Modern Trends in Analytical Chemistry - Analytical Chemistry (ACS


Modern Trends in Analytical Chemistry - Analytical Chemistry (ACS...

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A nalytical

Edition JULY 15, 1930

Volume 2

Number 3

SYMPOSIUM ON ANALYTICAL CHEMISTRY Papers presented under t h e auspices of t h e Division of Physical a n d Inorganic Chemistry a t the General Meeting of the American Chemical Society, Atlanta, Ga., April 7 t o 11, 1930

Modern Trends in Analytical Chemistry’ H. H. Willard DEPARTMEKT OF CHEXISTRY, UNIVERSITY OF BIICHIGAN, ANN ARBOR,MICH.

KALI’TICLIL cheniistry is the oldest branch of the subject and a t one time was the most important. I t is still of fundamental importance, particularly in connection with other lines of work. Little progress \vould be made in experimental research without suitable analytical methods to check the composition of the substances formed or the progress of the reactions. Some of our most startling discoveries in the field of medicine would have been impossible without the assistance of accurate analyses. Many a line of research has been abandoned because of a lack of suitable analytical methods. I n industry its value is well recognized. TYithout the science of analytical chemistry our basic industries could never have deT-eloped to their present state. K h e n a shorter and quicker inetliod is found to produce a certain material, the value of such a discovery is promptly recognized. K h e n a shorter and inore accurate method is found for analyzing the material to assure its uniformity and purity, there is likewise a saving in cost of manufacture. vi-hich is, however, not so promptly recognized. This saving results from fewer rejections by the consunier, as well as less waste by the iiianufacturer. If a melt of steel can be analyzed before pouring, any error in composition can be iiniiiediat ely remedied without m,iting for an analysis of the casting. I t is unfortunate that the economic feature of analytical clieinistry is not inore widely recognized. Our rapid industrial progress has made urgent denialids on this branch of chemistry. The slow nietliods used in the days of Fresenius, Rose, and other famous men are no longer permissible. We deinand rapidity and accuracy. Moreover, many elements, forinerly rare and merely curiosities, are now required in our everyday life. This means that suitable analytical iiietliods for them had to be developed. So great has been the deniaiid for analytical control of i1idus:rial processes that attempts are constantly being made to simplify such methods so that they may be used in a purely meclianical way by workmen who have no knowledge of clieinistry. Khether or not this is desirable from the point

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of view of the chemist does not enter into consideration. The demand is there and inust be met. This has resulted in the use of methods quite different from those employed by the older chemists.

Development of Analytical Chemistry I n the old days a large number of niethods were developed, most of which were rather tedious and subject to numerous errors, the cause and nature of which were not understood. The niechanisn of adsorption and occlusion was unknown; ions had not been heard of. Thirty-six years ago Ostwald published his book on “The Foundations of Analytical Chemistry’! and made the first attempt to put on a theoretical basis the mass of data which liad accumulated during the earlier years. This resulted in an effort to improve old methods by trying to eliminate their errors, which were then understood for the first time. S e w light was tlirown on the phenomena of hydrolysis, titration of acids and bases, use of indicators, precipitation, formation of complex salts, s d u bility, etc. The importance of this can hardly lie overestimated. I t not only showed how to improve existing methods, but also indicated new fields for research which later proved extremely fruitful. Since that time the application of physical and physicochemical methods has increased rapidly. Because the extent of these developments is not so generally known as it should be t o chemists a t large, we are laying particular stress on these things in this syniposium. 11-e are trying to make this a report of progress, incomplete to be sure, because there is not time enough to discuss all recent developments. Optical Measurements The ad\-ance in physical methods in recent years has been very rapid, Coiisidering first the optical ones, we find the use of the immersion refractometer a very rapid and convenient method for measuring the concentration of solutions, particularly where chemical analysis is too long or impracticable. I n common with some other methods it has the advantage that the sample is not destroyed. A more sensi-

