Construction and Operation of an Automatic Ozone A. D. KIFFER AND L. G. DOWELL Laboratory, Linde .4ir Products Co., Division of L'nion Carbide and Carbon Corp., Tonawanda, .%-. I ' Automatic equipment was needed for continuously measuring and recording the concentration of ozone in a stream of gas. This equipment was built and has been successfully operated for about 2 years. It depends upon the absorption by ozone of a selected band of visible light at odone concentrations in the range 0 to 10 mole % or higher. (.4t concentrations the absorption tube should greater than 10 mole be shielded as a safeguard against possible explo-
T
HE commercial use of ozone in the United States is now a
reality ( 2 , S), and it seems probable that, ozone xi11 be used on a much larger scale in the future. \\-hen ozone is produced and used, it is desirable to have reliable equipment for continuously measuring ozone concentrations over the range from 0 to 10 mole % or higher in a stream of gas. Apparatus for this purpose has been described ( 7 , 9, I O ) , but the method given below is believed to be the simplest yet devised. A sufficiently complete description has been given PO that the apparatus can be built and assembled without undue difficulty. The principle employed is the absorption of a portion of a beam of suitably filtered visible light by ozone, while the unabsorbcd portion of the light is continuously measured by a phototube having maximum sensitivity for the wave band employed. Ozone has much stronger absorption in the ultraviolet than in the visible portions of the spectrum, and the first units tried at this laboratory employed ultraviolet radiation. Difficulties were experienced in getting a sufficiently steady source of the ultraviolet and in preventing drifting of the phototube readings. Khile it seems certain that these difficulties could have been overcome, the equipment required for constructing apparatus based on the absorption of visible light was simpler, cheaper, and easier to obtain. Further attention was therefore devoted to this type of unit, and a sa ctorj- analyzer was developed. The main disadvantage is that a rather long absorption cell is required, whereas with ultraviolet radiation the cell would be only a few millimeters long. The strongest absorption of visible light by ozone takes place between 5600 and 6200 A. ( 6 ) . Ozoue is blue, and even in concentrations as low as 2% its blue color can readily be seen by sighting dowi a tube several feet lorip. DESCRIPTIOS O F TIlE APPARATUS
The ozonometer consists of a light source, a housing (including a lens and optical filters), an absorption tube 48 inches long, and a phototube housing also containing a lens. Auxiliary equipment includes a voltage regulator for the light source, a recording potentiometer, and a pon-er unit consisting of batteries and resistors for the phototube circuit. A schematic diagram of the apparatus is shown in Figure 1, and a detailed description of the parts used is given in Table I. I n building the apparatus, precautions ehould be taken to ensure constancy of operation: rigid mounting t o avoid misalignment of the optical system, soldered connections m-here possible, and proper shielding. The Sorenson voltage regulator should be grounded and properly loaded. Light Source. It is essential to keep the light intensity constant. The light source is a 100-watt spotlight lamp. Power is maintained at some constant voltage between 105 and 115 volt,s by ~t Sorenson electronic voltage regulator. T o help stabilize the voltage, a 300-watt incandescent lamp is connected in parallel with the light source.
sion.) Colored gases-e.g., chlorine, bromine, and nitrogen tetroxide-and fog or mist interfere with the analysis. The apparatus described in this report is reliable and relativelj simple. It is useful for recording and controlling the concentration of ozone in the gas from an ozone generator. It is also helpful in carrjing out chemical reactions of ozone by recording the concentration of the input ozone, and of ozone in the gas leaiing the reaction vessel. Absorption Tube. The absorption cell itself IS made of d a n d ard borosilicate glass tubing, 48 inches long and 38 mm. in outside diameter. Windows are made of borosilicate plate glass fused to the ends of the tube. Inlet and outlet tubes for the gas sample are sealed on about 1.5 inches from each end. The cell is wrapped in aluminum foil to keep out stray light and the ends are wrapped with rubber tape so they xi11 fit snugly into the filter, lens, and phototube housings. A frame supports the housings, absorption cell, and light source sockets, its construction is shown in detail in Figure 2. The phototube housing and the filter and lens housing are
Table I.
