V O L U M E 2 6 , NO. 1, J A N U A R Y 1 9 5 4
181
(130) Seliger, H. H., P h y s . Rea., 88, 408 (1952). (131) Seliger, H. H., and Cavallo, L., J . Research Natl. B u r . Standards, 47, 41 (1951). (132) Serrano, E. S.,BoZ. radioactividad ( M a d r i d ) , 24, 42 (1951). (133) Shirley, R. L., ANAL.CHEM.,22, 732 (1950). (134) Siksna, R., Arkiv Geofysik., 1, 123 (1950). (135) Spevack, J. S.,U. S.Patent Appl. 786,880; Oficial Gaz., 639, 1662 (1950). (136) Stanford University, “Annual Review of Kuclear Science,” Vol. I, 1952. (137) Stevens, G. W. W., Brit. J . Radiol., 23, 723 (1950). (138) Stevens, W. H., Pepper, J. M., and Lounsbury, M. J., Chem. Phys., 20, 192 (1952). (139) Straub, C . P., Nucleonics, 10, KO.1, 40 (1952). (140) Straub, C. P., Morton, R. J., and Placak. 0. R . , .I. Am. Water W o r k s Assoc., 43, 773 (1951). (141) Siie, P., B u l l . soc. chim. France, 1951, D9.
(142) Tompkins, P. C., Biszell, 0. M., and Watson, C. D., Nucleonics, 7, No. 2, 42 (1950). (143) Wahl, A. C., and Bonner, N. A., ed., “Radioactivity Applied to Chemistry,” New York, John Wiley & Sons, 1951. (144) Walker, D., Reu. Sci. Instr., 22, 607 (1951). (145) Way, K., Fuller, G., Wood, M., Thew, K., and Jurgens, .1., Natl. Bur. Standards, Suppl. 2 to Circ. 499 (1951). (146) Ibid., Suppl. 3 (1952). (147) Way, K., Wood, M., and Thew, K., Natl. Bur. Standards, Suppl. 1 to Circ.499 (1951). (148) Westermark, T., and Erwall, L. G., Research ( L o n d o n ) , 4, 290 (1951). (149) Wilkinson, G., and Grummitt, W. E., Nucleonics, 9, No. 3, 53 (1951). (150) Willard, J. E., J. P h y s . Chem., 57, 129 (1953). (151) Winteringham, F. P. W., Analyst, 75, 627 (1950). (152) Wyckoff, H. O., and Taylor, L. S., “X-Ray Protection Design,” Natl. Bur. Standards, Handbook 50 (1952).
[End of Review Section]
literature Problems in Analytical Photometry M. G . MELLON, Purdue University, Lafayette, h d .
A
S A result of many decades of work, the chemist now pos-
sesses a considerable number of different kinds of analytical methods, by means of which he has been able to accumulate a wealth of information concerning both the qualitative and the quantitative composition of vast numbers of substances. Classical examples of such procedures are the widely known processes of gravimetry and titrimetry. Another of these processes, now also widely used, is absorptiometry. In this type the physical property measured is the capacity of a given sample system to absorb radiant epergy. Because the literature for this area is extensive, fairly involved, and widely scattered, i t has seemed worth while to present some of the library problems encountered by the author during more than two decades of such work. A ndte of explanation of the methods may help in understanding the nature of the literature relating to them. First of all, the word “absorptiometric” is used in a general way to include two different types of procedures based upon the absorption of radiant energy. The first kind may be designated as comparometric, since a quantitative determination consists in comparing the intensity of the color of the unknown with that of a standard. Generations of chemists have used the terms “colorimetry,” “colorimetric,” and “colorimeter” here, most often with reference to standard series and balancing techniques. Such usage usually disregards physicists’ reservation of these terms for the measurement of color as color, without regard for the nature of the colorant or its amount. The second kind may be designated as photometric, which includes the use of both filter photometers and spectrophotometers. With these instruments one actually measures, in percentage or some related term, the absorptive capacity of the sample for given nave lengths of radiant energy. As the spectral region of 0.2 to 25 microns is a t present analytically most useful, the discussion is thus limited. Also, the emphasis is on photometry and its applications. Like most other methods of chemical analysis, photometric methods generally involve both chemistry and physics. The chemistry concerns the transformations required to produce a system fit for measurement. Representative common operations and processes are fusion, dissolution, oxidation-reduction, complexation, volatilization, precipitation, extraction, electrodeposition, and adsorption. The physics concerns the final operation of measurement-that is, principles and uses of the photometers. Much of the relevant chemistry has a long background. Spectrophotometers, in a t least primitive form, go back nearly a cen-
tury. Current applications have become very extensive and diverse. Consequently, only the general nature of the literature for each part of the total problem is considered. In general, the references cited are among the most important. If desired, they will lead the way to others. CHEMISTRY
