An Improved Method of Quantitative Spectrographic Analysis C. C. NITCHIE'AND G. W. STANDEN, New Jersey Zinc Company, Palmerton, Pa.
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the tips of the e l e c t r o d e s in N A previous p a p e r ( 8 ) The method described is an extension of one an erratic way. Constant, alert from this l a b o r a t o r y , a suggested by Gerlach and Schweitzer, in which the attention is required on the part general method of quantiamount of the element to be determined is estiof the operator to make sure t a t i v e spectrographic analysis mated by measuring the intensity of one of its t h a t t h e l i g h t f r o m the arc applicable to the determinaspectrum lines, using as a basis of measurement shall fall directly on the spectrotion of small amounts of minor graph slit t h r o u g h o u t an exc on8 t i t u e n t s a n d impurities the intensity of another line in the same spectrum posure-a condition that is vital was described. The method is due to another constituent known to be present in to t h e a c c u r a c y of the work. one which has stood the test of constant amount in all samples of the particular Even under the best possible several years of c o n s t a n t use A microphotometer is used f o r measmaterial. control there are still unavoidand is furnishing satisfactory uring the line intensities. The method offers able variations in the arc which results. The p r o c e d u r e conu n d o u b t e d l y result in slight sisted essentially in comparing additional reliability and a marked saving in variations in the line intensities the arc spectra of the substance time over the method previously described by one on the spectrum plates. For to be analyzed with the spectra of the authors. this reason there is always B of standard samples containing possibility of error when lines known amounts of the constituents to be determined. The experimental precision is in one spectrum are compared with those in another, even such that duplicate determinations ordinarily vary from though both may be on the same plate and thus have had the the mean by less than 10 per cent of the amount being same photographic treatment. estimated. Thus if 0.1 per cent is the mean of a series of I n the improved method, the lines of the element to be determinations, the maximum deviation from 0.1 will be determined and the comparison standard lines are taken *0.01 per cent. simultaneously from the same light source. Any variations, The method to be described in this paper is an extension of therefore, should affect both in the same way so that they are one suggested by Gerlach and Schweitzer (1,s). Its essential still comparable no matter what variations have occurred in feature is the introduction into the sample solution, in constant the arc. This improved comparability now justifies the use known amount, of an element not originally present therein. of the recording microphotometer for measuring line intensiThis added element is used to furnish spectrum lines of con- ties, a use which was not warranted previously when the errors stant intensity, which may serve as comparison standards for brought about by the causes mentioned were probably greater estimating the percentages of the constituents to be deter- than those involved in visual estimates of intensity. mined. The method here described, therefore, gives more reliable Gerlach and Schweitzer have used a similar method in which results. I n addition, there is a marked saving of time on the no added element is required, the comparison of line intensi- spectrograph over the previous method. A single spectrum ties being between lines of the main constituent and of the now serves for one determination where at least five were minor constituent to be determined, Pairs of lines are found formerly required. in each of which a certain line of the element to be determined is equal in intensity to a neighboring line of the main constituAPPARATUS ent, A series of such pairs is found showing equality a t different concentrations within the range to be expected. The spectrograph, arc stand, electrode holders, and conThe analysis then consists in observing which of these pairs densing lens system are the same as those described in the show equality in the spectrum of an unknown sample. Be- previous paper ( 2 ) . cause of the number and positions of the lines to be paired, it A rotating sector wheel with adjustable aperture, driven by is not always possible to find pairs of suitable intensities a small electric motor, is mounted between the light source close enough together on the plate to be used conveniently. and the slit so as to interrupt the light beam. This permits Usually it is better to add an element having a more conven- the reduction of exposures when necessary without decreasing ient spectrum than that of the main constituent. the angular aperture of the illuminating system below that The method of this paper is, briefly, as follows: For a given needed to fill the collimator lens, and without shortening the range of concentration of the constituent to be determined, total time of exposure to such an extent that the sample only one pair of lines is used. With the aid of a recording material is incompletely volatilized. A similar result could microphotometer, the relation of the intensities of the two be accomplished by using a smaller sample, but this introduces lines of the pair is determined for varying concentrations of additional possibilities