64
INDUSTRIAL AND ENGINEERING CHEMISTRY
Summary
A spectrophotometric study indicates that Saywell and Cunningham's method of using o-phenanthroline for the determination of iron is very satisfactory for small amounts. It is more sensitive than the more common methods, the range being from 0.10 to 6.0 p. p. m. in a 1.00-cm. comparison cell. Hydroxylamine is the best reductant studied. One-tenth milliliter of an aqueous 10 per cent solution is required to reduce each p. p. m. of iron completely from the ferric to the ferrous state. Sodium sulfite, sodium formate, and formaldehyde are unsatisfactory as reductants, owing to the formation of complexes with the ferric iron. Six milliliters of an aqueous 0.10 per cent solution of ophenanthroline are required to produce tlie maximuni color with 5.0 p. p. m. of iron. The color reaction conforms to Beer's law over tlie entire range of concentrations studied. Visual comparison is thus applicable. pH has no effect on the intensity of the color over the applicable range, 2.0 to 9.0. The color is stable and does not change over a peiiod of
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6 months, including 100 hours under ultraviolet radiation during an accelerated fading test. There are very few ions that seriously interfere with the production of the quantitative color reaction. A study of fifty-five possible interfering ions was made. Literature Cited (1) Blau, Monatsh., 19, 647 (1898).
(1.4) Hummel and Wllard, IND.E m . C m x . , Anal. E d . , 10, 13 (1938). ( 2 ) Mellon, "Methods of Quantitative Chemical Analysis," p. 413, New York, Macmillan Co., 1937. (3) Mellor, "Comprehensive Treatise on Inorganic and Theoretical Chemistry," Yol. XIV, p. 88, London, Longmans, Green &Co., 1935. (4) hlichaelson and Liebhafsky, Gen. Elec. Rev., 39, 445 (1936). ( 5 ) Sakwell and Cunningham, IND.EBG.CHEY.,Anal. Ed., 9, 67 (1937). (6) Smith, "Ortho-phenanthroline," p. 5 , Colunibus, Ohio, G. Frederick Smith Chemical Co., 1935. (7j Walden, Hammett, and Chapman, J . A m . Chenr. SOC., 53, 3908 (1931). RECEIVED December 28, 1937. Abstracted from a portion of 8 dissertation submitted by 1%'. B. Fortune t o the Graduate School of Purdue University in partial fulfillment of the requirements for the degree of doctor of philosophy.
Quantitative Spectrochemical Analysis Chemical and Metallurgical Applications J. S. OWENS X-Ray and Spectroscopy Department, The Dow Chemical Company, Midland, Mich.
Q
UASTITATIVE spectrochemical methods have been developed a t The Dow Chemical Company to a position of regular use for control analyses of many commercial products, including, among others, magnesium metal and its Dowmetal alloys, plastics, and pharmaceuticals. The speed and sensitivity of the spectrographic method contribute markedly also to the more rapid conclusion of research on new products and to the production of better materials through closer control of the manufacturing processes. The saving of time and expense and the particular fitness for the analysis of materials for elements present in extremely small concentrations constitute important advantages of the method. I n general, the method saves a considerable portion of the time and cost required for a chemical analysis. Moreover, the accuracy of spectrographic analysis of materials for elements occurring in concentrations of only a few thousandths of a per cent is extremely valuable, for in many instances this method provides the only practicable means of analysis. Suitable analytical spectral sources and technics are used for the analyses of different chemical and metallurgical materials to obtain maximum sensitivity, rapidity, and accuracy. The spectral sources used iiiclude condensed sparks, highvoltage alternating current ( 1 ) and direct current arcs, and the cathode layer of the direct current arc (6, 10). The quantitative analytical procedures are based upon the correct correlation of the concentration of an element in the specimen with the actual intensity of the radiation emitted by that element under controlled conditions of excitation in a luminous discharge ( 3 ) . These methods use internal standard elements (Q), intensity calibration of each plate by a
jtep-slit (5, II), and graphical conversion of microphotometer readings into percentage concentrations. Extremely pure graphite for use as spectroscopic electrode supports is obtained by the use of high-temperature heating in an evacuated furnace Several thousand quantitative determinations are ordinarily made each month in routine fashion upon chemical and metallurgical materials. The concentrations of the elements under analysis in these materials vary from 0.0001 per cent to several per cent. Quantitative Analysis i n Production Control The basis of the technic is the experimental determination of the relationship between the concentration of a constituent of a specimen and the relative intensity of selected lines of that constituent and of an internal standard element in the specimen. The internal standard element may be the major constituent of the