Few desired constituents can be mearmred as such in the usual environment in which they occur. The chemistry of concern t o us is that which is applicable and necessary to make measurement possible. In general, the kind of information needed, as far as it is collected and systematized, is to be found in the great treatises and in monographs. Subsequent to the time of their publication, one must turn, of course, to abstracting journals and thence to the original sources, such as periodicals, bulletins, and patents. For the general chemistry of organic compounds and their reactions, the incomparable source is the Beilstein treatise (49). and Grignard (16)treatises are valuable The newer Elsevier (4) complements to the Beilstein set, as is the set by Heilbron and Bunbury (17). As one turns from these comprehensive works to those more specialized, perhaps certain monographs are next in importance Thus, the ring index of Patterson and Capell (4%)serves for quick checking of particular structural types of organic compounds. Compilations such aa those of Yoe and Sarver (5J), Mellan (sa), and Welcher (61) summarize much information on organic reagents. Still more specialized is Martell and Calvin’s discussion of metal chelate compounds (3%). For inorganic chemistry the five treatises of most general value are those of Gmelin (59), Mellor (98),Friend ( l a ) ,Pascal (41),and Abegg and Auerbach ( 1 ) . There is often a long time lag in parts of these sets, the least serious perhaps being in the Gmelin set. Useful works specifically analytical are the new Fresenius and Jander (10) and the relatively old Rudisiile (45) treatises. The monograph of Hillebrand and Lundeli ( 1 9 ) is invaluable in being critical and up to date. The spectral range of 0.2 to 25 microns covers the nonvacuum ultraviolet, the visible, and the as yet analytically usable infrared regions. Very largely our interest in chemical transformations concerns the preparation of systems for measurement in the visible region-that is, colored solutions. Important requirements of such systems, and of reactions used for producing the solutions, have been summarized elsewhere (86). -4specific problem may emphasize the complexity of some of the
182
ANALYTICAL CHEMISTRY
chemical difficulties. Suppose one wishes to determine minor amounts of germanium in an alloy by photometric measurement of the heteropoly complex, molybdogermanic acid, or its blue reduction product. Assume, for simplicity, no interference by constituents other than possible small amounts of arsenic, phosphorus, and silicon, elements all likely to be present in trace amounts in many alloys. As these other three elements also form heteropoly acids, one has at once the likelihood of a polycomponent absorbing system containing all four complexes. As their individual absorptive characteristics are too much alike to permit simultaneous photometric determination, we must “hitlerize” (divide and conquer) the system. Chemically this means to discover and take advantage of any small differences in properties that are applicable. Various questions arise. What conditions, such as pH, affect the complexes? What is their stability under different conditions? Are they all colored? May they be selectively formed or reduced? Do they all form mixed acids, and, if so, are the properties all similar? If separations must be used, should the process (es) be applied before or after formation of the complexes? In either case, what is the possible applicability of common separative processes such as selective volatilization, precipitation, electrodeposition, liquid-liquid extraction, and adsorption? Only when the answers to these questions, and others of like nature, are a t hand can the analyst decide what can be done chemically to obtain a system susceptible to satisfactory measurement. The over-all problem here is not easy, for heteropoly chemistry extends back a t least a century and a quarter. PHYSICS
Next one comes to the means of measurement-that is, photometers. This term is used to include both filter photometers and spectrophotometers. Sometimes the former are called abridged spectrophotometers, and the latter spectrometers. Many writers use the term “photoelectric colorimeter,” instead of filter photometer, if the detector is a photocell. All this must be kept in mind in searching the literature, especially as some writers are not clear in usage of terms. The distinction between the two types of photometers is one of degree, or refinement, rather than of kind. In general, filter photometers employ broad range sources from which a filter isolatea and paasw a band of wave lengths, which in extreme cases may be as much as half the visible spectrum in width. Spectrophotometers, in contrast, incorporate a prism or grating to disperse the radiant energy of the source and then a mechanical slit to isolate and pass the desired spectral band. Usually