained on movement of the analyzer through 90". During this adjustment, the solution cell contains only water. The solution cellis a I-dm. Bausch & Lomb spectrophotometer absorption cell with removable end plates. The cell aperture is 26 mm. Six optically active solutions ranging from 8 to 75% suwose were prepared from roughly weighed, commercial sucrose to permit comparison of this instrument with a Bausch & Lomb quartz-wedge saccharimeter. Each solution was observed first in the photoelectric polarimeter where four readings were taken: two a t 45" and two a t 135'. The same solution was then placed in the 2-dm. saccharimeter tube and six settings were obtained, using unfiltered tungsten lamp illumination, and averaged together. A plot of values of a from the photoelectric polarimeter against 1/2 V"-Le., one half of the saccharimeter reading expressed in degrees Ventzke to con-
Data Obtained with Single-cell Polarimeter Showing Reproducibility and Comparison with Saccharimeter Photoelectric Polarimeter (Circular Degrees), 1-Dm. Tube 450135'setting setting Av. 1.55 1.55 1.53 1.49 1.52 2.81 2.67 2.79 2.81 2.67 5.83 5.86 5.83 5.80 5.83 8.01 7.95 8.00 7.98 8.04 9.54 9.39 9.50 9,48 9.57 13.66 13.73 13.66 13.56 13.79 Average deviation
Saccharimeter" (Ventske Degrees), 2-Dm. Tube 7.8
0.370 1.45
i P 0.08 0.04
Vo X
14.8
2.74
31.7
5.86
0.03
43.3
8.01
0.01
51.2
9.47
0.03
74.4
13.76
0.10
Each result is the average of six independent readings.
0.05
V O L U M E 27, NO. 10, O C T O B E R 195s
1647 -
described above hecame a primitive photoelectric odorimeter upon removal Angulaz of the polaroid plates. It seemed reaRotation, Average. sonable, therefore that a stable photoDegrees m De~rees electric colorimet,er might be con-6.30 -6.15 -6.00 verted to a polarimeter. A Lume-6.15 -6.30 tron, Model 402-E,photoelectric color-6.00 imeter was available and i t proved easy -6.18 -6.36 -6.00 to construct a simple adapter for in-6.18 -5.4s sertion into the cell eompsstment of -6.88 this instrument. The ultimate design of the adapter is shown by Figure 2, B photograph of the assembled adapter. Figure 3 showa the adapter disassembled, and Figure 4 shows the adapter in position in the cell compartment of the Lumetron photoeolarimeter.
Table 11. Showing Reproducibility of Two-Cell Polarimeter Slidewire Readinm Rr RL.
Run
R
A
50 50
60.9
B
50 50
60.9
C
50 50 50 50
61.0
D
45'
Rm/R
1.218 39.6
1.218 39.6
1.220
39.6~ 59.5
88.1
1.190
135' 0.792
0.792 0.792 0.762
The polarizer polaroid is fastened to the left end plate of the adapter. The aperture in this plate i s 6 mm. The analyzer polaroid is mounted in a square brass plate with a 4-cm. aperture, slotted on the four edges. This plate tits snugly into the vertical positioning braces a t the two ends of the adapter and a t either end, the analyzer may be held a t the four positions, 90' apart. Figure 2.
Adapter assembled
vert to 1-dm. basis-was linear and the value of the slope was 0.370. Browne (1) gives 0.398 as the conversion factor from degrees Ventzke, using unfiltered Wellsbach mantle light, to circular decree8 measured with the D line of sodium. Table I shows the reproducibility of photoelectric polarimeter settings and also compares the observed average values of a with corresponding oalculated values from saccharimeter settings. Linearity over a rotation range of 14' with a n average deviation of about 0.05" describes the performance of this one-photocell instrument. It became very clear that much of the uncertainty in readings was due to line voltage fluctuations which affected the readings in spite of the presence of the voltage regulator.
Figure 4.
Adapter in well of Lumetron colorimeter
The rectangular Bakelite plate seen st the extreme h Figure 3 has an aperture of 9 mm. This plate was inserted i slot ahead of the reference photocell of the Lumetron t o der the light striking the photocell and hence simplify the bala at a setting of 50 on the slide wire. It was necessary t o u initial setting less thrtn full scale hecause half of the rem R,, were greater than the initid reading. The slide-wire rea on the Lumetron are read as per cent transmittance. The i-a two transmittance value8 will equal the ratio of transmitted intensities with identioal inoident intensity:
t in the e?se cing 3 an pgs, ings io of ight
(8)
Figure 3.
