INDUSTRIAL A N D ENGINEERING CHEMISTRY
January 15, 1929
A quartz condensing lens serves to concentrate the light from the arc on the slit and to exclude that from the incandescent carbons, whose strong continuous spectrum would be objectionable, Suitable adjustments are provided, which permit the arc to be moved both horizontally and vertically so as to keep it properly centered during the exposure. Constant arc length is insured by keeping the tips of the electrode images always a t the same points above and below the aperture in the slit diaphragm. With a reasonably constant line voltage in the power supply, this control of the arc length is all that is necessary to maintain constant current and voltage drop in the arc. I n order that the operator's attention may be concentrated on keeping the arc properly centered, an automatic timing device has been installed. This consists of an electric clock so connected through a system of relays that it operates only while the arc is burning, and extinguishes the arc as soon as a predetermined exposure time has elapsed. This is particularly valuable in photographing the spectra of very volatile materials, which often yield almost explosive arcs, difficult to keep burning continuously. If it were necessary for the operator to keep track of the intermittent exposures that result under such conditions, he could not give proper attention to the centering of the arc, which is one of the most important factors in insuring that all spectra shall be accurately comparable. Using a large Littrow-type quartz spectrograph, with an objective of 70 mm. aperture ando170 cm. focal length, the spectrum from A2350 to A3400 A. is photographed on a plate 10 inches (25.4 cm.) long. This is the spectral range that has been found the most generally useful for much of the work that is encountered in the average metallurgical laboratory. Other ranges may be utilized, if desired, by a simple change in the adjustment of the instrument, but experience has shown that a t least 95 per cent of the work can be done within these limits. Persistent lines of most of the elements are found there, and the dispersion is great enough to insure accurate identification of lines. The most complete and accurate lists of lines are available for this region, and ordinary plates are sensitive to these wave lengths without special treatment, such as bathing in sensitizing dyes or fluorescent oils. Smaller instruments can be used for a great deal of work. Several makers put out quartz spectrographs in which the full length of the photographable spectrum is included within the length of a single 10-inch (25.4-cm.) plate, and excellent results can be obtained with them on many materials. Occasions do arise, however, when only the larger instrument
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can resolve closely coincident lines of two elements, which must be separated to permit the certain identification of one constituent and its estimation, free from the possibility of error due to an overlapping line of another. I n general, therefore, any laboratory which has a large amount of work adapted to the spectrographic method will find that the additional cost of the larger instruments is fully justified by the greater certainty of the results which may be obtained and by the wider range of possible applications. Three-minute exposures are given with most materials, using an arc carrying 10 amperes with a 60-volt drop across the arc terminals. This length of time is sufficient to insure the complete evaporation of the sample, or at least the expulsion of all of the constituents to be determined, It is quite necessary that none be left in the electrode, otherwise erratic results will be obtained. Slow photographic plates give the best results. Their fine grain makes it easy to compare line intensities, and their freedom from fog is an important consideration, particularly when comparing extremely weak lines. Conclusion
No hard and fast rules can be given for methods applicable to the determination of all elements in all kinds of materials. Some of the general methods have been sketched, which have been found useful in a laboratory devoted mainly to the study of problems connected with the production and use of zinc and zinc-bearing materials. Each type of analysis requires individual study to find the most suitable details of technic. The main purpose of this paper is to call to the attention of the chemists of this country the apparently littleappreciated fact that in the spectrograph they have a powerful tool, capable of yielding information of the greatest value, with a speed and certainty that cannot be matched by any other means. A few laboratories have installed the necessary equipment and are finding new uses for it almost daily. In that of The New Jersey Zinc Company several hundred spectrographic analyses are made each month, mostly quantitative estimations, and many of them are daily routine and control analyses. Acknowledgment
Grateful acknowledgment is made to George W. Standen, whose painstaking care and attention to details have been important factors in the progress of this work.
Simple Pressure Regulator for Vacuum Distillations' Henry L. Cox* MELLONINSTITUTE
O F INDUSTRIAL RESEARCH, UNIVERSITY OF PITTSBURGH, PITTSBURGH,
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HE chemical literature of the past two decades contains numerous descriptions of devices designed to maintain a constant pressure in a vacuum distillation apparatus. Most of these devices are not entirely satisfactory in that they are not sufficiently accurate for the intended purpose or the construction is so complicated as to discourage use in all except the most, exacting investigations. Several of them employ mechanically or electrically operated valves requiring a high precision in their manufacture and subject to the 1
Received August 3, 1928.
* Senior Industrial Fellow, Mellon Institute of Industrial Research.
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disadvantages inherent in such devices-for instance, corrosion and sticking or breakage. The apparatus described in this paper is to a large measure free from most of these disadvantages. It contains no valves or moving parts other than an efficient motor-driven pump and a relay, the armature contacts of which are of sufficient size to carry the electric current required to operate the motor. The apparatus is entirely automatic, requires little attention, and maintains any desired pressure constantly within *O.l mm. of mercury. A closed-arm mercury manometer containing a sealed-in
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A,YALYTIC'AL EDITIO-AT
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RELAY IIOYOLT LINE
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v MOTOR
contact and an adjustable contact is connected in series with a relay and a battery as shown in the accompanying figure. The armature of the relay is connected to the motor which drives the vacuum pump. When the manometer circuit is closed by the mercury rising t o the end of the adjustable contact, the relay is actuated and the motor is stopped. When the pressure within the system rises, thus breaking the manometer circuit, the pump is started and continues t o operate until the pressure is reduced t o the desired magnitude.