this band is between 1 and 100 A. in width. Ideally, perhaps, spectrophotometers should operate on monochromatic sources, but almost none do, No further attention will be given here to any differences between the instruments. With the first spectrophotometer usually attributed to Govi (14) in 1860, the literature now covers nearly a century. Prior to 1900, references appear largely in journals on physics or closely related subjects. Few composite instruments were in chemical use prior to 1915. ilssemblies consisted, in essence, of a source of radiant energy, a monochromator (or filter), a photometer, an absorption cell (for gases and liquids), and a detector. The literature concerns the development of a variety of each of these items, and their applications in different ways. Thus, 72 possible variations in spectrophotometric arrangements have been suggested (23). Many of the earliest contributions have been summarized by Gibson (IS), Kayser (%5),and Kriiss and Kriiss (28). The author’s brief review (56) covering the century 1852-1952, concerns both types of absorptiometers for the visible region of the spectrum. Two monographs cite many historically important references (6, 37). Several other monographs present different viewpoints ($8, $1, 33). What is more important to the chemist today concerns current
developments in instruments. New or improved devices come now from individuals having interests as varied as chemistry, biology, optics, electronics, and engineering. For all such items, then, one should be able to turn to abstracting services for the respective fields. In chemistry this means the general abstracting journals, Chemical Abstracts, British Abstracts, and C h e m i s c h Zentralblatt. In biology it is Biological Abstracts, in physics it is Science Abstracts, and in engineering it is Engineering Index. Selection of headings under which to search in subject indexes of such sources is important. The words “photometer” and “spectrophotometer” may be adequate, if complete instruments are described. More often some part of an assembly is concerned. Then one may need to have in mind specific items, such as; ( 1 ) sources of radiant energy, such as the hydrogen arc, different spectral lines, tungsten filament lamps, C.I.E. illuminants (A$, C ) , Nernst glowers, and Globar lamps; (2) monochromators, both grating and prism types; (3) filters; (4) photometers, such as the polarization, rotating sector, iris, and neutral wedge types; and ( 5 ) detectors, such as the photographic plate (film), various kinds of photocells (photoemissive, photovoltaic, photoconductive), the human eye, thermocouples, bolometers, and oscilloscopes. To these more or less standard parts should be added, for particular instruments, certain special attachments. Illustrative of such accessories are indicators, recorders, multiple-ordinate plotting cams, and automatic integrators for tristimulus calculations. All of this means, of course, that the searcher must be alert to all such instrumental details to avoid missing important headings in subject indexes. Even then, the specialist in photometry may profit from the expert advice on searching by E. J. Crane, editor of Chemical Abstracts ( 7 ) . Another point of interest in this connection may be mentioned. Because of lack of agreement on nomenclature, entries likely to be important €or instruments are photometers, spectrographs, spectrometers, spectrophotometers, and spectroscopes. Those dealing more broadly with the subject are photometry, spectrochemistry, spectrography, spectrophotometry, spectrometry, spectroscopy, and spectrum (absorption). Because wide appreciation of photometry as an analytical technique is fairly recent, one cannot be sure, especially in the older literature, that chemical journals carry abstracts on such instruments. All of these difficulties apply to annual and cumulative subject indexes of abstracting journals. There is still more difficulty for one trying to follow developments during a current year. For an example, we may consider the use of Chemical Abstracts. As it has no issue subject index, each number must be read. For photometers, both filter and spectro-, the author examines regularly the following sections; 1. Apparatus, Plant Equipmeut, and Unit Operations; 2. General and Physical Chemistry; 3. Electronic Phenomena and Spectra; and 7. Analytical Chemistry. An occasional reference may be located elsewhere. These abstracts cover journals, governmental bulletins, and patents. Occasionally manufacturer’s technical publications, which are not abstracted, appear long before any technical description of the instruments. Advertisements must be watched for such information. Also anyone interested should keep his name on the mailing lists of companies in the field. APPLICATIONS
