High Temperature Recording Polarimeter R. S. SALTZMAN, J. F. ARBOGAST,
end
R. H. OSBORN
Hercules Experiment Station, Hercules Powder Co., Wilmington,
i recording photoelectric polarimeter has been developed to meet the need for a means of continuously monitoring the optical rotation of a plant stream of molten resin at 180" C. The instrunient contains an arrangement of two photovoltaic cells in a balanced electrical network. A change in the optical rotation of the sample stream creates an unbalance signal in the network which, after amplification, actuates the balance motor of a recorder. This motor simultaneously moves the recorder pen and rotates a polarizer in the instrument until balance is restored. The polarimeter has been in service more than 2 years. I t is rugged and stab1e;and has an over-all accuracy within about 1%.
I
S CHEMICAL processes involving optically active materials,
the optical rotation of the product is often used as a primary measurement for quality control, However, the usual periodic optical rotation measurements by a control laboratory may introduce serious time lags betxeen the recognition and correction of unusual operating conditions. An instrument which would continuously indicate and record the optical rotation of the pi oduct would obviously provide a more reliable guide for close control of the process. Heller gives an excellent survey of the field of polarimetry ( 4 ) . Other polarimeters have been described ( 2 , 3, 6, 7 ) ; however, none of these could be easily adapted to meet the requirement of reliable continuous plant service a t elevated temperatures. Hence, a recording photoelectric polarimeter suitable for plant use was developed in this laboratory. It has been in operation on a plant stream of molten resin for more than 2 years.
Del.
liquid flowing through the sample cell, results in an off-balance signal in the photocell circuit which is fed into the recorder amplifier. The output from the amplifier actuates a balance motor wliich simultaneuusly moves the recorder pen and rotates the balancing polarizer until balauce is restored. At balance, the rotation of the balancing polarizer from its zero position is always equal and opposite to the optical rotation of the sample. THEORETICAL CONSIDERATIONS
From the law of Nalus:
z = I, COS? e (1) where Zo = intensity of the plane-polarized beam entering the analyzer, Z = intensity of the beam leaving the analyzer, and e = angle between the plane of polarization of the beam and the polarizing axis of the analyzer. dI Then = -210 cos e sin 8 (2)
ze
dl dzI The angle a t which - is a maximum is found by equating -2 = d0 de -21, (cos2&-sin20) to zero and solving for e. It is thus seen that with the polarizing axis of each analyzer a t 45" to the plane of polarization of the light beam, the response of the detecting circuit to a small change in rotation of the plane of polarization of the light beam is a maximum. The photocells are connected in the current-balancing circuit shown in Figure 2. By considering the characteristics of this circuit (8) and with the aid of the law of Malus and Lambert's law of absorption, an optimum cell length may be derived for a sample having a given absorption coefficient. For simplicity, assume that the intensities of the beams striking
PRIYCIPLE OF OPERATION
1
The instrument is similar in operation to an ordinary labor* tory polarimeter, except that an electromechanical feedback loop replaces the human operator. -4schematic diagram of the polarimeter is shown in Figure 1. Light from a tungsten lamp is collimated by a lens combination, monochromatized by a multilayer interference filter, and planepolarized by a rotatable Polaroid polarizer. The beam then passes through the sample cell and is split by a half-aluminized mirror. Each of the resulting two beams passes through a Polaroid disk used as an analyzer and finally impinges on a photovoltaic cell. The polarizing axes of the two analyzers are 90" apart. The beam reflected from the mirror is actually elliptically polarized. However, by mounting the mirror so that the angle of incidence is small, the beam may be considered as plane-polarized for all practical purposes. The photocells are connected in a current-balancing network. The condition of balance is attained when the plane of polarization of the light beam from the sample cell is in a unique position, approximately midway between the axes of the analyzers. At balance, there is no output from the photocell network. If the intensities of the two beams were equal, and if the trvo photocells were matched, the angle between the plane of polarization of the light beam from the sample cell and the axis of each analyzer would be 45" at balance. Rotation of the plane of polarization of the light beam from the balance position, caused by a change in optical rotation of the
2.
Figure 1.
Schematic diagram of recording polarimeter 1. 2. 3.
4. 5. 6. 7,8. 9,lO. 11. 12.
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A
Lamp Lens Filter Polarizer and worm gear Sample cell Half-aluminized mirror Analyzer Photocells Amplifier Balance motor
V O L U M E 7.7.
