January 15,1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACKNOWLEDGMENT The writers wish to express appreciation for the material cooperation of F. G. Cottrell and C. H. Kunsman in the preparation of this paper. LITERATURE CITED (1) Blacet and Leighton, IND. ENQ.CHEM.,Anal. Ed., 3,266 (1931). (2) Burk, 6.Phys. Chem., 34, 1174 (1930). (3) Burk, Ibid., 34,1195 (1930). (4) Burk, Proc. 3nd Intern. Soil Sci. Congress, Moscow, 1930. (5) Burk, Horner, and Lineweaver, J . Gen. Physiol. To be published, (6) Burk and Lineweaver, J. Bact., 19, 389 (1930). (7) Burk and Lineweaver, Arch. Mikrobiol., 2, 155 (1931). (8) Burk and Lineweaver, J . Bid. Chem. To be published. (9) Burk, Lineweaver, and Horner, Soil Science. In press. (10) Campbell, Proc. Phys. SOC.(London), 33,287 (1921).
7
(11) Krebs, Biochem. Z., 220,283 (1930). (12) Langmuir, J. Am. Chem. SOC.,35, 105 (1912); 40, 1378 (1918). (13) Lineweaver, Burk, and Horner, S. Gen. Phuswl. To be published. (14) Niederl and Meadows, Mikrochemie, 9, 350 (1931). (15) Pregl, “Quantitative Organic Microanalysis,” 2nd Engliah ed., Churohill. 1930. (16) Presoott, J: Am. Chem. SOC.,50, 3237 (1928). (17) Ryder, Ibid., 40, 1656 (1918). (18) Shepherd, Bur. Standards J . Research, 6, 121 (1931). (19) Spoehr and McGee, Carnegie Inst. Wash. Pub. 325 (1923). (20) Warburg, Emil, Sitzber. preuss. Akad. Fiss., 34,712 (1900). (21) Warburg, Otto, “Ueber den Stoffwechsel der Tumoren,” Springer, Berlin, 1926. (22) Warburg, Otto, “Ueber die Katalytisohen Wirkungen der Lebendigen Substanr,” Springer, Berlin, 1928. (23) Weaver, J. IND.ENQ.CHEM.,12, 359 (1920). R~CBIVE September D 12, 1931.
Microanalytical Methods as Time and Labor Savers E. M. CHAMOT,Cornell University, Ithaca, N . Y.
N
EITHER microchemical T H E MICROCHEMICAL and microscopical w i t h m o d e r a t e manipulative nor methods of analysis which are being developed For the information of those microscopy can be conpossess such potential economic possibilities that who have not yet looked into the sidered, as new tools. They are all analysts should become familiar with them, merits of microanalytical methnew only in the sense that they not only because they provide means f o r the are now a t t r a c t i n g far more ods, it canbe asserted that subattention than when they were stantially all standard methods solution of many problems which cannot be solved first devised and most chemof quantitative analysis can be by methods, but also because they provide ists are beginning to view them so modified as to be made apthe most practicable way of reducing costs and from a somewhat different angle. plicable to the analysis of very Of time in analyticaz and research labors- small amounts of material with I n these methods we find new tories. systems of procedure rather than no appreciable increase in error. new tools. What is essential, however, is American chemists have, for the most part, been slow to that the analyst must be a good manipulator and have dexrecognize the value of microanalytical methods because they terity and patience as well as steady nerves. I n other words, have regarded them merely as a means for dealing with he must be a good analyst. Handling the delicate, fragile utenamounts of material far too small to be handled by the ordi- sils and apparatus and supersensitive instruments is no job for nary methods of analysis and have failed to perceive that this a clumsy, careless, or impatient chemist. There is nothing very fact-ability to deal with small weights and small vol- inherently difficult in these methods and nothing which should umes-can be taken advantage of and a mode of procedure bar them from any well-conducted laboratory, for the requisite devised whereby chemical operations can be greatly expedited. manipulative technic is easily acquired. Since the weights and the volumes of reacting compounds Microquantitative methods are available for inorganic are reduced to a minimum, the sizes of the utensils required as well as for organic compounds (,2-5), although the latter for carrying on the various operations are correspondingly appear to be receiving more attention, probably because of reduced and the space necessary for a set-up is small indeed Pregl’s work and the fact that his book has been translated compared with that which is essential when standard methods into English and is therefore well known, whereas the contriare employed. It follows that time and labor are saved and butions of Emich, Donau, and others in the field of inorganic costs greatly diminished. The