ANALYTICAL EDITION
INDUSTRIAL AND ENGINEERING VOLUME7 NUMBER 4 *
CHEMISTRY
JULY 15, 1935
HARRISOX E. HOWE,EDITOR
General Microchemistry A. A. BENEDETTI-PICHLER, Washington Square College, New York University, New York, N. Y.
M
ICROCHEMISTRY may be defined as the systematic presentation of the technic involved in the performance of chemical experiments on an essentially smaller scale than is usually employed in laboratory practice. In accordance with the atomic concept of matter, the c:hemical phenomena and physical properties observed on the smallest quantities of matter handled up to the present t i n e in microchemical experiments gram or approximately 10l2 molecules) cannot be expected to differ essentially from those observed on a large bulk of material. (Phenomena belonging to the fields of colloid chemistry, the study of radioactive elements, etc., are not considered here.) It is obvious, however, that the technic of working and the methods of observation must vary with the amount of material under consideration.
Field of Microchemistry General microchemistry (3) deals with the development, testing, correlation, and systematization of methods for the handling of small quantities of materials, and for the observation and determination of their properties. It has the task of supplying the working technic used in applied microchemistry, of which microanalysis represents up to the present time the most important branch. Certainly more than nine-tenths of the microchemical methods carried out are used in microanalytical procedures. According to the definition, the upper limit for the size of material to be handled by microchemical methods is reasonably defined by stating that the quantity of material taken should be so small as to prohibit the use of traditional working methods. The lower limit is, of course, determined by thje state of advance of microtechnic. It is obvious that both liimits of the amount of material depend on the nature of the individual problem and will undergo changes in the course of time. If chemists should succeed, for example, in increasing the sensitivity of an analytical procedure by increasing the molecular weight of the precipitate to such an extenit that 1-mg. samples could be analyzed by the standard methods of today, then the use of microtechnic as applied today to the analysis of 1-mg. samples would probably permit the analysis of 0.01-mg. samples. Such an advance would simply effecta shift of both limits of the range of microanalysis in favor of still smaller samples. The example also illustrates the relation between the sensitivity of tests and the necessity of using microchemical technic for their performance: the more sensitive the test, the smaller the quantity
of material which may be analyzed without the use of a microchemical technic. The simultaneous use of sensitive tests and microchemical methods, however, promises the attainment of extremely low limit of identification. The principal task of most chemical work (analytical as well as preparative) is the isolation of the components contained in a mixture, and their subsequent identification or estimation. The isolation itself usually comprises the preparation of suitable compounds, followed by their separation and purification. Identification and estimation are nearly always based upon the observation or measurement of physical properties-states of aggregation, transition temperatures, shape, density, mass, volume, color, refraction and reflection of light, radiation, magnetic susceptibility, etc.-which can be directly or indirectly recognized by sensory impressions. When the size of the specimen under investigation becomes very small, a refined manipulative technic must be employed, and the manifestations of the physical and chemical properties must be amplified by suitable devices in order to be registered by our sense organs. General microchemistry provides these special technics. Micromethods must be rather closely correlated to the amount of material available, and the size of sample which may be taken for a microanalysis varies over a wide range. In general, approximately 10 to 50 mg. constitute the upper limit of sample size for qualitative and quantitative microanalysis. The smallest sample may a t present be considered 0.1 microgram (0.1 gamma, 0.0001 mg.), since it is possible t o collect and recognize quantities as small as 0.00003 y of hydrogen ion, 0.0002 y of hydroxyl ion, 0.0005 y of borate ion, 0.0003 y of cobalt or nickel ion, about 0.001 y of antimony or silver ion, or about 0.01 y of sulfide, arsenic, cadmium, copper, iron, lead, or mercury ion (2). Furthermore, it is possible to collect and measure such small quantities 8s 0,001 y of gold ( 5 ) and 0.01 y of mercury (9). As littIe as 0.00001 y of thorianite should be sufficient to detect the helium occluded in such material (7), and only 0.005 y of air is required for a quantitative analysis by the bubble method af Krogh (6). Gravimetric residue determinations with a few micrograms of sample have already been carried out in great numbers a t Emich’s institute (IO),and the use of microbalances of extreme sensibilities-0.0002 y and less-should make i t possible to work with still smaller samples. A variation in the size of a microsample from 0.1 y to 50 mg. corresponds to a variation in the weight of a macrosample from 1 gram to 500 kg. Consequently, the manipu207
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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.
VOL. 7 , NO. 4
“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).