of error. Such control of light intenthe constituent to be determined and a fixed concentration of sity is needed in those cases where the most suitable lines for the added element. A calibration curve is thus established quantitative comparisons are too strong, when full exposure is b y means of which the intensity ratio for unknown samples given, to show properly measurable variations in intensity can be converted to concentration. with variations in concentration. This is a condition freThe precision of the previously described method ( 2 ) is quently encountered, when the raies ultimes of an element are subject t o the limitation that the arc between graphite so much stronger than the other lines in its spectrum that they electrodes is not as steady as is desirable, but wanders about are too dense for measurement, even at concentrations so low that there are no weaker lines of that element left in the 1 Present address, Bausch & Lomb Optical Co., Rochester, N. Y. 182
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spectrum. The only recourse then is to weaken the entire spectrum to the point where these lines are measurable. High-purity graphite electrodes are prepared as described in the previous paper. The microphotometer may be of any suitable type, provided it is of sufficient sensitivity to give a throw of the recording galvanometer beam of at least 20 cm. on the recording sheet between the positions corresponding to complete darkness and
FIGURE 1. STANDARD PLATESHOWING MAGNESIUMBISMUTHCOMPARISON PAIR to full illumination through the clear portion of the spectrum background between lines on the spectrum plate. The one in use in this work was constructed in the instrument shop of the Research Division of The New Jersey Zinc Company. It consists essentially of a motor-driven carriage by which the plate can be moved lengthwise across an intense beam of light focused by a lens system into a small area in the plane of the plate. A second lens system focuses an image of the illuminaled area of the plate upon a diaphragm immediately in front of a photoelectric cell (a thermopile may also be used). This diaphragm has a slit somewhat shorter and narrower than the images of the spectrum lines focused on it,and parallel to them. This slit serves to admit light to the sensitive surface of the cell, the amount of that light being governed by the density of the portion of the plate whose image covers the slit. As the plate moves, this density varies and a record of the variations is made by a beam of light reflected from the mirror of a galvanometer in the photoelectric circuit, and falling on a sheet of bromide paper moving vertically behind a narrow horizontal slit along which the spot of light from the galvanometer mirror moves. It is also possible to use a visual or nonrecording type of microphotometer when the procedure would, of course. be modified accordingly. PROCEDURE The details of the method can best be described by taking a particular analysis as an example. The application of the method to other analyses can readily be made with only such minor changes in details as may be required by the cheniical and physical characteristics of the materials involved. The analysis to be described is the determination of magnesium in a zinc-base alloy. The magnesium is present in the amount of approximately 0.01 per cent. The added element t o furnish the standard comparison spectrum is bismuth. The magnesium line 27958 and the bismuth line 27808 are the two lines forming the comparison pair. These two lines satisfy the requirements for satisfactory comparison lines which may be listed as follows: 1. Under the conditions of the excitation and photography of the spectra, the composition and amount of the sample, and the photometry of the spectral intensities, the comparison lines must be sufficiently intense to make the height of the photometric record for each line great enough so that the photometric peak is clearly distinguishable from the fluctuations in the base
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line of the photometer curve. Furthermore, the lines must both be of an intensity which falls within the nearly straight-line portion of the exposure-density curve of the photographic emulsion used. 2. Each line must be separate and distinct from all other lines possibly present in the spectrum. The lines of the comparison pair should be separated sufficiently from each other and from other lines so that there is no possibility of halation effects modifying the relative intensities. 3. The lines should be in that portion of the spectrum where the continuous or background spectrum is very weak. 4. The lines should be close to each other for convenience in photometering and to ensure equal sensitivity of the photographic plate for the two wave lengths. PREPARATION OF SAMPLE.A known weight of the sample is dissolved in nitric acid and the volume adjusted so that the solution contains 0.25 gram per cc. Two cubic centimeters of this solution are taken and mixed with 2 cc. of a 10 per cent bismuth nitrate solution. With a capillary pipet exactly 0.1 cc. of this mixed solution is introduced into a hole in one of the drilled graphite electrodes. The electrode is placed in an oven and dried for 45 minutes at 110" C. EXCITATION AND PHOTOGRAPHY OF SPECTRUM. The procedure for mounting the electrodes, maintaining and focusing the arc, and timing the exposure is the same as that described in the earlier paper.