The application of photometers is considered only from the viewpoint of their concern to analysts. Broadly speaking, the three main analytical uses are qualitative analysis, quantitative analysis, and the determination of absorptive data as such. Inasmuch as Vierordt (60)is given the credit for the first analytical use of a spectrophotometer, in 1873, the work now spans eight decades. Chemical Analysis. The absorbing power of a selectively transmitting solution has both qualitative and quantitative as-
V O L U M E 2 6 , N O . 1, J A N U A R Y 1 9 5 4 pects. Let us assume an absorption spectrum of the system in the form of a curve having absorbing power as the ordinate and spectral region covered as the abscissa. Then the form or profile of the curve characterizes the system qualitatively; and the vertical position of the curve, with respect to the z-axis, characterizes the system quantitatively. The laws of absorption for such a solution may be summarized in the expression, loglo 1 / T = abc = A in which T = transmittance, a = absorptivity, b = thickness, c = concentration, and A = absorbance. Library difficulties arise a t once because of diverse usage in the literature, such as definitions, symbols, manner of plotting curves, and specifications of conditions of measurement. Usually the abscissa of curves give the spectral region covered, but the designation may be wave length ( p , mp, or A.), wave number, or frequency. Some are logz of wave length (mp). The ordinate may be transmittance (T), per cent transmittance (100 T), absorbance ( A = extinction, E = optical density, D),absorptance (1 - T ) ,molar absorptivity (aAM = molar absorbance index = molar extinction coefficient, e), or log of molar absorptivity. Occasionally one cannot be sure that an ordinate labeled absorption is not really transmittance. Usage may even be inconsistent in the same paper. When it comes to a physical constant, such as molar absorptivity, all factors affecting variability should be clear. Some examples for solutions are concentration, cell thickness, temperature, solvent, spectral band width, and wave length of measurement. Many current papers are analytically inadequate in this respect. I t should be obvious that one unaware of the importance of all such details is hardly in a position to evaluate !he significance of papers in this field. Actual preparation of a sample for measurement is generally just as much of a problem as for other kinds of methods. The alert searcher should be aware of the necessity of the requisite preparative processes and operations. Primary sources, such as periodicals, bulletins, and patents, contain increasing amounts of such information. Sections 2, 6, and 10 of Chemical Abstracts are most important in a general way; but many others, especially Section 11, should not be overlooked. A number of treatises and monographs, already mentioned, deal with such chemistry. In all of them the primary concern is the preparation of colored solutions. No comparable compilations relate to systems for measurement in the ultraviolet or infrared regions. In fact, little work has been reported dealing with the formation of specific compounds for measurement in these recions. The great wealth of information on applications consists largely of specific methods for given materials. Thus, one finds the ASTM all-photometric methods (4)for the minor constituents in magnesium alloys, and the APHA colorimetric methods ( 3 ) for a score of constituents in water analysis. The most comprehensive general compilation of such methods is by the Snells (47). A dozen other works cover less ground and may be more specific (8, 9, 16, 18, 10,81, %7,SO, 40, 46, 49, 52). The relative importance of the applications of photometric methods becomes most impressive when one follows current periodicals. The author recently compared (35) absorptiometric methods for the visual region with titrimetric and gravimetric methods. The change in two decades is noteworthy. An extensive bibliography (48) covers selected methods employing photoelectric instruments. Searching subject indexes of abstracting journals for such methods presents more difficulties. One needs another list of index headings and modifiers, such as ultraviolet, visual, and infrared. For the visual region the words “colorimetric,” “colorimetry,” and “colorimeter” may be best. Along with these may be
