clioaeii in the same manner as outlined above, but the expanded scale spectra then appear to be roughly mirror images of Figures 1 and 2, which represent the case of an impurity occurring on the short wave length slope of a major hand. The individual to n hom the mixtures nere submitted was given the problem or" determining the amount of minor coniponent. For this investigation the iiiisture and pure minor component nere supplied but not the pure major component. In each case the unexpaiided spectrum of the mixture showed no distortion of the major band that nould suggert the presence of an impurity.
Results of this Rtudy, including choice alveiit, concentration, and cell thicknesses, are shown in Table I. In mixture 2 the minor component is a strong absorber. In mixtures 1, 3, and 4 the niinor component in each case ahsorbs rather n-eakly. The determination of intensity value iq necessarily in terms of two points rather than the usual three points employed in the standard base-line measurement. The two significant points are the peak absorption of the minor component and the shoulder on the side farthest from the major band. The shoulder lying clorest t o the major band 01
of course, displaced from its normal position because of deliberate overcompensation. Figures 3 and 4 illustrate this situation. The degree of success in choosing a substitute reference for a specific problem is dependent on the size of the collection of reference spectra and the availability of the pure reference compounds. With large collections and a degree of patience and experience. the major absorber can be closely simulated. However, after examining a large number of bands under expanded scale, the authors believe that nature will seldom, if ever, exactly duplicate a particular absorption band contour. By expanding the wave length scale, it is ea3y to detect differences in a series of absorption bands that a t the standard 2 inches per micron appear to be virtually superimposable. Though a qaniple run a t standard scale may shonno indication of any distortion of the major band, it \vi11 often show a definite flattened area near the peak point of the minor component with the espanded scale. Thus, merely expanding the abscissa may be sufficient in many cases to detect the presence of a suspected impurity and to make a semiquantitative determination. The practicing spectroscopist is acutely aware of the limitation imposed
upon him by the degree of purity of thr best standard material available. T-irtually all determinations are in ternis of comparison to a like standard. In differential work particularly, this standard all too often leaves something to be desired regarding freedom from the contaminant being analyzed. Selection of R substitute reference known to be free of the contaminant to be determined should help to emancipate the infrared spectroscopist from dependence on questionable standards. The authors hope that this approach nil1 be considerably expanded in the near future, not only in quantitative studies but also in the study of overlapping bands in structcral investigations. LITERATURE CITED
Bard, C. C., Porro, T. J., Rees, H. L., h s . 4 ~ .CHEY. 27, 12 (1955). Beroza, M., I b i d . , 25, 112 (1953). Freeman, S. K., Ibid., 27, 1268 (1955). Ibid., 29, 63 (1957). McDonald, I. R. C., Watson, C. C., Ibid., 29, 339 (1957). Powell, H., J . A p p l . Chem. 6, 488 (1986). . 24, 619 Robinson, D. Z., ~ A L CHEX (1952). Washburn, W,H., A p p l . Spectroscopy 11, 46 (1957). Kashburn, W. H., Scheske, F. .4., -4s.4~.CHEM.29, 346 (1957). RECEITEDfor review rlugust 16, 1957. .kcepted January 8, 1958.
Determination of Carbon Dioxide in Automotive Exhaust by Means of Infrared Filter Photometer 1. L. PARSONS, J. C. NEERMAN, J. R. LIFSITZ, and F. R. BRYAN Scientific laboratory, Ford Motor Co., Dearborn, Mich. ,An infrared filter photometer has been designed for the determination of carbon dioxide in automotive exhaust. The instrument utilizes filters of the multilayer interference type which limit transmittance to the 4.29micron wave length a t which carbon dioxide absorbs radiation. Carbon dioxide concentrations from 1 to 18 volume % are determined within 10% of the amount present. Results obtained with the photometer on exhaust gases tend to b e slightly higher than Orsat results.
S
the recent advent of interference filters for use in colorimetry. there have been continued efforts t o extend the usefulness of such filters into the ultraviolet and infrared regions. Important applications of interference filter photometers have been reported for the 1- to Cmicron region of the inIh-cE
frared (2, 4). Improved evaporation techniques and new substrate materials are n o v providing interference filters capable of isolating narrow bands above -1 microns (1). An interference filter photometer for the measurement of carbon dioxide a t 4.29 microns has been designed and utilized as a monitor to indicate the carbon dioxide content of gasoline engine exhaust. DESCRIPTION OF ANALYZER
Interference Filter Assembly. The filter provides high transmittance at 4.29 microns and minimum transmittance a t all other wave lengths ( 3 ) through a combination of elements consisting of narrow-band-pass and long-wave-length-pass interference filters on a substrate which serves as a short-wave-length-pass element. Filter combinations of this type were obtained from A. F. Turner, Bausch & Lomh Optical Co.