economies which can thus microanalysis (2-5) are overlooked. be brought about are often of a startling nature, especially The errors alleged to be inherent in micro-sampling are of where rare and expensive materials or reagents are involved no greater relative magnitude than those to be found in our or when the preparation of a suitable sample is a time-con- macroscopic methods. If the sampling is performed under auming task. Many industries are finding that the adoption a low-power microscope-e. g., Greenough type-the acof microchemical methods results in the saving of days of time curacy is certainly in favor of the micromethods, for the microand many thousands of dollars. scope is almost certain to reveal any non-homogeneity in When comprehensive microscopical methods are combined the material in question. Moreover, because of the rapidity with microchemical methods, still further economies can be with which analyses can be made and their very low costs, effected. I n addition, the increased safety to the analyst of more samples can be analyzed in less time and there is less explosives and to the chemist dealing with dangerous reac- temptation to shirk. Thus any errors in sampling and any tions is an inestimable advantage. non-uniformity in the material are more likely to be brought We now have available, of American manufacture, satis- to light. factory and accurate microbalances and chemical microscopes, The development of microanalytical methods to suppleand tiny utensils are procurable for all analytical operations. ment most, if not all, our routine analyses is greatly to be We can carry out with commercially available apparatus prac- desired, not only because expenses are cut down, but also tically any chemical operation on a “micro” scale. More- because more analytical data will be secured, and all with the over, most of the utensils required can be made by any worker expenditure of less time and less labor.
8
ANALYTICAL EDITION
Vol. 4, No. 1
These microanalytical methods differ from our so-called standard methods of quantitative analysis in substantially one particular only-they deal with very small amounts of material. The basic principles involved and the reagents employed are the same as when large samples are treated. It is coming to be generally accepted that the term “microchemical” should be restricted to methods in which about 10 mg. of sample are treated and a microbalance employed. When the weight of the sample approximates 50 mg., the method becomes “semi-micro,” and an ordinary sensitive analytical balance can be employed. These semi-micromethods are especially well suited to industrial control laboratories where speed is essential and the lowering of costs desirable.
surfaced and etched. Fragmental materials can be embedded and sectioned or surfaced. The ratios of components are determined by comparison with standards, by counts of particles, by the sum of the intercepts along a given line across the field of the microscope, by the use of a reference substance, or by measuring the areas of the different fragments in a suitably prepared section. The counting of the different components is greatly facilitated by judicious selection of a mounting medium of the proper refractive index, or by counts or comparisons made between crossed nicols. Microscopical methods of quantitative analysis are worthy of far more attention of chemists than they have received, for in the majority of cases these methods are capable of yielding results with great rapidity and a t substantially no cost for ADVANTAGES OF MICROSCOPICAL QUANTITATIVE ANALYSIS reagents and with the expenditure of but little labor. But But there is a phase of microanalysis in which our proce- the analyst must acquire the requisite microscopical technic dures are radically different from those which we term chemi- and learn to illuminate his preparations properly and intercal methods. This is a field quite apart and one which is pret the images which he sees when looking into his microsadly neglected. I refer to microscopical quantitative analy- scope, images obtained with polarized light or with ultraviolet light, as well as with ordinary light. sis ( I , 6, 8). While I recognize the importance of microchemical PREVALENCE O F MICROSCOPICAL QUALITATIVE ANALYSIS analyses and can fully appraise their value both in industry and in research, I am nevertheless firmly convinced that Microscopical qualitative analysis no longer requires any microscopical methods offer even greater possibilities in the propaganda in its