Continuous Ultrasonic Nebulization and Spectrographic Analysis of Molten Metals SIR: Historically, the spectrographic analysis of metals has been based primarily on using various electrical discharges to vaporize metal from pin or disk electrodes and to use the same discharge to excite the atomic spectra of the vaporized sample. Although remarkable speed, accuracy, and precision have been achieved, this approach has several basic limitations familiar to all emission spectroscopists. The primary limitations are the time required to take the sample and to cast, cool, and fabricate the solidified metal into appropriate electrode forms. The direct examination of the metal in its molten state offers attractive possibilities of overcoming these limitations and of providing analyses with sufficient speed to allow continuous composition control. Several schemes for performing molten metal analysis have already been suggested (1-8). However, the problems encountered in actually performing the analysis under the high temperature and corrosive environment of a molten metal stream have not yet been resolved. In this paper, a sampling and excitation technique which circumvents some of these basic problems is described. Ultrasonic energy is employed to nebulize the molten metal. The finely divided aerosol quickly solidifies into a metal dust which is transported to an excitation source by a flow of gas. In the high temperature environment of an appropriate excitation source, the metal dust is vaporized and its constituents are continually excited.
ARGON SCREEN
ARGON
G AEROSOL
ULTRASONIC PROBE
Figure 1. Ultrasonic nebulization and excitation assembly BASIC PRINCIPLES
When ultrasonic energy encounters the surface of a liquid, capillary waves are produced. According to Kelvin (9), the wavelength of these waves is = ( 3 ’ 3
where X CT
=
wavelength
= surface tension
p = liquid density f = ultrasonic frequency
(1) V. N. Balandin and S . L. Mandelshtam, Zacodsk. Lab., 23, 545 (1957). (2) Federated Republic of Germany, Patent No. 1,006,039 CI. 421, Gr. 3/08 24.3 (1960). (3) 0. 1. Nikitina and S . D. Gerasimova, Sb. Tr. Ukr. Nauchn-lssled. Inst. Metal, No. 8, 361 (1962). (4) I. M. Kustanovich, Fiz. Sbornik. L’vov Univ., 4, 451 (1958). ( 5 ) A. B. Shaevich and S . B. Shubina, Zavodsk. Lab., 28, 447 (1962). (6) A. B. Shaevich and S. B. Shubina, Trans. of 2nd Conf. Nouosibirsk, Siberian Brarich of Academy of Sciences, USSR, 1962, 352. (7) A. B. Shevich, S. I. Melnikov, and V. V. Danilevskaya, Zacodsk. Lab., 31,169 (1965). (8) E. F. Runge, R. W. Minck, and F. R. Bryan, Spectrochim. Acta, 20, 733 (1964). (9) J. W. S . Rayleigh, “The Theory of Sound,” Vol. II, Dover Publications, New York, 1945, p. 344.
If sufficient ultrasonic energy is applied, the capillary waves are ruptured at the liquid surface to produce droplets. The size of the droplets has been experimentally determined by Lang (10) to be: D = 0.34 X (2) where D = median droplet diameter X = capillary wavelength according to Equation 1. The gas flow necessary to transport the aerosol droplets must be in excess of the droplet terminal velocity, which can be readily calculated using Stokes’ law. EXPERIMENTAL
Among several schemes which can, in principle, be employed for nebulizing a molten metal (11-13) and exciting the emission spectrum of the aerosol, the simple device shown in Figure 1 has adequately demonstrated the practical feasibility of this approach. A 20-KH8 ultrasonic generator with probe assembly and amplifying step horn (Biosonik I1 Ultrasonic Probe, Bronwill was used to nebulize the molten Scientific, Rochester, N. Y.), (IO) R. J. Lang, J . Acoust. SOC.Am., 34,6 (1962). (11) T. F. Hueter and R. H. Bolt, “Sonics,” Wiley, New York, 1955. (12) J. R. Frederick, “Ultrasonic Engineering,” Wiley, New York, 1965. (1 3) B. Carlin, “Ultrasonics,” McGraw-Hill, New York, 1949. VOL 40, NO. 1, JANUARY 1968
247
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Figure 3. Analytical curve for the determination of copper in tin-base solder
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Figure 2. Analytical curve for the determination of arsenic in tin-base solder
to photograph the spectra. Microphotometry and intensity ratio calculations followed standard practices (20). RESULTS AND DISCUSSION