N O . 9, S E P T E M B E R 1 9 5 5
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the two photocells are equal a t halanee, and that the currene illumination characteristics of the two photocells are equal. For a small change ( A a ) in the specific rotation of the sample:
i
= c(r, -
I$)
(3)
where
i
= off-halance ourrent e = constant I , = intensity of light beam striking photocell 1 IZ = intensity of light beam striking photocell 2
PHOTOVOLTAIC CELLS
Lenses. Light from the lamp is collimated hy two achromatic lenses, 54 mm. in diameter and 140 mm. in focal Icngth. Filter. A 2 X 2 inch Baird multilayer interierenw filter hnvina a transmittance ai 73% a t the peakwave length of 5%) mp ana a. half-hand width of only 5 mp monochromatises the light beam. Polarizers. Type H, high-transmittance Polaroid polmizers are used in this instrument. The rotatable balancing polarizer is a 2.5-inch-diameter disk mounted in a housing which is press-fitted into the inner race of 8. New Departure No. N5L16-A double-row angular contact hall hearing. The two analyzers are L i 5 X ' / 8 inch rectangles with palarizing axes 45" to the edges. One is mounted in front of each phatacell SO that t,he polarizing axes are 00" apart. Sample Cell. The sample cell is designed to handle molten resin a t 180" C.-the average temperature of the resin in the plant stream. The stainless steel body of the cell is channelled for the
psrtment is sealed with 2.75-inch optically pohhed borosilicate glass windons pressed against Teflon paskets.
panded vekiculite. I n addition, the cell is GGed with double windowa, separated by a dead air space, and the cell assembly is mounted within a compartment, the walls of which me covered by a layer of magnesia. Beam Splitter. The beam splitter is B 2 X 2 inch, half-aluminized mirror which reflects approximately 40% of the incident light to one analyzer and transmits 40% to the other. A piece of optical glass is cemented over the aluminized surface in order to protect it from corrosiou and mechanical injury.
2. Photocell current-balancing
Figure
circuit
From the law of Malus: It =
0.5 Is cos'(45"
I,
0.5 Ia cosa (45'
=
- AaL)
(4)
+ AaL)
(5)
where I S = intensity of light leaving sample cell, a = specific rotation of sample, and L = length of sample cell. From Lambert's law:
I, = roe-= whereIo = intensity of lightenteringsample cell, and k tion coefficient of the sample a t 589 mp.
=
(6) absorp-
Combining Equations 3, 4, 5, and 6
i
=
0.5 ~ I ~ e - ~ ~ [ (45' e a s *- A a L ) - cos* (45'
+Ad)]
(7)
Simplifying Since AarL is ve Therefore:
where K = do The value of L t o make
_1.
A0
a maximum is desired; therefore:
(L) Ke-"((l
d dL Am
=
- kL) = 0
(10)
Fie
Solving for L:
L = -1
(11)
k
From measurements of the spectral transmittance of the resin a t 589 mp, the ah8orption coefficient IC was found to he 0.67 per inch. Therefore, the optimum value of L becomes 1.5 inches.
hermetically sealed photovoltaic cells are used as deteotors. They are mounted in Dural holders which are wrapped with copper tubing through which cooling water flows. Since photovoltaic cells must not he operated continuously above 50' C., cooling is necessary. ELECTRICAL, BAJANCING, AND RECORDING SYSTEMS
OPTICAL
UNIT
The optics of the recording polarimeter are entirely enclosed under a positive pressure of dry, clean air in a housing which is mounted on the back of a Mimeandis-Honeywell circular chart recorder as shown in Figure 3. Light Source. The light source is a General Electric ZOO-watt, 120-volt projection lamp with a Type 2CC8 filament and medium prefocused base. The lamp housing (I) is both water-cooled and air-ventilated.
Power Supply. A Sola, 115-volt, 250-watt constant voltage transformer regulates the power for the prAjeetion lamp. A parallel combination of a resistor and an indicator lamp in series with the projection lamp drops the voltage across it to approximately 100 volts. The life of the lamp varies with the applied voltage as follows:
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ANALYTICAL CHEMISTRY
n here LV is the life of the lamp with the applied voltage, V , and L, is the life of the lamp with the applied voltage, v ( 5 ) . Hence,
this 20-volt drop from the rated voltage increases the nominal life of the bulb from 50 to 550 hours. The indicator lamp is mounted in the front of the recorder and acts as a warning signal t o the operator if the projection lamp burns out. Photocell Circuit. The photocells are connected in the currentbalancing circuit shown in Figure 2. Photocell fatigue is a t a minimum as a result of effective zero external resistance a t balance (8). The output of the photocell circuit is connected directly t o the input of a high gain amplifier associated with a Minneapolis-Honeywell recorder. Recording and Balancing System. The amplifier converts the direct current signal from the photocell circuit to an alternating current, which is then amplified. The output of the amplifier drives a balancing motor, which is coupled not only to the indicator pointer and recorder pen, but also, through a universal joint and long shaft, to a worm and pinion gear combination on the balancing polarizer assembly. The worm gear is mounted on a tube press-fitted into the inner race of the ball bearing. The worm is mounted between tmo small ball bearings located on the base of the large bearing assembly. An unbalance signal detected by the amplifier causes the motor to rotate the polarizer in such a direction as to restore balance. The angle through which the polarizer rotates from its zero position is equal and opposite to the optical rotation of the sample. Since the polarizer is mechanically linked to the recorder pen and pointer, the optical rotation can be read directly from the recorder scale. SAMPLE HANDLING SYSTEM AND TEMPERATURE CONTROL
The sample handling system is designed to deliver a flow of molten resin to the polarimeter a t a relatively constant temperature of 180' C. The flow diagram in Figure 4 shows the details of the system.