specimen or may be an element added in constant concentration. The relative intensity is obtained by means of an intensity calibration pattern placed upon each plate with a step-slit according to one of the accepted methods (5, 11). S o one spectral source is best adapted to the analysis of all typeP of materials; hence the source used for the analysis of any specified material should be that one found by experiment to possess the optimum qualities for that analysis. The blackenings (defined as the difference between the peak of the galvanometer deflection of the microphotometer for the spectral line and the deflection for the transparent photographic plate) of the steps of the intensity calibration pattern and of the spectral lines are obtained with a photo-
FEBRUARY 15, 1938
ANAL\-TICAL EDITIOK
65
some or all of the alloying constituents and impurities in the following usual concentration ranges: j4
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1
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Manganeoe Silicon Copper Aluminum
0 0 0 0 ( 2
Per cent 001 to 3 001 t o 1 01 to 0 01 t o 0 0 t o 13
0
5 10 10 0
Zinc Cadmlum Calclum Iron Sickel
0 1 0 0 0
Per c e n t 10 t o 8 0 t o 6 06 t o 0 001 t o 0 001 t o 0
0 0 50 045 05
Solid alloy electrodes are used. The analyses for manganese, silicon, copper, aluminum, zinc, cadmium, and calcium are made with a condensed spark spectral source. The analyses for iron and nickel are made with a direct current arc source, for which the electrodes are supported in watercooled holders. TABLE 1. COMPAR-LTIVE ASALYSES 5Ianganese Spectrographic Chemical
%
FIGURE 1. AxiLi-rIc.iL CURVEFOR AXALYSISOF hfAGSESIUM A"LL0E.S FOR
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CURVEFOR ANALYSIS OF FIGURE 2. ASALYTICAL MAGXESIUY ALLOYSFOR ZIXC electric, nonrecording microphotometer. From the blackenings of the steps of this pattern a blackening-logarithm of intensity, characteristic curve for the plate is drawn. The relative intensity of the selected line pair is determined from this curve. This procedure, carried out for a series of specimens of known composition in which the concentration of the element under analysis varies over the desired range, yields a n analytical curve for the analysis for this element (9). Representative analytical curves for the analysis of magnesium alloys are shown in Figures 1, 2, and 3. The analytical curves made for each element under test serve as the sole basis for future analyses of similar specimens. The speed of routine analysis is increased by reducing to a single, graphical step the conversion of spectral line blackenings, obtained with the microphotometer, into percentage concentration of the element under analysis. Quantitative spectrochemical methods are used for production control analyses of several metallurgical and chemical plant products. Illustrations of these materials comprise magnesium metal and its alloys, plastics, and pharmaceuticals. I n addition to the analyses now in regular use, the development of analyses of other plant products is being carried on. MAGNESIUM METAL AXD ALLOYS. Control analyses of magnesium metal and of its alloys are made chiefly b y spectrochemical methods (8). These materials are analyzed for
0 0 0 0 0 0 0 0 0 0 0 0 1 1 1
125 14 17 166 20 152 179 22 23 26 23 273 08 20 43
O F hI.IGXESIE31 .&LLOYS
Zinr Spectrographic Chemical
%
%
%
0.118 0.14 0.16 0.160 0 17 0.160 0.175 0.21 0.24 0.26 0.27 0.280 1.07 1.23 1 49
1.07 1.12 2.13 2.23 2.64 2.45 2.98 2.81 2.77 2.82 2.76 3.01 2.92 3.95 4.09
1.09 1.09 2 13 2 17 2 59 2 62 2 72 2 72 2 74 2 77 2 89 3 04 3 24 4 20 4 20
The majority of the specimens to be analyzed are received by this laboratory in groups of ten or more. An average of four determinations is made upon each specimen. Under the present experimental conditions the time required for a duplicate determination is 5 man-minutes. This is roughly one-sixth of the time required for a chemical analysis. I n the usual concentration ranges of the elements under analysis, the accuracy of the routine spectrographic method is comparable with that of the routine chemical methods, except in the case of the analysis for high aluminum. I n this instance the small change of aluminum spectral line intensity with concentration decreases the accuracy t o some extent. However, the accuracy of the spectrochemical analyses for low copper and especially for nickel is considerably greater than that of the routine chemical methods. Table I shows comparative analyses for manganese and zinc by spectrographic and chemical methods made in ordinary routine practice. These analyses mere chosen a t
FIGURE3. ASALYTICALCURVEFOR ANALYSIS OF MAGNESIUM METAL .4ND ALLOYS FOR NICKEL
INDUSTRIAL AND ENGIIVEERISG CHEMISTRY
66
random from a list of about 100 for each, made over a period of 3 months. With the exceptions of the analyses for elements present in concentrations of the order of 0.001 per cent and for high aluminum, the average error of the spectrochemical analysis amounts to approximately 5 per cent of the amount present.