metal. When the flat tip of the step horn was brought into contact with the molten metal surface, aerosol formation occurred. In the assembly shown, a 2-liter/minute flow of argon transported the solidified metal dust into an induction coupled plasma (14-19). The characteristics of the plasma were as follows: POWERSUPPLY. Lepel High Frequencies Laboratory, Woodside, N. Y.,Model T-2.5-1-MC2-J-B generator, frequency variable from 23-48 MHz and attached tuning and coupling unit; 2.5 kW, nominal output. The frequency was set at 30 MHz. COIL. Two turns of 5-mm 0.d. copper tubing, 24-mm i.d. COOLANTTUBE. Clear fused quartz, 18-mm i.d., 20-mm o.d., 12.5-cm total length, extending 8 mm beyond the coil. PLASMATUBE. Clear fused quartz, 13-mm i.d., 15-mm o.d., 13-cm total length, terminating 2 cm below top of coil, centered within coolant tube. AEROSOL TUBE. Borosilicate glass, 5-mm i.d., 7-mm o.d., terminating at bottom of plasma tube. BASECONSTRUCTION. Brass; double O-ring seals on each quartz tube; optional screen to ensure near laminar flow. FLOW RATESTO DISCHARGE. Coolant, 20 liters/minute of Ar; plasma, 0.6 liter/minute of Ar; aerosol, 2.0 liters/minute of Ar. IGNITION,Graphite rod, not grounded; lowered into high field region until plasma is formed, then withdrawn. A uniform flow of nebulized sample into the plasma for over an hour was achieved in this manner without adjustment of the liquid metal surface to the probe tip. For the early exploratory experiments involving observations on the nebulization and excitation of Woods metal, a Jarrell-Ash Model 82000, OS-meter Ebert mounting scanning spectrometer was used. The photocurrent from an EM1 6255B photomultiplier was recorded with a Leeds and Northrup Speedomax recorder. A Hilger-Engis Model 1O00, 1-meter Czerny-Turner mounting, scanning spectrometer with a photographic attachment was employed for the quantitative analytical calibrations. Kodak SA #1 plates were used (14) T. B.Reed, Intern. Sci. Techno/., 6 , 42 (June 1962). (15) T. B. Reed,J. Appl. Phys., 32,821 (1961). (16) T.B. Reed, Proc. Natl. Electron. Conf., 19, 654 (1963). 37,920 (1965). (17) R. H.Wendt and V. A. Fassel, ANAL.CHEM., (18) Ibid., 38, 337 (1966). (19) R. H. Wendt, Ph.D. Thesis, Iowa State University, Arnes, Iowa, 1965.
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ANALYTICAL CHEMISTRY
No difficulties were encountered in reproducing spectral intensity ratios when molten Woods metal-an alloy of bismuth (50%), lead (25%), cadmium (12.5%), and tin (12.5%) - wasFxamined. For this alloy 21 repeated scans of the Sn 3034.1 A and Bi 3024.6 A lines over a period of one hour gave a relative standard deviation of 4.9% for the Sn 3034.1 &Bi 3024.6 A intensity ratio. When higher melting point alloys were examined, the probe assembly tended to overheat during continuous nebulization periods exceeding 15 minutes. These observations suggest that intermittent on-off operation-for periods on the order of 15 to 30 seconds-may be necessary in some applications. This mode of operation presents no problems when integratingtype, direct-reading spectrometers are employed for quantitative measurements of intensity ratios, because integration periods of this magnitude are commonly used. With the present spectroscopic experimental facilities, quantitative intensity ratio measurements could only be performed by photoelectric, spectral-scanning techniques or by photographic recording of the spectra. Because the spectralscanning approach required continuous nebulization periods longer than 15 minutes when the spectral lines of interest covered a wide wavelength region, recourse was taken to photographic recording for most of the quantitative experiments. In order to demonstrate the applicability of the ultrasonic nebulization technique to the quantitative determination of constituents in a molten alloy, a series of tin-base solder standard samples (Standard Numbers 583-87, 584-69,585-75, P939-14, P940-13, and P941-13 obtained from Morris P. Kirk & Sons, Inc., Los Angeles, Calif.) were examined. The melting points of these alloys were 190" f 10" C. The exposure periods for this alloy varied from 20 to 30 seconds. The analytical curves shown in Figures 2 and 3 are typical of those obtained for the impurities in the series of standards examined. The average relative standard deviations for 10 observations at each of the concentrations plotted in the figures were 6.1 % and 12.2%, respectively,for copper and arsenic. (20) American Society for Testing and Materials (ASTM), Cornmittee E-2,"Methods for Emission Spectrochemical Analysis," 4th ed., Philadelphia, 1964, pp. 56-90.