FIGURE2. PHOTOMETRIC RECORDOF STANDARD PLATE OF FIGURE 1 (REDUCED) Using a 3-minute exposure, which is the minimum time allowable for the sample to be completely volatilized, the rotating sector wheel is set for a 10 per cent opening. Under these conditions the selected magnesium and bismuth lines are of suitable intensity for photometry. The extent of the sector opening must, of course, be selected with proper regard to the size, type, and light-transmitting power of the spectro-
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corresponding to the standard reference line shall have a definite and constant height.) In this way a compensation is brought about for slight variations in conditions of exposure. An important point to be noted is that measurements are purely relative, requiring constancy in the photoelectric cell and its circuit for only the few seconds during which a single record is being made. The plate is now returned to the starting position, j?st short of the first of the two lines to be compared, the bromide paper is placed in the holder, the motor started, and the marked portion of the plate recorded. As soon as the second of the two spectrum lines has been passed the motor is stopped and the second spectrum on the plate is brought into position in the light beam. Adjustments are made as before and the record taken in the same way on an adjacent portion of the same sheet of bromide paper. All spectra on one plate are recorded on a single sheet. The paper is then developed, fixed, washed, and dried. Figure 2 shows a photometric record of the magnesium-bismuth pair in the three standard spectra of Figure 1.
EVALUATION OF PHOTOMETRIC RECORD The evaluation of a record is made by measuring the difference in height of the two peaks in the microphotometer trace, the one due to the magnesium line, 27958, the other due to the
graph, the speed of the photographic plates, and the intensities of the spectrum lines to be compared. PHOTOMETRY OF COMPARISON PAIR. After the spectrum plate has been processed in the dark room and dried, it is ready for the photometering operation. The portion of each spectrum which contains the magnesium line, 27958, and the bismuth line, 27808, is marked so as to be readily observed without the aid of a magnifier. Figure 1 is a reproduction of a portion of a spectrum plate showing the spectra of three standard solutions corresponding to 0.027,0.009, and 0.003 per cent magnesium. The bismuth line, 2780A, has equal intensities on the three spectra, since the bismuth is present in equal amounts in the three standard solutions. The magnesium line, 27958, shows a gradation in intensity. The plate is mounted on the carriage of the microphotometer and the marked portion of one spectrum is brought into the illuminated field and adjusted so that the lines are vertical and parallel with the slit aperture before the photoelectric cell. They must, of course, be sharply focused on this slit diaphragm, With the aperture covered so that no light can reach the receiving surface, but with all electrical connections closed, the galvanometer mirror is adjusted to bring the recording light beam to a marked zero position on the front of the holder which is to carry the bromide paper. The plate is then moved until the image of the bismuth line, 2780A, is nearly up to the photoelectric-cell slit. The motor drive is started and the line image permitted to cross the slit while the throw of the light beam from the galvanometer mirror is closely observed, and an iris diaphragm on the plate-illuminating lamp is adjusted so that the maximum throw of the galvanometer beam for the 2780 line brings it to a marked point exactly 80 mm. from the zero (dark) position. This may require several trials. (This distance of 80 mm. from the zero applies to the instrument now in use in this laboratory and to this particular determination. It may be varied as required for other instruments and other analyses. The essential point is to set the adjustment so that the peak in the record
FIGURE4. PHOTOMETRIC RECORDOF FOUR SPECTRA FROM SAMPLES OF SAMEMATERIAL
bismuth line, 27808, and correlating this distance with concentration. Table I gives the measurements made on this record. The definite relation between per cent magnesium and difference in height between the magnesium and the bismuth peaks is determined by measurements of records made from the spectra of solutions prepared for calibrating purposes and
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INDUSTRIAL AND ENGINEERING CHEMISTRY
containing known amounts of magnesium. The data from several plates are averaged and used in the preparation of a calibration curve on semi-logarithmic coordinate paper, with linear measurements of differences between peaks on the direct scale and corresponding per cent magnesium on the logarithmic scale. This yields very nearly a straight-line graph which may then be used for interpolation in evaluating the results on analytical samples.
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TABLE111. DATAOBTAINEDFROM FOURSPECTRA No.