T'ol. 1, s o . 1
Satisfactory regulators have been constructed using commercial laboratory pumps driven by '/c-horsepower, 110-volt, a. c. motors. In the writer's laboratory, one of these maintained a constant reproducible pressure of 15 mm. with the pump operating less than 1per cent of the time. Any desired pressure between atmospheric and approximately 2 to 3 mm. may be maintained if care is taken t o prevent leaks in the system. Obviously, this device is limited to the use of mechanically operated pumps which may be stopped without allowing air to leak back through the pump mechanism into the system. The manometer should have an internal diameter of about 8 mm. in order t o minimize the curvature of the mercury meniscus. The use of a reservoir between the pump and the distillation system reduces the fluctuation in pressure to such an extent as to render it almost unobservable on the manometer. Obvious changes in the construction and the use of a pressure pump will enable pressures above atmospheric pressure to be readily obtained. The apparatus as described was designed by the writer when a t the laboratories of the Standard Oil Company (Indiana) a t Whiting, Ind., in 1923, and has given satisfactory results during the past four years.
Analysis of Maple Products',' D. E. Fowler and J. F. Snell MACDONALD COLLEGE, MCGILLUNIVERSITY, MACDONAL COLLEGE D P. O., P.
X-Study
Q.,C A N A D A
and Modification of the Canadian Lead Method
ETHODS for the detection of adulteration of maple products, dependent on measurements of the quantity of precipitate produced by treatment of the diluted sirup with basic lead acetate, were first proposed in 1904 by Jones3 and by Hortvet14who measured the volume after centrifugal collect,ion. In 1906 the more accurate methods at present recognized by the L4ssociat'ionof Official Agricultural Chemists3 were originated by Winton and Kreidern and by t'he chemists of the Laboratory of the Canadian Inland Revenue Department17particularly A. valin. Studies on three sirups by Snell and Scott8indicate that in falling off more rapidly in adulterated sirups than the maple content, the Canadian lead value has an advantage over all the other values used for the detection of adulteration with refined sugar. But that the method lacks in precision, as evidenced by comparison of duplicates, is well known,8 mean differences of 0.10 and 0.09 between duplicates by a single
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1 Received August 16, 1928. For reference to previous papers, see IND.ENG.CHEM.,19, 275 (1927). 2 T h e experimental work reported and much of the comment are derived from a thesis presented b y M r . Fowler t o t h e Faculty of Graduate Studies and Research of McGilI University in M a y , 1928, in partial fulfilment of t h e requirements for t h e M . Sc. degree. 3 Jones, Vermont Agr. Expt. S a . , 17th Ann. R e p t . , 1903-4, p. 446. 4 H o r t % e t ,J . Am. Chem. Soc., 26, 1523 (1904). :Assocn. Official Agr. Chem., Methods, 1926, p. 204. I n 112 t h e quantity of water t o be used in t h e preparation of the basic acetate solution should be 600 cc. e Winton and Kreider, J . Am. Chem. Soc., 28, 1204 (1908). 7 Laboratory Inland Revenue Dept. (Ottawa), Bull. 120 (1906); 140 (1907). T h e Food and Drugs Laboratories of t h e Department of Health are successors t o this laboratory. 8 Snell and Scott, J. IND. ENG.CHEM.,5, 993 (1913). Laboratory Inland Revenue Dept. (Ottawa), Bull. 140 (1907); 228 (1911).
observer, of 0.13 between the means of two observers, of 0.16 between duplicates in another series of experiments (printed figures give 0.19) and occasional differences as great as 0.20 (equal t o 10 per cent in a lead number of 2.0) being found. The experiments of Valin, reported by Snel1,lo indicated that actually different quantities of lead mere precipitated in the duplicate determinations. On the other hand, Lancasterll has expressed the opinion that the variations between duplicates are probably due mainly t o irregularities in the washing. I n the present work the effectsof temperature and quantity of reagent, as well as methods of washing, have been studied with a view t o improving this simple and useful determination. Materials The lead subacetate solution was made from Horne's salt according to the A. 0. A. C. directions. It contained 0.2243 gram lead per cubic centimeter. The ratio of neutral to basic leadlo was 1.88. The sirups were collected in Ontario and Quebec by district representatives of the Provincial Departments of Agriculture, instructed to collect only samples of unquestionable purity. A few (4 to 9) were supplied by A. Valin, analyst of the Dominion Department of Health in charge of the Montreal Laboratory. Time Required for Maximum Precipitation Except for the time of standing, the prescribed method6 was closely followed. The results are recorded in Table I. Except in sirup 2 a t one-half hour, the results show no varia10 11
Snell, J . Assocn Oficial A g u Chern , 4, 428 (1921). Lancaster, Ibzd , 8, 372 (1926).