183 needed the whole range of kinds of materials, such as steel, water, and petroleum products, as well as specific desired constituents, such as iron and silica. In this kind of searching a particular difficulty concerns the lack of suitable naming or classification of analytical methods. As an illustration of what this deficiency means, the fourth decennial index of Chemical Abstracts may be taken as an example. Under the heading, “Nickel, detn.,” are 70 entries, and under the heading, ‘LNickel,detn. in, steel,” are 60 entries. I n no case is there anything to indicate which of perhaps a half dozen methods of measurement is described. Thus, if one wants only the absorptiometric methods, all abstracts must be checked. The abstracts thus found may yield the desired information, although often they do not. The only recourse, then, is to consult the original publicatjons. Then following the current issues of Chemical Abstracts for the chemistry of such methods and their applications is even worse than for instruments alone. One reading the 33 divisions, in order, usually finds nothing in Section 1, and only a small number in Section 2. Section 3 has many, especially on the absorptive nature of systems in the ultraviolet and infrared. Section 3A yields little. Section 4 may include strictly analytical papers, but there is not much probability of any in Section 5. Sections 6 and 10 are of fundamental chemical interest. Section 7, Analytical Chemistry, is the principal source for general methods and often for applications to specific materials. Unfortunately, one cannot depend upon cross references here for relevant work in other sections. It seems that the number of these citations might well be greatly increased. Sections 8, 9, 11, and on to 31, deal primarily with some aspect of applied chemistry, but papers strictly analytical in nature may be included. Excellent examples are Sections 11B and 14. Mostly such papers are not cross indexed in Section 7. In view of this situation, the only way to avoid nlissing references is to examine practically every abstract of each issue. Often titles are irksome in being noncommittal on the kind of method. Thus, expressions such as a micromethod and a method for trace amounts reveal nothing as to whether the procedure is photometric. One concerned with qualitative aspects of absorption spectra has great interest in collections of data, such as tabulations giving the wave lengths (frequencies, wave numbers) of the peaks of absorption bands and spectrophotometric curves. Brode ( 5 ) has cited references to the older collections. Three new American collections are significant. The one published by Friedel and Orchin (11) includes ultraviolet curves for 579 aromatic compounds. Curves covering the infrared region and also the ultraviolet in many cases, for many organic compounds, have been issued in loose-leaf form under sponsorship of the American Petroleum Institute ( 2 ) . A noteworthy COoperative effort is the joint project of the National Research Council, the National Bureau of Standards, and the American Society for Testing Materials. The most useful infrared characteristics of many hundreds of organic compounds are being coded on IBJI cards under the general direction of Kuentzel(d9). Color Analysis. An important use of spectrophotometric curves in the visual region of the spectrum is to designate the color of a system. The curve itself is considered fundamental. Conventional plotting is transmittance (for transparent systems) or reflectance (for opaque systems) as the ordinate vs. wave length, in millimicrons, as the abscissa. I n 1931 there was international agreement to use such curves as a basis for calculating the C.I.E. trichromatic specification of a color in terms of three selected stimuli (6). Two recent monographs summarize this broad and involved subject and include valuable bibliographies. The first (IS) is the famous report of the colorimetry committee of the Optical Society of America. The second ($4) is the result of a quarter of a century of work on color a t the National Bureau of Standards.
184
ANALYTICAL CHEMISTRY (2) American Petroleum Institute Research Project 44, Carnegie
Table I. Number of Periodicals with 1 to 5 References Each References No. of No. of per Journal Journals References 88 38 27 36 25 214
.-.
Table 11. Periodicals with More than 5 References Each No. of No. of Name Journals References Analyst ANAL. CHEM. Anal. Chim. Acto J . Am. Chem. Sac. J . Chem. Sac. Japan J . Opt. Sac. Amer. Magyar Kdm. Folyrdirat Z . anal. Chem.
Total
1 1 1 1 1 1 1 1 8
21 127 11 826 9 22 8 12 236
-
-
Table 111. General Distribution of Kinds of Publications Publications No. of Kind Number References Periodicals 138 450 Patents
Books
Manufacturers’ technical publications
Institute o f Technology, Pittsburgh, Pa. (3) American Public Health Association, Iiew York, “Standard Methods for the Examination of Water and Sewage,” 1953. (4) American Society for Testing Materials, Philadelphia, “ASTM Methods for Chemical Analysis of Metals,” Materials, 1950. (5) Brode, W. R., “Chemical Spectroscopy.” New York, John Wiley & Sons, 1943. (6) Commission Internationale de l’Eclairage, Proceedings o f 8th Session, Cambridge, England, pp. 19-29, 1931. (7) Crane, E. J., J. Ind. Eng. Chem., 14, 901 (1922). (8) Delory, G. F., “Photoelectric Methods in Clinical Biochemistry,” London, Hilger and Watts, 1949.
