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INDUSTRIAL AND ENGINEERING CHEMISTRY
lative technic and methods of observation used in microanalysis must undergo vast changes as the amount of material decreases. These changes must be even more radical than those taking place when reducing the quantity of the sample from 500 kg. to 1 gram, as the corresponding range in microanalysis extends below the limit where the palpable properties of matter can be directly observed by the human sense organs. This fact, together with the observation that a reduction to one one-hundredth of the original mass manifests itself in a less obvious way when working on a small scale (compare the reduction of a mass from 10 to 0.1 mg. with the reduction from 100 to 1 kg.) necessitates a close correlation between mass and technic in microanalysis, a fact which is usually overlooked but which cannot be emphasized too much.
Microtechnic The development of a system of microtechnic for chemical work was begun by Emich in 1900 and essentially completed by 1911, when the first edition of his Lehrbuch (3) appeared. This part of Emich’s work has not only contributed toward the rapid progress of microchemistry in the last two decades, but was actually the foundation of microchemistry (8). The technic of quantitative analysis and of preparative work, and the methods for the observation of properties (physico-chemical work) have been developed with proper consideration of adaptability to certain size ranges of matter. Emich must have recognized this requirement a t a very early stage of his research, and he repeatedly emphasizes the importance of its strict observation. When a sample of a few centigrams is taken for analysis (semi-microanalysis,or centigram method), the use of centrifuge tubes of 3- to 15-cc. capacity is recommended. In most cases it will be possible to effect the separation of solid and liquid phases by simply pouring out the filtrate after centrifuging. Filtration through paper could be used, but is less efficient. Spot tests are suggested as confirmatory tests. With samples of 0.5 to 1 mg., small centrifuge tubes (microcones) of 0.7-cc. capacity are most efficient for the separation of solid and liquid phase. The use of a microscope is still not necessary as the presence and color of precipitates 5 to 10 y in weight, or 01 colorations caused by these amounts, can be observed with the unaided eye. Spot tests suggest themselves as confirmatory tests here; however, the certainty of identification i s increased by the use of tests carried out with microscopic observation.
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“pure” reagents, should never be taken as proof of their presence in the sample. A technic of working in capillaries of 0.5- to 1-mm. bore has been developed (4) for the analysis of 0.05- to 0.5-mg. samples (a centi-milligram procedure), whereby practically all the processes required in qualitative analysis can be carried out with ease and certainty. A magnifying lens, or a low-power microscope becomes a necessary tool for the detection of precipitates and colorations. The use of laboratory apparatus of the ordinary type becomes absurd when the size of the sample decreases to a few micrograms or less, since the small quantiti or the tiny drops of solution cling to any surface. Continuous control of all operations by means of the microscope becomes imperative, magnifications of 50 X to 100 X being necessary for the observation of the confirmatory tests. Emich succeeded in handling such small quantities of matter by absorbing them on the ends of textile fibers. Precipitates, formed by dipping the fiber into droplets of reagent solutions, cling to the fibers and can be transferred without difficulty to other solutions which may serve for washing the precipitates, extracting parts of them, etc. At the same time, the material under investigation always remains concentrated in a very small area, thus preventing its loss and facilitating observation. Microtechnic has already contributed greatly to the progress of science, but one cannot expect it t o realize its full potentialities in that direction until the great majority of research men-not only the chemists but also biologists, geologists, archeologists, etc.-are informed as to the possibilities of microchemical work. Even Pregl would not have begun the development of organic quantitative microanalysis had it not been for the knowledge of the previous work of Emich which indicated the feasibility of the proposed task
(8)’
Literature Cited (1)
Benedetti-Pichler, A. .4.,and Spikes, W. F., “Introduction to
(4) (5)
the Microtechnique of Inorganic Qualitative Analysis,” Douglaston, L. I., Microchemical Service, 1935. (In press.) Emich, F., Ann., 351, 426 (1907). Emich, F., “Lehrbuch der Mikrochemie,” p. 3, Munich, Bergmann, 1926. Emich, F., 2. anal. Chem., 54, 489 (1915). Haber, F., Jaenicke, J., and Matthias, F., 2. anorg. Chem., 155,
(6)
Krogh, A., Skand. Archiv. Physiol., 20, 279
(2) (3)
A complete scheme for the isolation, identification, and estimation of the commoner cations has been worked out, and will be presented soon in book form (1). The results, which may be obtained without difficulty, may be illustrated by the analysis of a 1-mg. “unknown” sample: Al, 3 per cent found (3 per cent given); Fe, 0.1 (none); Cr, l(l.5); Mn, 1 (1.5); Zn, 1.5 (1.5); Ni, 0.7 (1.5); Ca,0.4 (1.5); Pod, not estimated (1.5). The iron found was introduced by the rereagents used. If the purity of the reagents has not been tested, the detection of less than 2 y of iron, calcium, or sodium, which constitute very common impurities of the
RECEIVED &May25, 1936. Presented, with demonstrations by H. K.dlber, before the Division of Physical and Inorganic Chemistry, Symposium on Recent Advances in Microchemical Analysis, at the 89th Meeting of the .%mericanChemical Society, New York, N. Y., April 22 to 26, 1935.
Etching Stainless Steels. A new method for etching stainless steel, prior t o microscopic study of grain structure, has been developed in the Xational Bureau of Standards. All metals are composed of small, imperfect crystals known as grains, the size, shape, and structure of which are of great importance in the study of any metal and its application in service. To reveal this grain structure it is necessary t o etch the metal with a chemical reagent. The appearance is then studied under the metallurgical microscope at suitable magnifications. Certain metals are difficult t o etch satisfactorily because of their compositions. Stainless steels are among the most troublesome, since they resist all ordinary reagents. In the past it has been necessary t o use strong, mixed acids to reveal the structures
of stainless steels, and these mixtures require great care in handling and in disposing of them afterwards. The new method was worked out in connection with a study of the changes induced in stainless steels by welding. The stainless steel is etched electrolytically in oxalic acid (10 grams dissolved in 100 milliliters of water), the s ecimen being the anode and a piece of platinum the cathode. Burrent is supplied from four dry cells in series or from a &volt storage battery. The carbides are revealed in from 15 t o 30 seconds’ etching time, while an additional 30 to 45 seconds will reveal also the grain boundaries of the “18-8” (18 per cent chromium, 8 per cent nickel) type of stainless steel. The solution is relatively rapid in etching action and does not stain the specimen.
177 (1926).
(1908). (7) Paneth, F., and Peters, K., Z . phys. Chem., 134, 353 (1928). (8) Pregl-Fylemann, F., “Quantitative Organic Microanalysis,” p. 1, Philadelphia, P. Blakiston’s Son & Go., 1930. (9) Stock, A., et al., Z . angew. Chem., 44, 200 (1931); 46, 62, 187 (1933). (10) Wiesenberger, E., Mikrochernie, 10, 10 (1931).
