Recording Chlorine Analyzer J. F. ARBOGAST AND R. H. OSBORN Hercic les Experiment Station, Hercules Powder Go., Wilmington, Del. Because infrequent spot checks of chlorine concentration in process gases cannot give a complete picture of the condition of chlorination operation, and because the determination of chlorine by chemical methods is time-consuming, an instrument which continuously records chlorine concentrations in a flowing gas stream has been developed. This analyzer has been especially designed to withstand severe corrosive atmospheres, and to give long periods of trouble-free operation. It consists essentially of a photoelectric photometer, the output of which feeds into a recorder reading directly in per cent
chlorine concentration, and a gas-handling system to maintain the sample gas at constant temperature and pressure in the optical absorption cell of the photometer. The estimated maximum error of the instrument at 40Yo chlorine concentration is " 5 % of the amount of chlorine present. This instrument should be useful to the chemical process industries for the determination not only of chlorine, but also of other colored gases such as nitrogen dioxide. The analyzer could be modified to handle liquids, making possible continuous colorimetric determinations by adaptation of well-known techniques.
T
tion. A gas-handling system provides a flov of filtered gas a t constant pressure and temperature to the absorption cell.
H E chemical methods for determining chlorine concentration in process gases are laborious, and they yield only spot checks on the condition of a chlorination operation. Spot checks can indicate, only very ineffectively, fluctuating chlorine concentrations il-hich often accompany erratic operation. A device that would continuously record chlorine concentration would obviously be of considerable value from the standpoint of process savings. It would enable the operator t o adjust chlorine feed rate t o suit the demands of the reaction, and thus avoid unnecessary waste. d number of instruments based on the measurement of the optical absorption of gases and vapors have been described (3-5, 79). Most of these were designed t o detect small concentrations of the gas, and for one reason or another it was felt that none could easily be adapted t o the reliable measurement of large concentrations of chlorine under corrosive plant conditions. ilccordingly, an experimental instrument, capable of continuously analyzing and recording chlorine concentrations between 0 and loo%, was designed and built in the authors' laboratory. The analyzer, which is described in this paper, has been especially designed to withstand severe corrosive atmospheres, and t o give long periods of trouble-free operation. One of the instruments, built according to the design, has been in continuous operation on a plant chlorinator for over tiyo years.
PHOTOMETER UNIT
The photometer unit is shown in Figures 1 and 2. The entire unit is enclosed in a light-tight case which also affords some measure of protection against corrosion.
BALANCING PHOTOCELL
Q I BEAM S P L I TTER
MEASURING PHOTOCELL
I
'
I
I SAMPLE CELL LIGHT SHIELD A N 0 APERTURE
Figure 1.
PRIKCIPLE OF OPERATION
\
/
VIOLET FILTER LENS
\
HEAT ABSORBING F I LTER
Optical System of Chlorine Analyzer
Light Source. The light source is a 2OO-watt, 120-volt, General Electric projection lamp with a Type 2CC8 filament and medium prefocused base. The lamp and socket are mounted in a watercooled housing consisting of two concentric copper pipes silversoldered together on a mounting plate. A cover is provided to prevent leakage of stray light. A Lucite rod, 0.375 inch in diameter, which is connected to the lamp housing a t one end, and to a small warning reflector on the instrument case a t the other end, indicates lamp burnout. Lens. A lens 40 mm. in diameter and of 50-mm. focal length is mounted in a holder which, in turn, is silver-soldered t o the lamp housing, and focuses the lamp filament on the photocells approximately 12 inches away. Originally, a General