(9) Fister, H. J., “Manual of Standardized Procedures for Spectro(10) (11) (12) (13) (14) (15) (16) (17)
7 11
7 11
(18)
16
16 -
(19)
Total
484
(20)
DATA ON A REVIEW ARTICLE
The literature for most specific analytical problems will vary because of differences in nature, objective, and other factors. However, a general idea of the type of literature involved in photometry is illustrated by a review just completed by the author. This review covers advances made during a Zyear period for the analytical applieations of absorptiometry in the visual region of the spectrum. Tables I, 11, and I11 sumxmrize certain data for the 484 references selected as representative examples. A few items are of special interest. Thus, one journal has more than 25% of all references, and eight of the 138 journals cited have nearly half of t,he total number of references. The journals listed in Table I1 include four for analytical chemistry, three for general chemistry, and one for physics. Of the seven patents one is for a method of analysis (a process) and six are for instruments (machines). The books, either new or new editions, relate to the field reviewed. The manufacturers’ technical publications are all for instruments and are not abstracted in Chemical Abstracts. There are no governmental bulletins. To locate this material Chmical Abstracts was indispensable. The News-Let& of the Inter-Society Color Council was of value for some borderline material. Some references came from general reading, and a few were suggested by fellow workers in the field. CONCLUSION
These comments obviously reflect one individual’s experiences in his efforts to follow developments in one division of analytical chemistry. Without the kinds of publications mentioned, both original and secondary sources, the problem of keeping up to date would be too formidable to continue. With them one needs knowledge of the subject, including its peculiarities, along with skill in the art of searching. Finally, perseverance is indispensable, not only in collecting references, but in selecting from them relevant desired details. LITERATURE CITED (1)
Abegg, R., and Auerbach, F., “Handbuch der anorgenischen Chemie,” Leipzig, S. Hirzel, 1908f.
(21) (22)
photometric Chemistry,” Xen- York, Standard Scientific Supply Co., 1950. Fresenius, R., and Jander, G., “Handbuch der analytischen Chemie.” Berlin, Julius Springer, 1940+. Friedel, R. A., and Orchin, M., “Ultraviolet Spectra of Aromatic Compounds,” New York, John Wiley & Sons, 1951. Friend, J. N., “Textbook of Inorganic Chemistry,” London, Chas. Griffin and Co., 1914-37. Gibson, K. S.,et al., J . Opt. SOC.Amer., 10, 169 (1925). Govi, G., Compt. rend., 50, 156 (1860). Grignard, V., “Trait6 de chimie organique,” Paris, Masson et Cie, 1935+. Haywood, F. W., and Wood, A. A. R., “Metallurgical Analysis,” London, A. Hilger, 1944. Heilbron, I. XI., and Bunbury, H. M., “Dictionary of Organic Compounds,” Iiew York, Oxford University Press, 1943. Heilmeyer, L., “Spectrophotometry in Medicine,” tr. by -4. Jordan and T. L. Tippell, London, 8 . Hilger, 1943. Hillebrand, W. F., Lundell, G. E. F., Bright, H. A,, and Hoffman, J. I., “Applied Inorganic Analysis,” New York, John Wiley & Sons, 1953. Hoffman, W. S., “Photelometric Cliniral Chemistry,” New York, W. Morrow and Co., 1941. Hunter, F. T., “Quantitation of Mixtures of Hemoglobin Derivatives by Photoelectric Spectrophotometry,” Springfield, Ill., C. C Thomas, 1951. Institute of Paper Chemistry, Paper Trade J . , 105, No. 18, 135;