Adapter disassembled
A power supply consisting of two batteries in parallel was suhstituted for the alternating current supply and although a slight improvement in stability of light intensity was noted, the discharge rate to produce adequate light intensities was so high that the over-all precision was not noticeably improved. DESIGN OF A TWO-PHOTOCELL POLARIMETER
A two-cell, helanced circuit instrument seemed to he the answer for line voltage fluctuations. It was clear that the instrument
It therefore follows that the theoretical principle of this two-cell design i s hasically the same as that summarized earlier in Equation 7. The light filter used was a Farrand interference filter which has maximum transmittance a t 597 mp with a half-band width of 15 mw. This gave nearly monochromatic light of wave length near that of the sodium D line. RESULTS
Table I1 shows the results of four independent observations on the same solution of levulose. I n run D, the analyzer was deliberately rotated several degrees from its initial setting a t approximately 45' to the polarizer. This instance of ohtaining the same average as with the earlier setting agrees with the general conclusion finally adopted that this 45' setting is not critical in the present design.
1648
ANALYTICAL CHEMISTRY
Table I1 also illustrates the rule for the sign of the rotation. Here the ratio at 45' (clockwise on the optical axis from lamp to photocell) is greater than unity and the ratio at 135"-i.e., 90' clockwise movement-is less than unity which corresponds to levorotation. Dextrorotation gives the reverse order of ratio magnitudes.
Table 111. 3Ieasurement of Rotation by Substances of Known Optical Activity Sample a Obsd. a Calcd. &Tartaric acid 1.35 1.39 Sucrose (B.S.) 1.69 1.735 Dextrose (B.S.) 2.11 2.165 Dextrose (B.S.) 2.84 2.93 Maltose 3.56 3.65 Lactose 6.78 6.98 Resolving agenta 8.10 8.31 Resolving agent" 8.46 8.64 Maltose 14.30 14.76 Average Resolving agent is d-a-bromodicamphor sulfonate.
Calcd./Obsd. 1 ,030 1.026 1.026 1.030 1,026 1.026 1.026 1,022 1.026 1.026
To test the linearity of this polarimeter design, rotation ohservations (the avenage of two determinations, one a t 45" and one a t 135") were made on a series of 26 optically active solutions with arbitrary concentrations, and compared with average readings obtained on the saccharimeter previously described using a 1-dm. tube in both instruments. This series consisted of three aqueous solutions of d-tartaric acid, six aqueous solutions of da-bromodicamphor sulfonate, two aqueous solutions of maltose four aqueous solutions of levulose, two aqueous solutions of lactose, six aqueous solutions of dextrose, one aqueous solution of sucrose, and two ethyl alcohol solutions of turpentine. The quartz-wedge saccharimeter was illuminated by a sodium vapor lamp, Ten readings were taken on each solution and averaged together. Figure 5 is a plot of the saccharimeter readings against the photoelectric polarimeter readings. A clearly linear relation holds up to 15". The points marked by arrows are levorotations, all the remainder are dextro. The slope of the straight line as determined by the method of least squares, is 3.05 =t0.08. The reciprocal, which gives the conversion factor, is 0.328. This is to be compared with the factor 0.346 cited by Gibb (6) for sodium light. To test for accuracy, photoelectric polarimeter readings were talien on a series of precisely prepared solutions of purified, optically active compounds. Two were Bureau of Standards samples. The rotations of these solutions were calculated from the known specific rotation values. Table I11 gives the results along with a column of ratios of calculated to observed values. The average value for the correction factor is 1.026 which in the light of present knowledge must be considered as an empirical correction. DISCUSSION
There are several obvious sources of error: The illumination is neither monochromatic nor the same wave length as the D line of sodium, the Polaroid plates do not completely polarize the light, the beam of light is not parallel, fatigue effects in the two photocells might not cancel since the reading photocell is illuminated with polarized light and the reference photocell receives unpolarized light, and temperature was not controlled any closer than 25" i 3'. Further investigation is planned along these lines: observations with controlled temperatures, attempts to improve the precision by means of Nicol prisms and other types of photocells, and attempt t o construct a similar adapter which could fit into the optical path of a photoelectric spectrophotometer to produce a device for rotatory dispersion observations.
0
0
I
,
4
8
a
, tz
16
Figure 5. Plot of saccharimeter readings us. photoelectric polarimeter readings for 26 optically active solutions ACKNOWLEDGMENT
The authors %