For the tin-base solder alloy, Stokes’ law calculations showed that solidified particles with a maximum diameter of 12 to 14 microns should be transported to the plasma by the 2 liters/minute argon fiow. These predictions were confirmed by direct measurement of particle size distribution. The majority of particles were 12 microns or less in diameter, and the largest particles observed were in the 14- to 15-micron diameter range. Undoubtedly, larger aerosol particles were produced but were not transported to the plasma by the gas flow employed. The flow of powdered aerosol to the plasma was not determined, but was estimated to be in the 1-10 mg/minute range. With appropriate adaptations of this or other ultrasonic nebulization techniques, continuous composition monitoring of molten metal in either static or flowing systems is now in the realm of possibility. Other materials in liquid form, such as
oils, paints, and process liquors, should be equally amenable to continuous analysis by this technique. ACKNOWLEDGMENT The authors thank Bronwill Scientific for the loan of their Biosonik-I1 Ultrasonic Probe, C. C. Hill for constructing the needed glassware, R. H. Wendt for performing preliminary studies on metal nebulization, and W. B. Barnett for his assistance in operating the induction coupled plasma. VELMER A. FASSEL GEORGE W. DICKINSON Institute for Atomic Research and Department of Chemistry Iowa State University Ames, Iowa RECEIVED for review May 29, 1967. Accepted October 30, 1967.
The Effect of Static Magnetic Field on Polarography SIR: Besides theoretical analysis of the Hall effect of magnetic field on the electrolytic ions in solution ( I ) , a few measurements of this effect have been reported (2, 3). However, no experiment has been done with respect to the effect of magnetic field on polarography. Antweiler tried but failed to observe this effect (4). This effect was examined with the use of the dropping mercury electrode polarograph, where magnetic field, H , ranging from 0 to 18,000 oersteds was applied perpendicularly to the electrodes. No effect was observed in the recorded polarogram with respect to the intensity of the residual current, half-wave potential, and the diffusion current, but a sizable effect was observed in the current intensities of the maximum waves of the first and the second kinds, .,i and L,, 2nd. Namely, when H is applied, ,i is reduced. Figure 1 reproduces the results of measurements in typical cases. (1) H. L. Friedman, J . Phys. Chem., 69,2617 (1965). (2) D. Laforgue-Kantzer,Electrochim. Acta, 10,585 (1965). (3) N. I. Pekhteleva and A. G. Smilnov, M a g n h . Gidrodinam., Akad. Nauk. Larv., SSR,1965(2),89; C.A., 64,279h (1966). (4) H. Antweiler, 2.Ekkrrockem., 44,836 (1938).
Table I. Chemical Systems Whose Polarograms Show (a) an Effect of Magnetic Field, and (b) No Effect of Magnetic Field Depolarizer Maxima (a) MnCh (I1 0) 2nd max NiCh (I1 0) 2nd max (11 0) 2nd max [CO(”3)61Ch 2nd max [Co(NH3)jCl]Clz (I1 + 0) cos04 (11 0) 2nd max 2nd max Cd(N03)~ (I1 0) FeC12 (I1 + 0) 2nd max AlC13 (111 0) 2nd max (111 0) 1st max Cr(NOd3 cuso* (I1 + 0) 1st max 1st max (NH&M070z4 (VI --c V) (1 0) 1st & 2nd TlCl (1 0) no maxima (b) HCI (111 11) no maxima [Fe(CN)J3(IV + ?) no maxima Th(SOn)z (111 0) no maxima Bi(NO& uozso4 (VI V) no maxima no maxima [CO(NH3)8]C13 (111 + 11) no maxima [Co(NH,)CI]Clz (111 + 11)
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Figure 1. Effect of magnetic field on polarogram Mn(I1) 0.5mM KCI 1M Table I shows the classification of chemical systems which do or do not show this magnetic field effect. This effect becomes unobservable even with the systems cited as (a) in Table I, when the concentration of the supporting electrolyte is much reduced. Surface active reagents such as polyacrylamide and gelatin suppress this effect linearly with their concentrations. Just as in the general case with no values do not show the root square magnetic field, the ,i dependence on the mercury reservoir height in magnetic field. The effect cited above will be useful for the elucidation of the mechanism of the oxidation and the reduction at the sucface layers of mercury electrode, and its application can be pursued. Further investigation is in progress. SHIZUOFUJIWARA HIROKIHARAGUCHI YOSHIOUMEZAWA Department of Chemistry Faculty of Science The University of Tokyo Hongo, Bunkyoku, Tokyo Japan RECEIVED for review August 18, 1967. Accepted September 25,1967. VOL. 40, NO. 1, JANUARY 1968
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