behalf, for it has come to be accepted in matter of time- and labor-saving. Any competent analyst most laboratories as a most welcome substitute for the less trained in the use of the microscope can devise new procedures, rapid, less easily interpreted and far more costly chemical new tests, and new applications for chemical microscopy. methods. Chemical-microscopical methods are applicable to synAlthough the recognition of cations and anions is a comthetical as well as analytical studies. I n the former cases, paratively simple matter, the identification of compounds the rate of formation and change of character or the trans- usually calls for a rather comprehensive knowledge of optical formation of a compound can often be followed, and much new crystallography, a knowledge which unfortunately is seldom light can thus be thrown upon a process and possibilities acquired by the chemist. This, in a way, is a new tool for of improvement revealed, facts which could not readily have the chemist and it is quite important that he possess it if he been brought to light by other means. Studies of relative is to make full use of the polarizing microscope in the solusolubilities are best made with the microscope. Neither high tion of the intricate problems arising in his investigations. magnifications nor elaborate instruments are essential for the This is especially true if his field is organic chemistry or in the study of the great majority of problems arising in chemical border-land between chemistry and biology. laboratories. The application of the micromanipulator in chemical miUnlike those methods which we have considered above and crurgy (7) opens up untold possibilities in the analysis of subwhich are of general applicability, microscopical quantitative stances present in such minute quantities as to be invisible analyses are restricted to certain specific types of material. t o the naked eye. The time has come when it is no longer They were originally devised to enable the analyst to deter- necessary that material to be analyzed must be visible to the mine the probable percentage composition of materials not eye, If it can be rendered visible under the microscope, the susceptible of analysis by ordinary chemical methods-for ex- probabilities are that it can be analyzed. ample, mixtures of starches, meals, crude drugs, paper pulps, textile fibers, rocks, etc. They are by no means restricted, LITERATURE CITED however, to such materials, and are now being applied with (1) Chamot and Mason, “Handbook of Chemical Microscopy,” great success to ceramic materials, cements, mortars, mixtures Wiley, 1931. of powdered minerals and ores, percussion powders and other (2) Dafert, “Quantitative Mikroanalyse,” Abderhalden’s “Handbuch d. biolog. Arbeitsmethoden,” Vol. IV, Pt. 4, Urban & explosives, alloys, etc. That is, their special field is the analySchwartzenberg, Berlin, 1929. sis of heterogeneous mixtures of materials of complex chemi- (3) Donau, “Arbeitsmethoden d. Mikrochemie,” Franckh, Stuttgart. cal composition. 1913. Microscopical quantitative analyses can be successfully (4) Emioh, “Mikrochemisches Praktikum,” Bergmann, Munich, 1931; “Lehrbuch d. Mikrochemie,” Bergmann, Munich, applied to any materials provided the following requirements 1926; “Methoden d. Mikrochemie,” Abderhalden’s “Handare met: buch d. biolog. Arbeitsmethoden,” Vol. I, Pt. 3, No. I, Urban 1. The components of the mixture must differ sufficiently in appearance t o permit easy recognition under the microscope or must be susceptible of being rendered different. 2. The different components must bear a reasonably constant relationship t o the physical and chemical properties. 3. The specific gravities of the components must be nearly the same or must be known. 4. The sample must be representative of the material.
I n general but little preparation of the sample is required when it is in the form of a powder. The chief difficulty lies in obtaining a uniform distribution of the particles in a suitable mounting medium upon an object slide. Alloys must be
& Schwartzenberg, Berlin, 1921; “Umsetzungen sehr kleiner
Stoffmengen,” Stiihler’s “Handbuch d. Arbeits. in d. anorganischen Chem.,” Vol. 11, Pt. 2, deGruyter, Berlin, 1925. (5) Pregl-Fyleman, “Quantitative Organic Microanalysis,” Blakiston, 1930. (6) Schneider, “Microbiology and Microanalysis of Foods,” Blakiston, 1920. (7) Titus and Gray, IND. ENQ.CHEM.,Anal. Ed., 2,368 (1930). (8) Wallis, “Analytical Microscopy,” Arnold, London, 1923. R E C ~ Y V September ZCD 22, 1931.
A
.