polarimeter was made by carefully measuring the optical rotations of samples of resin with a laboratory polarimeter a t 25" C., and comparing these measurements with the recording polarimeter readings of the same samples a t 180" C. A linear relationship between the instrument readings and the optical rotations was found. The instrument scale was finally calibrated to read specific rotation a t 25' C. directly. ADJUSTMENTS
The range of the polarimeter is fixed by the chosen gear ratios and sample cell length. Inside the recorder case a zero adjustmentis provided. This consists of a knurled knob and set-screw arrangement a t the universal joint by means of which the coupling shaft may be disconnected from the balance motor and rotated to a new position. With a liquid of known rotation in the Pample cell and the coupling shaft disconnected from the balance motor, the worm shaft is turned until the recorder indicates the correct rotation. The shaft is then reconnected to the balance motor by means of the set-screw. SAFETY PRECAUTIONS
The optical unit, the recorder, and the constant voltage transformer are air-purged for safety as well as for cooling purposes, A pressure relay is connected in the air line and will disconnect the electrical pon er to the instrument should the air pressure fail. A pressure relay is also installed in the steam line. Should the steam pressure in the jackets fail, the relay will disconnect the sample pump to prevent possible damage to the system. PERFORMANCE
P R O C E S S STREAM
FL. O W R A T O R
I
f Figure 4.
Sample handling system
The sample is tapped from the process, flows through the sample cell, then through a steam-traced Fischer and Porter Flowrator to a Marco pump which pumps it back into the process stream. The sample cell and Flowrator are kept on the suction side of the pump to avoid the possibility of exposing the Flowrator and the sample cell windows to excessive pressures in case of an obstruction on the discharge side. Several points in the sampling system are tapped to provide for blowing out sections of the sample lines with steam. Primary temperature control is attained through pressure control of the steam used to heat the sample lines and the sample cell. I n this application, a K-Master regulating valve reduces 250 pounds per square inch steam to a controlled pressure of about 140 pounds per square inch. The sample lines consist of a/,-inch tubes mounted inside 0.i5-inch tubes. The steam flows in the annular space between the two tubes. The lines are lagged with magnesia insulation. Swadgelok heat exchanger tees are used for the connections. With this arrangement, the temperature of the resin within the sample cell is controlled well within the required limits of 15' C. CALIBRATION
Previous plant experience has been based on the measurement of the resin a t 25" C. As the resin is an amorphous solid a t 25" C., however, any continuous measurement must be made on the molten plant stream at 180" C. The calibration of the recording
The recording polarimeter is a rugged and relatively troublefree instrument designed for a high temperature application where a high precision is not necessary. It has been in plant operation for over two years and has shown excellent stability and more than adequate sensitivity for a required accuracy of f0.5' specific rotation over a range of 40". The precision of the instrument is limited primarily because of lack of close temperature control and because of fluctuations in readings due to vapor bubbles in the sample stream. However, under the more ideal conditions of the laboratory, the polarimeter is capable of considerably higher precision. With sugar solutions a t 25" C., the recording polarimeter is precise to 5 0 . 2 " specific rotation, being limited by the precision of the recorder. Somewhat higher precisions could be obtained by using a longer sample cell with a decreased range of rotations. However, for very high precision (better than f0.02" specific rotation), very careful optical alignment and elimination of fluctuating strains, although minute, in the borosilicate glass windows would be necessary. The excellent stability of the recording polarimeter is indicated by the fact that, although the zero setting is checked daily, zero adjustments have not been required for month-long periods of continuous operation. LITERATURE CITED
(1) Arbogast, J. F., and Osborn, R. H., 4 x a ~CHEM., . 23,950 (1951). (2) Bruhat, G., Blanc-Lapierre, A., Schilta, J., and Raoult, G., Compt. Tend., 214, 616-17. (1942). (3) Ebert, L., and Kortiim, G., Z. physilz. Chem., B13, 105-22 (1931). (4) Heller, W., and Weissberger, A, "Physical Methods of Organic Chemistry," vol. 11, Interscience, New York, 1946. (5) Illuminating Engineering Society, New York, S . Y., "I. E. S. Lighting Handbook." Sect. 6. D. 10. 1947. (6) Levy, G. B., Schwed, P., and Fergus, D., Rev. Sci. Instr., 21, 693-9 (1950). ( i )Willey, E.J. B., J . Sci. Instr., 20, 74-5 (1943). (5) Wood, L. A, Rev. Sci. I w ~ T 5 , .295-9 , (1934). RECEIVED for review June 8. 19.54. Accepted February 21, 195.5