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a direct current arc. The error in these analyses does not exceed 10 per cent of the amount present. Accessory Technic The problems confronting an industrial spectrographic laboratory consist primarily of the development of analyses of greater sensitivity, rapidity, and accuracy, and of the practical use of these analyses on an economical, routine basis. The solutions of many of the problems which have confronted this laboratory have required the development of special equipment and technic.
PURIFICATION OF GRAPHITE E L E C T R O D E SA.c c u r a t e analyses of many materials for small concentrations of metallic or metalloid impurities require the use of very pure graphite electrode supports. Commercial graphite of spectroscopic quality contains appreciable amounts of iron, silicon, boron, magnesium, calcium, copper, aluminum, t i t a n i u m , and vanadium. The best grade of commercial FIGURE 4. SPECTROSCOPIC SOURCE EQUIPMEST graphite can be obtained only For production of condensed sparks, direct current arc, and high-voltage alternating cuirent arc a t considerable cost and even its purity is not completely PLASTICS. The spectrographic method is well adapted for satisfactory. A simple, effectivemethod of graphite purificaproduction control analyses of plastics for metallic impurities. tion has been developed in this laboratory which comprises heating a t a temperature of about 2500" C. in an evacuated By this method ethyl cellulose is analyzed for the following impurities in the concentration ranges given: furnace ( 7 ) . By this treatment graphite can be produced which contains practically no traces of any impurity, except Per cent Per cent boron, and is of higher purity than the best spectroscopic Iron 0 001 t o 0 02 Copper 0 001 t o 0 03 Nickel 0 001 to 0 02 Sodium 0 005 t o 0 50 graphite previously commercially available. SPECTRAL SOURCE DEVELOPMEET. I n order to aid in the The sample is prepared for analysis by digestion in concensolutions of several analytical problems, considerable work trated nitric acid and an internal standard element is added has been carried out on the development and refinement of to the resulting solution. A few drops of this solution are spectral sources most suitable for the analyses of different, evaporated on graphite electrodes and the spectrum of the types of materials in which the constituents occur in concendry salt residue is produced by excitation in a direct curtrations varying from 0.0001 per cent to several per cent ( 7 ) . rent arc. Sources now in use consist of condensed sparks powered by Under the usual conditions of analysis of a batch of six samples, the time required for one determination is about 17 man-minutes. The development of this analysis was carried out only t o the point a t which the accuracy required was obtained. This accuracy corresponds to possible errors of 10 per cent in the analysis for sodium and of 20 per cent in the analyses for the other metals. If required, these limits of error could be reduced by further refinement of technic. Styrene and other plastics are analyzed for metallic impurities in a similar manner. PIURMACEUTICALS. Control analyses for the production of sodium and potassium bromides provide an illustration of the use of spectrochemical methods in the manufacture of pharmaceuticals. The analyses of sodium bromide for potassium and of potassium bromide for sodium are carried out in the following concentration ranges. Potaasium Sodium
P e r cent 0 05 t o 2.0 0 01 t o 2.0
Small portions of an aqueous solution of the bromide, made up to a definite concentration, are evaporated to dryness upon graphite rods which then serve as electrodes in
FIGURE5. GRAPHICAL PLATECALCULATOR Coordinates of characteristic curve: ordinates, blackening: abscissas, logarithm of intensity
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,4NALYTIC4L EDITION
transformers yielding potentials up to 50,000 volts, the direct current arc, the cathode layer of the direct current arc, and the high-voltage alternating current arc. Figure 4 shows the equipment for these sources set up in a compact, switchboard-controlled form. Condensed spark spectra have been found to yield p e s t e r accuracy than direct current arc spectra in the analyses of magnesium base alloys for the alloying constituents, but direct current arc spectra are required for the determinations of traces of some impurities. Similar results have been obtained by other investigators in the analysis of alloy cait iron (IS). The applicability of the high-voltage alternating current arc for analytical purposes was shown by Duffendack and Thomson (1). It has been found, both in the laboratory of Duffendack and his co-workers (2, 1.2) and in this one ( 7 ) , that the sensitivity and accuracy of the high-voltage alternating current arc make this spectral source particularly well adapted to the analyses of many chemical materials. For the analyses of many industrial chemicals the sensitivity of the alternating current arc is comparable with or greater than that of the cathode layer of the direct current arc, while that of either is, in general, greater than that of the whole direct current arc. The advantages of the alternating current arc over the cathode layer for quantitative analysis consist of uniformity of spectral line intensity throughout the entire arc length ordinarily employed, much weaker background, and elimination of any optical system for accurately focusing a restricted portion of the arc upon the slit of the spectrograph. GRAPHICAL PLATE CALCULATOR.In order to increase the speed of quantitative analysis in which a n internal standard element is used, apparatus has been developed for graphical conversion of spectral line blackenings, obtained with a microphotometer, into percentage concentration of the element under analysis ( 7 ) . The calculator, illustrated in Figure 5, consists of a drawing board equipped with a straight edge constrained t o move only in a direction perpendicular to its length. An analytical scale is prepared from the analytical curve for the analysis of the material for each element by projecting the ordinates in percentage concentration upon the abscissas in logarithm of relative intensity of the comparison spectral line pair. The preparation of an analytical scale for the analysis of magnesium alloys for zinc is shown in Figure 6. The percentage concentration of each element under analysis is obtained by the procedure presented herewith.