KO.19. 27 (1937). (23) Jones, L. A . , et al., “The Science of Color.” New York, Crowell Go., 1953. (24) Judd, D. B., “Color in Business, Science and Industry,” Yew York, John Wiley & Sons, 1953. (25) Kayser, H., “Handbuch der Spektroscopie,” Vol. I, pp. 1-128, Leipzig, S. Hirzel, 1900. (26) Kortum, G., “Kolorimetrie und Spektralphotometrie,” Berlin, Springer-Verlag, 1948. (27) Krebs, W., “Clinical Colorimetry with t,he Pulfrich Photometer,” Jena, C. Zeiss, 1936. (28) Kruss, G., and Kruss, H., “Kolorimetrie und Spektralanalyse,” Leipzig, L. Voss, 1909. (29) Kuentzel, L. E., “Codes and Instructions for Wyandotte Punched Cards Indexing Infrared Absorption Spectrograms,” Wyandotte, hlich., Wyandotte Chemicals Corp.,
1951. (30) Leitz, E., Inc., “Handbook for Rouy Photometer,” New Ywk, E. Leitz, Inc., 1948. (31) Lothian, G. F., “Absorption Spectrophotometry,” London, Hilger and Watts, 1949. (32) Martell, rl., and Calvin, M.,“Chemistry of the Metal Chelate Compounds,” New York, Prentice-Hall, Inc., 1951. (33) Mayer, F. X., and Luszczak. A,, “.4bsorptions-Spektralanalyse,” Berlin, W. DeGruyter & Co., 1951. (34) Mellan, I., “Organic Reagents in Inorganic Analysis,” Philadelphia, Blakiston & Co., 1941. (35) Mellon, M. G., AXAL.CHEM.,24, 924 (1952). (36) Mellon, M. G., Proc. Am. Soc. Testing itfaterials,44, 733 (1944),; (37) Mellon, M. G., et al., “Analytical Absorption Spectroscopy, New York. John Wiley & Sons, 1950. (38) Mellor, J. W., “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” London, Longmans, Green and CO., 192237. (39) Meyer, R. J., “Gmelin’s Handbuch der anorganischen Chemie,” Leipzig-Berlin, Verlag Chemie, 1926+. (40) Morton, R. A., “Application of Absorption Spectrophotometb to the Study of Vitamins, Hormones, and Coenzymes,” London, -4.Hilger, 1942. (41) Pascal, P., “Trait6 de chimie minbrale,” Paris, Masson et Cie., 1931-34. (42) Patterson, A. M., and Capell, L. T., “The Ring Index,” New York, Reinhold Publishing Corp., 1940. (43) Prager, B., et al., “Beilstein’s Handbuch der organischen Chemie,” Berlin, Julius Springer, 1918f.
V O L U M E 2 6 , NO. 1, J A N U A R Y 1 9 5 4 (44) Radt, F., “Elsevier’s Encyclopaedia of Organic Chemistry,” New York, Elsevier Publishing Co., 1948-t. (45) Rudisiile, A., “Nachweis, Bestimmung und Trennung der chemischen Elemente,” Bern, M. Drechsel, 1913-t. (46) Sandell, E. B., “Colorimetric Determination of Traces of J l e b als,” New York, Interscience Publishers, 1950. (47) Snell, F. D., and Snell, C. T., “Colorimetric Methods of rlnalysis,” New York, D. Van Nostrand Co., 1948-53. (48) Stillman, J. W., and Dunlop, E. C., -4m. SOC.Testing Illaterials, Spec. Tech. Pub. 125 (1952). (49) Tintometer, Ltd., “A Handbook of Colorimetric Chemical Analytical Methods,” 1953.
185 (50) Vierordt, K., “Die Anwendung des Spektralapparatus sur Photometric der Absorptions-spektren und sur quantitativen chem-
ischen Analyse,” Tubingen, H. Laupp, 1873. (51) Welcher, F. J., “Organic Analytical Reagents,” New York, D. Van Nostrand Co., 194748. (52) Yoe, J. H., “Photometric Chemical Analysis,’’ Vol. I, New York, John Wiley & Sons, 1928.
(53) Y o e , J. H., and Sarver, L. A., “Organic Analytical Reagents,” New York, John Wiley & Sons, 1941. for review October 20, 1953. Accepted December 4, 1953. RECEIVED Presented before the Division of Chemical Literature at the 124th Meeting SOCIETY, Chicago, Ill. of the AMERICAN CHEMICAL
Kjeldahl Method for Organic Nitrogen R. B. BRADSTREET The Bradstreet Laboratories,
he., 1356 North Broad St., Hillside, N. 1.