Part Light source Socket for light source
I3allast lamp
Check List for Parts Specification Supplier G 16-1/2 spotlight n'estinghouse Electric lamp 100 watts, and Manufacturing 120 volts co. Spencer 3783-24,50- . . . . . . . . . . ... cp. single-contact bayonet-base Bakelite lamp socket 300-watt incandes- . . . . . . . . . . . . . . rent lamp
....... Sor.ket for 3CO-watt lamp Sorenson voltage regu- Model 500, 0.5 kv.amp. lator ., , .,,, . . Recording potentiometer 20,000-ohm Helipot Iiesistcr, RI 1000 to 10,000 ohms Resistor, R2 (see text and Figure 1) 100,000 ohms Itesistoi , R3 45 volts B hatteiies (2) Red-sensitire. S o . Photo tube 930 Standard octal Tube socket See text Ahsorption cell 3i-mm. diameter 5Lenses (2) or 10-cm. focal length S o 9780 standard I'iltei thicknew, 2 inches ,
, ,
,
Rorenson and Co., Stamford, Conn. Brown Instrunieiit Co.
'
................ .................
................ . . . . . . . . . . . . . . . ................ ................. ................. Central Scientific Co.
Corning Glass TVorks
SQ.
I'ilter
S o . 3480 standard sq.
.kbsorption tube sup- See Figures 2 and 3. Channel of steel, port and phototube other parts of and lens housings brass Two-conductor shielded wire JIiscellaneous wire and connectors 1796
Corning Glass Works
thickness, 2 inches ..................
*
V O L U M E 24, NO. 11, N O V E M B E R 1 9 5 2
1797
la= intensity of incident light = intensity of light trans mitted when only colorless gases are in the cell cy = extinction coefficient] sq. cm. per niiiiole C = ozone concentration, mmoles per cc. of space L = length of absorption cell, cm. Ii-ith the filters recommended, a is approximately 1.22 at a sample temperature of 25" C. and 760-mni. pressure. The cell length for maximum sensitivitj. at any concentration can be calculated by subst i tuting t,he proper values in the equation:
!am?
a
cyCL = 0.434 Phototube. The phototube is operated from B batteries a t 90 volts. When the analyzer is operating, thc current drain is only 2 to 6 pa., so the life of the H batteries is nearly as long as shelf life. The photocurrent flows through the resistor conibination s h o m in Figure 1 and yields a voltage drop to operate the recorder. (.MI leads in the phototube circuit should hare grounded shields. Instead of the combination of resistors R, and RP,a General Radio Type 602-5 decade resistance box, 0 t o 10,000ohms, can be employed. The resistancc I)os gives a greater range and minimizes trialand-error matching of the resistors with the phototube.) The resistor values shown i n the sketch are for a 0- t o 2.5-mv. potentiometer recorder, but t,he combination can be changed to operat,e a 0- to 5-mv. or higher range recorder if desired. Brown Elektronik recording potentiometers have operated sat isfactorily for this service, but models produced by other manufacturers would probably function equally well.
Figure 1. Schematic Diagram of Ozononieter
OPERATION Pmo70'usE lfOUS/N$
-I Figure 2.
MA EEL C H ~ H N E L Ozonometer
J"V
l\.ith connections made as in Figure 1, turn on the Sorenson regulator. This should light the lamp bulbs. Turn on the recorder. After it' warms up, a deflection should be observed. Turn the control on the Helipot t o obtain about half-scale deflection. Loosen the screws holding the light source and adjust to get, maximum recorder reading. If this is above full scale, turn the Helipot control to get the reading back on scale. iVhen the maximum reading is obtained, tighten the screws. I t is advisable to have a microammeter in the circuit for t,his adjustnient. I3rown Elektronik recorders respond
shown in detail in Figures 3 and 4. Two Corning Glass FVorks light filters are used. They are obtained as 2-inch squares and can be ground t o fit the housing. This combination transmits light with an effective wave-length range of about 5650 t o 6100
A.
The light from the source is rendered parallel by lens KO.1 and then passes through the tn-o filters and the absorption tube. I t is focused on the photosensitive surface of a red-sensitive phototube. The optimum cell length depends upon the concentration of ozone. For ozone concentrations usually obtained from laboratory-type ozonizers, a cell length of about 48 inches is satisfactory. This will give a useful range between 0 and 10 mole % ozone in colorless diluent gas at atmospheric pressure. .it the lower concentrations the sensitivity may be increased by using a longer cell. Higher concentrations of ozone can be analyzed by this method using a shorter absorption cell, but such a procedure is attended by some risk, since ozone will explode a t concentrations as low as 11 mole % if sufficiently stimulated. Khile visible light has not been found to be a sufficient stimulus to set off an explosion in high-concentration gaseous or liquid ozone, It is nevertheless recommended t h a t if ozone concentrations greater than 10 mole % are t o be analyzed by the light absorption method, the glass absorption cell be shielded so that any possible explosion cannot harm personnel or equipment. Light absorption has been found t o follow the Lambert-Beer law:
Ill0 = 10-aCL vhere Z = intensity of light transmitted when ozone is in the cell
Figure 3. Phototuhe Housing of Ozononieter
readily if the photocurrent is from 2 to 6 pa., but become slugywh if the value is much below 2 pa. Adjust t'he control on the power unit t o obtain full scale (or other desired reading) and continue t o adjust until the drifting has ceased. This usually takes 3 or 4 hours. A satisfactory point for the adjustment when drifting has ceased is a t about 90% of full scale with only colorless gases-e.g., oxygen-in the cell. This is zero ozone concentration.