The narrow-band-pass element consists of a multilayer dielectric interference filter of germanium and cryolite deposited on one side of a thin glass substrate. Transmitting side bands of longer wave length than 4.29 microns are suppressed by this glass substrate. The side bands a t lower wave lengths ( I to 3 microns) are masked by an interference-type long-wave-length-pass filter used in conjunction with the narrowband-pass filter. The spectral transmittance of this combination of filters, as measured on a Perkin-Elmer Model 13 spectrometer, is shown in Figure 1. Peak transmittance a t 4.29 microns is 37yc and pass band half-width is 0.069 micron. Sample Cell. The relatively high concentrations of carbon dioxide in automotive exhaust necessitate a sample cell of short path length. Absorption measurements on carbon dioxide indicate that a cell path length of 1 cm. is appropriate for the concentrations encountered. VOL. 30, NO. 6, JUNE 1958
1055
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A continuous flow cell was designed with 1-cm. path length and Zcm. aperture, with inlet and outlet fittings placed opposite one another in the cylindrical brass wall. The extended brass cylinder was threaded on each end to allow mechanical attachment to both source and thermopile. Sheet mica was selected as window material because of its high trausmittance as well as good mechanical strength and chemical stability. The high moisture content of exhaust gases prrcludrs the use of salt windows, and infrared absorption distinctly limits the usefulness of quartz. A pair of 0.002inch sheets of mica transmits approximately 90% of 4.29-micron radiation. Source. The source of infrared radiation is an electrically heated coil of Chrome1 A ribbon wound on a ceramic support, and provides a source area of about 1 square inch. Power is supplied from a 6-volt transformer operating a t 36 watts. Operating the source a t 800' C. providps long filament life. Detector Circuit. The detector is a Reeder temperature-compensated m c u u m thermopile having a sensitivity of 2 pv. per pw. and a resistance of about 200 ohms. Area of the goldblack absorbing surface is approximately '1, X 9/4 inch. The window of the thermopile is calcium fluoride. Total signal from the detector is about 50 pv. with the filter in the system. A null-balancing potentiometer circuit (Figure 2) is used to measure transmittance ratios. In principle, an electrical balance is made with a flowine nonahsorbing gas to fix a 100% transmittance point; a second balance is then made to establish the transmittance with the sample flowing. Transmittance is read from the measuring scale of Rs,which is a linear 10-turn helipot with limits standardized by associated voltage dividers. R, balances out the portion of signal due to ambient temperature; R, spreads the signal change due to sample absorption over the greater part of the scale on R,; and R4compensates for source intensity changes. A gang switch is used to operate the photometer shutter and the sampling valves in proper rrlationship to the electrical measuring sequence. Compensation for ambient temperature changes can be made with the selector switch a t position 1 or 4. This is necessary b e cause zero drift occurs whenever there is il laree chanee in radiant Dower on the thermopile,- necessitating' separate ambient temperature corrections depending on whether sample or flushing gas is in the sample cell. Photometer Controls. Measurements are made from a control box which houses switches, batteries, and potentiometers. Photometer shutter and inlet valve are controlled from this point, and bucking voltages applied. The amplifier and null indicating metcr are contained in a separate compact commercial unit, which allows the photometer to be placed in a convenient sampling location with controls and meter operated and read remotely. 1056
ANALYTICAL CHEMISTRY
Figure 1. Transmittance characteristics of combination long-wave-length-pass filter and narrow-band-pass filter on thin glass substrate
The indicating meter can be replaced with a recorder for measurements under transient conditions. The photometer unit is shown in detail in Figure 3. The shutter mechanism is an aircraft fuel gate valve operated by means of a 6 v o l t direct current reversible motor. Power is s u p plied from a transformer and dry disk rectifier in the control box. Limit switches prevent motor loading mhen the shutter is not in motion. ~
Table I. Calibration Readings from Known Nitrogen-Carbon Dioxide Blends
The three-way solenoid valve is activated by 6 volts of direct current and can be operated from the control box to allow either sample or reference gas to pass into the cell. Pressure of the engine exhaust forces sample through the photometer system. A small electric pump is used to flush the cell with nitrogen or air. Gases entering the photometer are passed through a glass fiber filter to remove particles. The particle filter assembly is on the side of the photometer box. CALIBRATION
Flowmeters
-
Orsat
Reading
Orsat determinations not made.
Figure 3.
The analyzer is calibrated by passing known blends of carbon dioxide and nitrogen through the instrument a t atmospheric pressure. A series of concentrations from 2.0 to 18.0 volume yo carbon dioxide in nitrogen was used to determine the relationship between transmittance and concentration. Blends were prepared by allowing the two gases to flow from cylinders through rotameter-type flowmeters which had been prcviously calibrated against both
Components of photometer unit
Table II.