Electric Type H-4 mercury lamp, equipped with a filter which transmitted onlv the 4358 and 4046 A . lines, was used as a source of radiation. Insufficient intensity and instability in the arcs of some lamps prompted the change t o an incandescent source. Filters. A Corning KO.3966 Aklo filter is mounted in the lens holder to reduce heat radiation. h Corning No. 5113 violet filter, having a peak transmittance a t 407 mp is mounted in a holder directly in front of the lamp housing. The maximum absorption of chlorine gas occurs a t 332 mp. However, the energy output of a tungsten lamp a t this wave length is very low. Hence, a wave length was chosen where the lamp energy is adequate, and where chlorine still absorbs strongly. A light shield is built into the filter holder to protect the photo-
I n the search for a means of determining chlorine quantitatively in a manner suitable for automatic recording, consideration was given t o a number of methods based on physical properties such as density, electrical properties, and refractive index. However, such methods necessarily involve more or less complete removal of interfering gases. Ideally, it would be desirable to use a method based on a unique property of chlorine not shared by any of the other gases present in the reaction. One such property is that of color. Chlorine is a greenish-yellow gas, while other gases normally present in process gas are colorless. Hence, an instrument based on photometric analysis was designed. In essence, the chlorine analyzer consists of a tungsten lamp, a violet filter, a lens to collimate the filtered beam from the lamp, a glass-windowed absorption cell through which the process gas is continuously assed, and a measuring photocell to receive the collimated Ii&t beam after its passage through the absorption cell. A balancing photocell, which receives reflected light from a partially transmitting mirror placed in the main beam! provides a means of minimizing the effect of voltage fluctuations in the lamp supply. A specially designed recorder unit measures the photocell output, which varies inversely \\ ith the chlorine concentra-
950
V O L U M E 23, NO, 7, J U L Y 1 9 5 1
951 era1 Electric, Type 8PVZF.44, hermetically sealed, barrier-layer cells with standard octal bases. They are mounted in brass holders which are positioned in such a manner that the light beam is
ELECTRICAL AND RECORDING SYSTEMS
The power-supply circuits for the chlorine analyzer are shown in Figure 3.
A Sola, Catalog No. 3080i, l l b v o l t , %&watt, constant-voltage transformer supplies current to the lamp. A &ohm resistor is placed in series with the lamp to decrease the voltage across it. By operating the lamp at reduced v o h g e , the lamp life is extended from a rated 50 hours to about 250
Figure 2. A. B. C.
D. E. F. 6. H.
Photocell leads Measuring ghotaodl Hareg terminal block Pmcess gas linea Airinlet Waterlines Sample oell Beam rplitter
Photometer Unit J.
Balancing photocell Filter holder and light shield Lamp housing LuciWrod Warning light Power w i t c h Power leads
K. M.
N.
0. P.
R.
s.
Series resistor
BROWN RECORDER
115 Y 60%
*c
I
Figure 4.
I
I
Detail of Recorder Balancing Circuit
SOLA CONSTANT YOLIAGE
TRANSFORMER
5-
The standardizing rheostats, 1 and 2 in Figure 4, are an integral Dart of the recorder. They are coupled to the balancing motor by .
Figure 3.
Sohernatio Wiring Diagram
~~~
~~~
sembly. When~thestandardTzing button is puslied, the balance motor engages the standardizing rheostats, and the standardizing switches are actuated. Switch A , normally closed, is open during standardiaation, so that variable resistor 5 is in series with
952
slidewire 3. Switch B, normally open, is closed, and switch C, normally closed, is open during standardization, disconnecting the slidewire contact, and connecting the negative terminal of the balancing photocell and the upper end of shunt 6 to the upper end of the slidewire. A single standardization a t the beginning of each &hour shift is sufficient for satisfactory operation of the instrument.