T
H E extent to which the Kjeldahl method has been applied to organic analysis has led to the publication of many data. From time to time, reviews ( 1 4 , 1 6 , 6 6 ) have been published covering both macro- and micromethods. Comprehensive reports on microchemical methods have been made by Willits and Ogg (?’I, ’72). In a previous paper (9), the method was reviewed from its inception to 1939. This report covers the period from 1939 to the present date. The Kjeldahl method, through the efforts of countless workers, has been modified to include many types of nitrogen compounds, and given a much wider scope than Kjeldahl could have anticipated when he devised the method which was primarily to follow the course of protein changes in grain. The considerable use of the method arises mainly from the fact that it is a relatively uncomplicated procedure. The ease with which the determination can be made has undoubtedly led workers to look upon it as a possible cure-all for problems in nitrogen analysis. It is obvious, however, from published data, that all forms of organic nitrogen cannot be converted quantitatively to ammonia. I n order to correlate the available data, the method has been broken down into its component parts-digestion, catalysts, distillation, titration, etc. DIGESTION
The Kjeldahl method ( 2 7 )is a wet oxidation using concentrated sulfuric acid. Originally, sulfuric acid alone was used, but because oxidation under such conditions was time-consuming, particularly if large samples were used, the method was soon modified by the addition of potassium sulfate (21). With the advantage of a higher boiling point, oxidation was possible in a much shorter time. Ogg and Willits ( 4 6 ) measured the temperature of boiling digestion mixtures a t six different points, and having a potassium sulfate concentration from 0.25 to 0.875 gram per ml. of acid. Using nicotinic acid, they found that results varied from less than 1% to 11%, and attributed this difference to the boiling temperatures or insufficient heating. Milbauer (44) also studied the acid digestion, using diphenylamine as a type compound with various additives, and found that a mixture of 25 ml. of sulfuric acid, 1 gram of mercury, 20 grams of (K2SZOB) and 25 ml. of hydrogen peroxide gave complete conversion in 4 minutes. In the determination of nitrogen in foodstuffs (1j , a digestion mixture of 40 grams of sodium sulfate and 1.6 grams of copper sulfate per 100 ml. of acid is recommended, with a digestion time of 6 hours. For the microdetermination of protein in 50% glycerol (51), bromine is used as an oxidizing agent, supplemented by 30% hydrogen peroxide. Rosenthaler (57j , who studied the formation of ammonia by oxidation of nitrogen-containing compounds, stated that the final conversion to ammonia is caused by hydroly-
sis. Quartaroli (53)considered the digestion to take place in two steps: oxidation of the organic matter, and reduction of all nitrogen to the ammonium salt. From his experiments with pure compounds, he concluded that when oxidizing action is weak and slow, the accelerating action of the oxidizing phase is slight. Strong oxidizing agents are injurious in the reduction stage, resulting in the loss of nitrogen. Powdered pyrolusite is recommended as the most useful oxidizing agent. At one time or another, there has been controversy as to the use of perchloric acid in the Kjeldahl digestion. By many workers its use is considered hazardous. However, shorter digestion times, with no loss of nitrogen, are reported. Pepkowitz and Shive (49) advocated the use of perchloric acid as a wet digestion micromethod, and claimed that for very resistant materials, the digestion is approximately four times as fast as the official method of the Association of Official Agricultural Chemists. Kaye and Weiner (26‘), however, stated that low results with heterocyclic compounds were obtained, although no loss of nitrogen from ammonium sulfate was observed. Koch ($88)reported its use in the digestion of grass, grain, and potato. In conjunction with selenium (68)for the digestion of soils and animal foods, it is said to reduce the digestion time from 4 hours to 15 minutes. On the other hand ( 6 5 ) , it is reported that the method of Mall01 (39), using this combination, gave good results with the hydrochlorides of aniline and ephedrine, but poor results with such compounds as asparagine, aspartic acid, acetanilide, antipyrene, and caffeine. Wicks and Firminger (69) discouraged the use of perchloric acid, particularly in the micromethod, because large losses are developed by the use of a so-called excess which may not b? too apparent with a macromethod. It is an extremely powerful oxidiaing agent and must be used with caution, particularly from the standpoint of loss of nitrogen either through formation and subsequent decomposition of ammonium perchlorate or oxidation of ammonium sulfate to nitrogen. Pepkowitz et a2. (48) obtained good results on addition of 10 drops of perchloric acid after strong heating of the sample in concentrated sulfuric acid, and cooling before the addition. Hydrogen peroxide, also, is used frequently in both micro- and macro-Kjeldahl determinations as an oxidizing agent. Various workers (12, S4, 54, 6 9 ) report favorably on its use. Marquee and -4llista (42) found that the use of potassium pyrosulfate in conjunction with hydrogen peroxide gives rapid oxidations. It is obvious that by increasing the severity of the reaction, oxidation will be accelerated. Disregarding for the moment the effect of catalysts, this is accomplished by the use of oxidizing agents and by increasing the salt content, with the consequent raising of the boiling point. The advantage of increasing the amount of potassium sulfate used, other than shortening of the digestion time, is that in many cases theoretical results are obtained which are not possible with the use of the conventional