1798
ANALYTICAL CHEMISTRY
T o minimize drifting, leave the power on the Sorenson regulator, the light source, and the recorder amplifier when the oaonometer is not in actual use; only, the chart drive is turned off. Then the equipment is always ready for use without a warming-up period. Calibrate the instrument by blowing ozone-oxygen mixtures of known concentrations, analyzed by the “neutral” potassium iodide method ( I , 4 , 8) throuxh the cell and note the scale readings. In carrying out the calibration, a dilute (2 t o 5%) aqueous solution of potassium iodide should be used without any buffer. Buffers that adjust the pH to values under 7 give high results ( 6 ) . , Use of boric acid (pH about 4.3) as a buffer has been found in this laboratory to indicate ozone concentrations up t o 16% too high. Use of buffers at pH’s greater than 7 appears t o contribute nothing t o the accuracy of the results. Typical calibration figures are: Recording Potentiometer Scale Reading 92.0 61.0 44.2 42.5 33.0 26.2 23.8
Mole % 0 1 in Gas 0.0 3.03 4.85 5.18 6.96
8.36 8.57
These numbers, plotted in Figure 5, indicate t h a t individual determinations fall within about 0.1 yo ozone of the calibration curve.
&#L€
Figure 5.
,&flOING
Sample Calibration Curve
The phototube can be removed by pulling out the shell on which the tube socket is mounted. The sensitivities of phototubes vary and a new tube may give more or less current output than the one i t replaces. If this is the case it may be necessary to change the value of resistor Rs (Figure I ) . The polarities must be observed \Then batteries are replaced. ACKNOW LEDGll EST
Figure 4.
Filter and Lens Housing for Ozonometer
Although the calibration data given above stop at 8.57 mole % ozone, the instrument has been used (with “spot” calibrations) in this laboratory for analyzing gas mixtures containing up to 12 mole % ozone. Gas mixtures containing over 25 mole % ozone have been passed through the cell without explosion. There seems t o be no reason why, with proper choice of cell length and suitable shielding, the instrument could not be used with ozone concentrations approaching even 100%. When the ozonometer is in use, i t is a good idea about once per hour to adjust the control on the power unit while a colorless gas (oxygen, nitrogen, or air) is passed through the cell, t o ensure that 0.0% ozone is always a t the same place-e.g., 92.0 on the scale. When this precaution is observed, the results obtained with the automatic ozonometer have been found t o be reproducible. MAINTENANCE
The lamp bulbs burn out after some use and must be replaced. Their lives can be extended if the apparatus operates satisfactorily with the voltage regulator output a t about 105 volts.
The use of suitably filtered visible light for use in the ozonometer was suggested by Gerhard -1.Cook, and the project was carried out under his direction. Valuable suggestions were also made by George M. Skinner and Robert L. Wehrli. A part of the work was done by Robert L. Wehrli and 11y Edward Spadinger. LITERATURE CITED
Birdsall, C. hl., Jenkins, A. C., and Spadinger. E., ANAL.CHEM., 24,662 (1952).
Chem. Eng., 56, 168-71 (November 1949). Hann, Victor, C h a . Znd., 67, 386-9, 515 (1950). Ladenburg, A,, and Quasig, R., Ber., 34, 1184-8 (1901).
Rollefson, G. K., and Burton, M.,“Photochemistry and the Mechanism of Chemical Reactions,” p. 143, New York, Prentice-Hall, 1939. Ruyesen, R., Natuurw. Tijdschr., 14, 245-53 (1932); 15, 12530 (1933).
Ibid., 15, 6-13 (1933). Treadwell, F. P., and Anneler, E.. 2. anorg. Chem., 48, 86-97 (1905).
Van den Akker, J., U. S. Patent 2,380,001 (May 31. 1944). Van Suchtelen, H., Warmoltz. X., and Wiggerink. G. L . . Philip.$ Tech. Ret)., 11, 91-7 (1949). R E C E I V Efor D review June 9, 1932.
Accepted .iugiist 2.5 1932.