CONDENSER
ANALYTICAL CHEMISTRY pound which will not undergo appreciable physical elmnge during long expasure to A mercury manomr?ter is used to set the manostrLt st 8, given Pressure, and Serves to check its 'peration. The tting is ordillarib made once each day. An airir-purge RYsteN1, utilizing a Moore constant-differential relay, Berves t o bleed air slowlg into that leg of the manometer urhich is conneeted t o the aystem. In thki manner the mercury is protected from cor;rosion. Thc manometer is located downstream from the AIR sample cell to prevent purge sir from miring with process gas in the cell. CALIBRATTON
The instrument is calibrated by introdueing pure chlorine into the mmple cell, aiid TRAP varying the pressure fromseverr~1millimeters of mercury t o the operating pr,essure of 460 mm. of mercury. The sample cell is maintained a t an operating terniperature of 45" C. The recorder reading MANOMETER .aij 460 mm. of nirroury corresponds t o 100% dhlorine. .owDiagram of Gas-IIandling System In Figure 7 isshown a typical calibration curve. From 0 to 60% concentration, A I ~ U U ~ ~ DL~ U ~ WUU wmtronik, potentiometer-type, circular t,here is a oneto-one correspondence between recorder readohart recorder is connected to the photometer as shown in Figure ings per cent chlorine conoentration, ~ ~ fro,n 3. It contains an amplifier unit that converts a direct current lineal rity occurs between 60 and loo%, but this is outsignal to alternating current, amplifies the signal, and causes it t o drive a bdancing motor. The balancing motor is mechanically side t he normal 01perat,ing range. By properly adjusting the value coupled t o the slidewire contactor, recorder pen, and indicator of peziistor 6 (Fipure 4), a better correspondencebetween chlorine Pointer. .-L,"&L"" "~ electrica1 .unbalance detected by the amplifier conceLruLrruAulra =,id recorder readings can be ohtained in the upper osuses the balancing motor to move in such a direction as t o posirange of concentrations. However, the correspondence in t.he &ionthe s]ida-wire contactor the point of electrical Because the photocells work a t relatively low light intensities, a lower range is then sacrificed. high-gain amplifier, having a sensitivity Of 1 microvolt, was SUPThe instrument odibmtion may be checked periodically by inplied m-ith the recorder used in this work. h d u c i n a-. oure chlorine into the ass-handling-system through the . air-inlet of the three-way stopcook. GAS-HANDLING SYSTEM The oalibration has been found to remain fair1y constant during np. ture and pressure. In addition, a three-way stopcock is provided for flushingthe sample cell with air during the period of stitndardization. The Row diagram of Figure 5 indicates the details of this system. An air-driven Haveg aspirator, designed by the Schutte and Koerting Co., is used t o pull the gas from the process through the cell, and exhaust it to the atmosphere. Any convenient pressure can be chosen for the gas in the absorption cell. Pressures close to atmospheric are somewhat easier to handle instrumentally. Therefore, the pressure has been fixed a t approximately 460 mm. of meroury absolute, The adjustment for this pressure is made by varying the air flow through the aspirator. Initially, the gas is passed through a condenser maintained a t the proper temperature to prevent condensation in the absorption cell. The gas is next passed through a threa-way stopcock, open to the prooess for normal operation, and open to the atmosphere to allow air to flush the cell during standardization. A glass u-001 filter, inserted in the gas line, minimizes the deposition of dirt on the sample cell windows. Neat, the gas flows through a restrict ine orifice which orotects the samule cell aminst excessive ures-
era1 return-bend passsges drilled in t6e Karbate brock. Both the accuracy and precision of the instrument depend largely on the degree t o which constant temperature and pressure are realized. Constant pressure is achieved by the use of a laboratory model Cartesisn manostat ( d ) redesigned t o withstand contact with chlorine (Figure 6). This manostat was manufactured by the Emil Greiner Co. In the redesign, all-glass construction has been substituted for metal, Teflon is used as an orifice seat instead of rubber, and the mercury ordinarily used has been replaced by highly chlorinated dimethylpentane--a liquid oom-
Figure 6 . Cartesian Manostat
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V O L U M E 23, NO. 7, J U L Y 1 9 5 1
953 this cause. In addition to placing the recorder case under positive air pressure, all electrical components of the measuring circuit, including those outside the recorder, are coated with a ParIon (chlorinated rubber) lacquer. The amplifier components are sprayed with a protective coating (tropicalized) by the Brown Instrument Co. Saran tubing (0.375-inch) and fittings are used to connect the components of the gas-handling system. In the construction of the analyzer, Teflon, Tygon, Karbate, chlorinated dimethylpentane, glass, Haveg, and Parlon, all chlorine-resistant materials, are used. DISCUSSION
Per Cent Chlorine
Figure 7.