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The appropriatr analytical scale is placed upon the straight edge so that a fiducial mark on the scale intersects the characteristic curve of the plate, which is plotted on cross-section paper fastened to the draming board, at the blackening of the selected line of the internal standard element. The straight edge is then moved until the scale intersects the characteristic curve at the blackening of tihe selected line of the element under analyeic. The value read from the scale at this point of intersection is the percentage concentration of the latter element. The complete analysis of the specimen is made by repeating this procedure with the analytical scales prepared for the analyses for the different elements under test. Use of this apparatus in routine control analyses has shown that i t saves a t least one-half the time required by arithmetical conversion of microphotometer readings into percentage concentrations, and that i t reduces the probability of error.
Summary The quantitative spectrochemical method has proved its worth as an analytical tool in the metallurgical and chemical industries. Important advantages of the method include economy of time and material, and fitness for analysis of materials for elements present in extremely small concentrations. As illustrations of the several thousand quantitative determinations made each month, the analyses of magnesium metal and its alloys, plastics, and pharmaceuticals are briefly described. The economy of the method is shown by the fact that the average time required for one determination, taking into account the analyses of all the chemical and metallurgical materials, is only 7 man-minutes. The corresponding time required by chemical methods is a t least four times as great. The concentrations of the elements under analysis vary from 0.0001 per cent to several per cent. The development of special equipment and technic, important for the attainment of greater sensitivity, accuracy, and rapidity of analyses in routine industrial use, is outlined.
Acknowledgment The writer wishes to express his appreciation to the staff of the X-Ray and Spectroscopy Department, particularly J. D. Hanawalt, Director, T. &I.Hess, L. G. Reinhardt, R. G. Fowler, and J. S. Peake, for valuable suggestions and experimental assistance in this work.
Literature Cited (1) Duffendack, 0. S., and Thomson. K. B., Proc. A m . Soc. Tesling Materials, 36, Part 11,301-9 (1930). (2) Duffendack, 0. S., and Wolfe, R. A,, to be published. (3) Duffendack, 0. S.,Wolfe, R. A,, and Smith, R. W., IND.ESG. CHEM.,Anal. Ed., 5 , 226-9 (1933). (4) Gerlach, W., 2.anorg. allgem. Chem., 142, 383-98 (1925). Hansen, G., Z.Phusik, 29,356-9 (1924). Mannkopf, R., and Peters, CI., Ibid., 70, 444-53 (1931). Owens, J. S.,Metals & Allog8, Vol. 9, 15-19 (1938). Owens, J. S.,and Hess, T. M., Proc. Am, SOC.Testing Materials, 35,Part 11, 61-70 (1935). and Neuhausser, A,, 2.angew. Chem., 41, 1218-22 Scheibe, G., (1928). Strock, L. W., "Spectrum Analysis with the Carbon Arc Cathode Layer," London, Adam Hilger, Ltd., 1936. Thomson, K. B.,and Duffendack, 0. S., J . Opticd SOC.Am., 23, 101-4 (1933). Thomson, K. B., with Duffendack, 0. S., and Sawyer, R. A., "Spectroscopy in Science and Industry," Proceedings of the Fifth Summer Conference in Spectroscopy and Its Applications, Massachusetts Institute of Technology, Technology Press and John Wiley & Sons (in press). Vincent, H. B., and Sawyer, R. A , , J . A p p l i e d Phys., 8 , 183-73 (1937).
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FIQDRE6. PREPARATION FOR GRAPHICAL PLATECALCUWTOR OF ANALYTICAL S c m FOR ANALYSISOF MAGNESTUM ALLOYSFOR ZINC
RECEIVED December 16, 1937. Presented before the Division of Physioal and Inorganic Chemistry, Symposium on Quantitative Spectrographic Analysis,a t the 94th Meeting of the American Chemical Society, Rochester N. Y., September 6 to 10, 1937.