Comparative Photometer and Orsat Determinations of Carbon Dioxide in Engine Exhaust
Intake Vacuum, Inches of Hg 18.6 17.1 16.9 13.9 Standard idle 16 8 Idle rich 15.8 13 4 Idle lean 13 5 Dederation 18 0 20.0 22.0
Test Condition Road load
MOLE PER CENT C A R B O N D I O X I D E
Figure 4. Calibration curve for analysis of carbon dioxide in automotive exhaust
wet-test and dry gas meters. I n addition, Orsat analyses were used to check the blending system and the resulting calibration curve. The relationship between transmittance scale reading and mole per cent carbon dioxide is shown in Figure 4. Precision in determining calibration readings was found to be t o i 1% of the measuring scale, which is equivalent to about +5% of the carbon dioxide present. ANALYTICAL RESULTS
Comparative determinations were made on exhaust gases from an engine on dynamometer test. Pressure at the engine exhaust was regulated by a gate
Fuel Flow,
Lb./Hr.
6.5
R.P.M. 1000
6 5 15.5 29 0
42 62 5 2 3 2 1-4 5
9.1 2.9
1000
2000 3000 466 445 355 310
2000
2000 2000
bverage T ~ mittance 26.3 26.3 23.3 25.0 31.0 27.3 43.0 37.0 26.5 21.0 26.0
~COn Concn., ~ ~ Rlole - % Photometer Orsat 10.6 11.3 11.3 11.1 12.7 11.9 11.9 11.8 9.2 9.4 4.9 4.4 5.8 5.4 7.2 6.6 11.2 10.4 13.7 13.0 11.4 11.0
valve to obtain a flow of sample through the analyzer. Three photometric determinations for each engine test condition indicated the degree of analytical precision, which was again t o i l % of the measuring scale. Variance between photometer and Orsat results averaged 5.6% of the amount of carbon dioxide present, the maximum difference being lO.8Y0 of the amount present. Photometric results tended to be somewhat higher than Orsat results over the entire concentration range, with individual test conditions and comparative results presented in Table 11.
red Transmitting Interference Filters,” Paper 10, Fort Belvoir Infrared Colloquium, Conference on Infrared Optical 3Iaterials, Filters, and Films, Feb. 10, 1955. (2) Seerman, J. C., Parsons, J L., Bryan, F. R., J . Air Pollution Control Assoc. 6 , 38 (May 1956). (3) Twiss, S. B., Teague, D. M., Bozek J. W., Sink, I f . F., Ibid., 5 , 75 (August 1955) (4) Wood, R. C., Foskett, L. W.,Foster, X. B., “Infrared Absorption Hygrometer, Progress Report,” First International Congress and Exposition, Instrument Society of America, Philadelphia, Sept. 13-24, 1954.
LITERATURE CITED
RECEIVED for review September 23, 1957. Accepted February 10, 1958.
(1) Mooney, C. F., Turner, A. F., “Infra-
Determination of Catalyst Water by Exchange with Deuterium Gas JAMES K. LEE and SOL W. WELLER Houdry Process Corp., linwood, Pa.
,A method of determining water (or its equivalent as hydroxyl) in oxide catalysts is based on the isotopic exchange of hydrogen in the water with gaseous deuterium a t 500” C. The method is nondestructive, by contrast with the ignition loss method, and it compares with the latter in accuracy. Another advantage over ignition loss is the absence of interference from volatile components other than water.
T
of small aniounts of water influences greatly the catalytic properties of oxide catalysts such as alumina and silica-alumina. It is frequently important, therefore, to determine the amount of water (including structural water) present in such catalysts after various pretreatments. The conventional method involres ignition HE PRESENCE
of the sample a t high temperature (> l l O O o C.) to constant weight; the weight loss on ignition is identified as the water content. This procedure has two disadvantages: The results are high if volatile components other than water are present, and the method is destructive, because the sample becomes seriously sintered during ignition. ,4n alternative approach is to determine the amount of exchangeable hydrogen by isotopic exchange t o equilibrium with either D 2 0 or Dz. The water content is computed from the amount of exchangeable hydrogen. Because the exchange reactions proceed a t 500” C. or lower, the method is essentially nondestructive for refractory oxides. Use of DzO for this purpose was reported by Alills and Hindin ( d ) , and the agreement betxveen results
obtained by this way and by ignition loss was recently shown by Haldeman and Emmett ( 2 ) . Use of Di rather than DzO has the advantage that the pre-existing water level is frequently not changed by the exchange reaction. This might be useful when further experiments are to be done with the same catalyst sample. Taylor and Diamond (6) showed qualitatively that exchange occurred between catalyst water and D2 for a variety of oxides, but they did not attempt quantitative determinations. This paper summarizes the results of a quantitative study of the DTcatalyst water reaction for several samples of alumina, silica, and silica-alumina. EXPERIMENTAL
Materials. Deuterium gas, 99.5% pure, obtained from t h e Stuart OxyVOL. 30, NO. 6, JUNE 1958
* 1057