Calibration Curve
concentration of 40% chlorine, is no greater than &5% of the amount of chlorine present. PROTECTION FROM CORROSION
Extremely corrosive plant conditions prompted special precautions for the protection of the various components of the analyzer. The recorder, constant-voltage transformer, and photometer are primarily protected by maintaining the cases surrounding the units a t positive air pressures. An air-failure warning device is employed for added protection. The units comprising the analyzer are housed in a wooden enclosure near the process control panel. This enclosure, too, is maintained a t a positive air pressure. Experience h:tq shown that the recorder is the portion of the analyzer most vulnerable to corrosion. A number of steps have been taken to reduce the possibility of instrument failure from
Although the apparatus described in this paper was designed for the analysis of gaseous chlorine mixed with colorless gases, it should be equally useful in the analysis of other colored gases, such as nitrogen dioxide. With suitable modification of the sample-handling equipment, the instrument could be adapted to liquids. Then, by proper choice of filters, a large variety of chemical colorimetric determinations could be carried out in continuous processes. Continuous recording of color, per se, would also be possible by rearranging the optical system. Such a rearrangement would comprise removing the violet filter from the main beam (Figure l),locating the sample cell between the lamp and the beam splitter, and placing appropriate tristimulus filters directly in front of the two phot,ocells (6). LITERATURE CITED
(1) Brice, B. A , . Reu. Sci. I n s t r u m e n t s , 8,279-85 (1937). (2) Gilmont, R., IKD.ENG.CHEW,ANAL.ED.,18,633-6 (1946). (3) Hanson, V. F., Ibid., 14,258-60 (1942). (4) Harris, L., and Siegel, B. RI., Ibid., 14,258-60 (1942). (5) Klots, I. M., and Dole, RI.,I b i d . , 18,741-5 (1946). (6) Osborn, R. H., U. S. P a t e n t 2,382,439 (Aug. 14, 1945). (7) Silverman, S., 1x0. ENG.CHEM.,A N ~ LED., . 15,592-5 (1943). (8) Willey, E. J. B., and Foord, S. G., Proc. R o y . SOC.(London), A135, 166 (1932). (9) Koodson, T. T., Rev. Sci. Instrwnents, 10,305-11 (1939).
RECEIVED October 17, 1950.
Infrared Analysis of Pharmaceutical Products Acetylsalicylic Acid, Phenacetin, and Cageine, and Combinations of These with Codeine or Thenylpyramine m
1.
., v . PARKE, A. &I. RIBLEY, E. E. KENNEDY,
AND
W. W. HILTY
Eli Lilly and Co., Indianapolis, Ind.
A
CETYLSALICYLIC acid, phenacetin (acetophenetidin), and caffeine can be determined simultaneously in pharmaceutical products even in the presence of codeine phosphate or thenylpyramine hydrochloride [rY,N-dimethyl-N’(2-thenyl)-N’(2-pyridyl)-ethylenediaminehydrochloride] without separation and with a minimum of mutual interference by infrared spectrophotometry. Determination of these components is not complicated by the addition of codeine phosphate or thenylpyramine hydrochloride to the mixture. Previously described chemical methods for the analysis of mixtures of acetylsalicylic acid, phenacetin, and caffeine (1-3,9) have required extensive extraction procedures for the separation of the individual or a pair of components prior to their final estimation. -4pparently any one or any combination of these chemical methods has not given sufficiently good precision to warrant adoption
as a general method of analysis by the Sational Formulary, which recognizes these mixtures a8 official preparations. A method has been developed ( 4 ) which utilizes ultraviolet absorption characteristics of phenacetin and caffeine for their simultaneous determination and for determination of the acetylsalicylic acid separately. Washburn and Krueger ( 7 ) describe a method using infrared absorption, in which considerable preparation of sample is required and calculations must be made by successive approximations because of mutual interference of the components. These authors have also published a method for analysis of combinations of these three components with thenylpyramine hydrochloride, in which the thenylpyramine is separated and determined as the reineckate (8). The present method has the great specificity inherent in infrared absorption analyses, accuracy equivalent to
