Quantitative Spectrochemical Analysis of Dilute Solutions A Safe Alternating Current High-Voltage Arc Circuit A. E. RUEHLE AND E. K. JAYCOX Bell Telephone Laboratories, New York, N. Y.
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INCE the early days of chemical analysis with the spectrograph the greatest difficulty has been to obtain
A safe and convenient alternating current high-voltage arc circuit together with its tie-in with the ordinary direct current arc and condensed spark circuits is described. The alternating current arc is a reproducible source for determining barium, strontium, tellurium, and phosphorus in dilute solutions. A precision of 50 to 100 parts per thousand was obtained using the comparison standard method.
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reproducible spectrograms on identical samples. Meggers (6) introduced the practice of photographing the spectra of a graded series of standards on the same plate with the sample and thus reduced to a minimum the errors due to photography. Gerlach's ( 3 ) concept of homologous pairs was another step forward, in that it reduced errors due to variable excitation. These workers, however, used spark excitation, which is not sufficiently sensitive t o meet the demands of many modern analyses. The direct current arc has been used in much recent work
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T R A N STAT V 0 LTAG E REG U L A T 0 R 1.65 KVA. 0 - 6 6 V RANGE BRUSH AMPS 2 5
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FOR ALTERSATING AXD DIRECT CGRRENT ARCSAND CONDENSED SPARK FIGURE1. WIRINGDIAGRAM
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MAY 15, 1940
ANALYTICAL EDITION
for both “comparison standard” and “internal standard” methods. While highly sensitive, the direct current arc is somewhat unreliable for comparison standard methods, showing a n average error of * 10 to 20 per cent of element determined. When internal standardization is employed this error can be reduced, especially when only the straight-line portion of the characteristic curve of the plate is used. For best results i t is necessary to calibrate the response of each plate ( 2 ) in order to use only the straight-line portion of the curve or to correct for deviations from it. Thus the procedure becomes somewhat involved if high precision is to be attained for determination of the smaller amounts of the metallic ele, . ments.
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the condensed spark; consequently, extra precautions must be taken in making it safe for regular use. The authors’ circuit and its tie-in with the direct current arc and the spark circuits also used in this laboratory are shown in Figure 1. Both transformers are caged in with a grounded expanded metal guard beneath the viorking table. The arc and spark stand is similarly caged. Entry is effected for changing electrodes, etc., through a door which mechanically obstructs the main alternating current switch when open. The direct current can be used wit,h the door open. All secondary circuit controls in the alternating current lines are eliminated. The spark is controlled by resistance in series with the primary. The alternating current arc is controlled for both voltage and current by a transtat in the primary line. The electrode separation can be adjusted while operating by means of an insulated flexible shaft on the ground side, the length of the gap being measured by an optical gage. The clock indicated is an inexpensive Telechron, the second hand of Tvhich times the direct current arc exposures by turning only when the arc current is passing.
Results I n Figure 2 are shown typical working curves for the comparison standard method using this source. Aliquots of solutions of the concentration indicated were dried on flat-top graphite electrodes of 0.19 or 0.25 inch in diameter. Both upper and lower electrodes were first coated with a waterproofing material (collodion is suitable) and preheated to 100” C. before the solutions were transferred, in order to prevent their soaking into the electrodes. Exposures were for 120 seconds, during which time the lovier electrode was rotated at 600 r. p. m. to steady the mean position of the arc column. Densities were measured with a projection densitometer made by modifying a Moll instrument. In practice, the maximum sning of the galvanometer is set to a standard value by adjusting the area of the photocell surface illuminated, complete blackness corresponding to zero sm-ing, so that the galvanometer reading, G, is proportional to the transmission.
As can be seen from the working curves, t’he precision of a comparison standard met’hod with the alternating current arc is * 5 t’o 10 per cent, which is about as good as the authors have been able to secure on run-of-the-mill work with internal standardization and the direct current arc source (4). Undoubtedly some of the improvement is due to being able to measure density under the conditions of low background intensity characteristic of this source. Whether the alternating current arc will give better precision than the direct current arc when using internal standardization is still under investigation. Literature Cited
FIGERE 2. TYPICAL WORKING CURVES,COMPARISON STANDBRD h l E T H O D
(1) Duffendack, 0. S., and Wolfe, R. A,, IKD, ESG.C ~ ~ % f . , A Ed., nal. 10, 161 (1938).
Several workers have recently reported very high spectral sensitivity for metals in the 2000-volt alternating current arc. Duffendack and Wolfe (1) determined minute traces of various metals in caustic liquors, and Owens (6) reported greater sensiti\