OF
DIFFERENCE BETWEEN SPECTRUM PEAKS Mm. 1 2 3 4
41 38 41 39
MAQNESIUM % 0.0097 0.0106 0.0097 0.0103 Av. 0.0101 f 0 0005
The maximum deviation of the analyses from the mean is about one part in twenty. I n routine practice the error TABLEI. MEASUREMENTS OF RECORD OF MAGNESIUM-BISMUTH ranges from one part in twenty to one part in ten. COMPARISON PAIR With less expenditure of time greater reliability can be DIFF.BETWEEN RECORDNo. MAQNESIUM PEAKS obtained with this method than with the other method deMm. % scribed by one of the authors (2). Thus, four check analyses 1 0.027 4 2 are taken on one plate consisting of four exposures. By the 0.009 44 3 84 0.003 older method four check analyses require four plates of five exposures each. ACCURACY OF METHOD The chief error in the method seems to be connected with The recording microphotometer has been carefully con- the excitation of the spectra. The arc between graphite structed and the technic of using it perfected so that the error electrodes is extremely variable and difficult to maintain to be expected from this step of the procedure is less than the centered on the slit. If, during the photography of one of the other errors present. Figure 3 shows five successive records four check spectra, the arc becomes most erratic while much of the same pair of lines from the same spectrum. The of the magnesium is being volatilized into the arc, and during the photography of another spectrum the arc remains steady differences between peaks are given in Table 11. or becomes erratic only after the magnesium is almost entirely expelled from the electrode, the two spectra may not check BETWEEN PEAKSFROM SAMEPAIR TABLE11. DIFFERENCE each other. It is wise, therefore, to take as many as four OF LINES check spectra in order to give a higher probability of correct(Five successive records) RECORDNo. DIFB.BETWEEN PEAKS ness to the average analysis obtained. Mm.
1 2 3 4 6
24.7 26.0 24.3 24.0 25.0 Av. 24.6 =t0.6
I n the routine practice of this method, four electrodes containing the sample to be analyzed are prepared. The four spectra are photographed on the same plate and a photometric record is made of each. ‘ The data obtained from a typical case are given in Table 111. The photometric record is shown in Figure 4.
ACKNOWLEDGMENT The authors are deeply indebted to M. L. Fuller for his painstaking efforts in editing the material. LITERATURE CITED (1) Gerlach, W., and Schweitzer, E., “Die Chemische Emissionsspectralanalyse,” Leopold Voss, Leipzig, 1930. (2) Nitchie, C. C., IND.ENO.CHEM.,Anal. Ed., 1, 1-18 (1929). (3) Sohweitzer, E.,2.anorg. allgem. Chem., 165, 364 (1927).
RECEIVED June 12, 1931.
.Analysis of Nitrous Oxide by Solubility in Water ALBERTL. CHANEYAND CHARLESF. LOMBARD, Los Angeles County General Hospital, Los Angeles, Calif.
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LTHOUGH the U. S. Pharmacopeia does not specify a t present the percentage purity of nitrous oxide . suitable for anesthesia, nitrous oxide is only effective as an anesthetic a t concentrations of 90 per cent or higher, and it is therefore desirable a t times to ascertain the actual nitrous oxide content of such gas. As has been shown recently by Bennett (I), the principal impurity, nitrogen, may sometimes be found in concentrations of 10 per cent or more in the gaseous phase of full cylinders of commercial nitrous oxide. After some consideration of the various physical and chemical methods of determination of nitrous oxide, such as explosion with hydrogen, fractionation a t low temperatures, or “washing out” with water, as described by Bennett, a modification of this last method was found to be both accurate and rapid.
METHOD The modified apparatus for absorption of nitrous oxide in water is shown in Figure 1. The method differs from that described by Bennett in using water saturated with air, nitrogen, or oxygen in place of air-free water for absorption, and also by introducing a correction factor for the effect of this dissolved gas, instead of a graphical calculation of the percentage of nitrous oxide. A 10-cc. sample of gas is introduced through stopcocks 8, and 8 2 into the calibrated 10-cc. pipet P , and the pressure and volume adjusted by means of the mercury leveling bulb L. If the pipet is washed out with water before filling with the gas, sufficient water remains on the walls of the pipet and the mercury surface to keep the sample saturated with water vapor. Water saturated with air is then admitted from a flask and siphon, and is allowed to flow through the pipet at