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tive type of refractometer, the interferometer, is being used for the measurement of very small concentrations or differences in concentration-as, for example, in work on adsorption. It is especially useful where chemical analysis is impossible. The use of the polariscope is so universal in certain types of analysis as to need no comment. Spectroscopic methods in qualitative analysis have been in use for many years, but only recently have they been brought to a degree of accuracy and convenience which makes them equal to or even superior to quantitative chemical analysis where very small amounts of a n element are concerned. Spectral analysis is used to a large extent in industrial work. X-ray methods are much 111x8 modern than the spectroscope. It is truly amazing what can be accomplished in analytical chemistry by x-rays. A striking illustration is the use of x-ray spectroscopy by Hevesy in his famous work on hafnium. B y this means he was enabled to determine the percentage of hafnium in various zirconium minerals before it had ever been separated. I n the discovery of new elements, x-ray spectroscopy has played a n important part, particularly in the rare earths. This new tool may be put to many diversified uses and industrial laboratories have been quick to recognize its value, for it reveals secrets which cannot be disclosed by any other means. Although the microscope has long been used to a limited extent in qualitative and quantitative analysis, its usefulness hss in recent years been greatly extended and the apparatus much improved. It affords a n entirely different means of identifying substances and, while chemical analysis cannot always tell us the actual compounds present in a mixture, the microscope with its accessories is of great value in this respect. Microscopic tests are often of much more .value than any other. So important have microscopic and microanalytical methods become that we now have a journal devoted to them. To the biological chemist v e owe most of the development of nephelometric and. to a considerable extent, colorimetric methods. In fact, the nephelometer and colorimeter are indispensable to all who have to deal with plant and animal fluids and tissues, because of the small amount of material available and the impossibility of applying gravimetric or volumetric methods. These instruments save an enormous amount of time, but in recent years they have been extended to the determination of hydrogen-ion concentration, and new types of colorimeters have been devised to meet these needs. Electrical Measurements

Another class of physical methods includes electrical meaurements. Nothing has contributed more to volumetric analysis than have potentiometric and conductometric titrations. The older chemists were limited to those reactions for which indicators could be found. No longer are me so limited. Not only has the scope of volumetric analysis been greatly increased, but its accuracy as well, because many indicator methods are not entirely satisfactory. Potentiometric methods are more specific than are conductometric, and are simpler to carry out; therefore, they are more widely used. The research in this field has been extensive. Many types of electrodes have been described and the apparatus required has been made so simple and convenient that it is in daily use in many industrial laboratories. It is used for titrations of all sorts and, more often, for determining the hydrogen-ion concentration of solutions. The hydrogen and quinhydrone electrodes are constantly used where indicator methods cannot be applied or are not accurate enough. Conductometric methods can be applied to many titrations where potentiometric methods fail, and would be more widely used if certain disadvantages could be overcome.

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One of the great advautages of electronietric method5 is the readiness with which they lend themselves to automatic control. The change in potential or conductance may be made to operate a suitable mechanism. I n these days of automatic machines such a mechanism is greatly desired. It is a simple matter to arrange the stopcock in the buret so that it will be automatically shut off a t the end point of the titration, and the chemist can come back and read it a t his leisure. Moreover, this can now be done even when a n indicator is used, the end point being judged, not by the human eye, but by an electric eye or photo-electric cell. There is an apparatus on the market which automatically tests the hardness of the water delivered by a softener and, when it reaches a certain value, regenerates the material. Another apparatus tests water for free chlorine, and automatically adds the proper amount to give a good chlorine highball. Perhaps the analytical chemist may be afraid of losing his job, but he can comfort himself with the thought that the mechanical chemist is doing only what the office boy can and often does do equally well. I t takes brains to devise the simplest test, but carrying it out may be largely mechanical. Radioactive Indicators

Mention should be made of the work of Paneth on the use of radioactive isotopes as indicators in analytical work. H e has shown their value and others have extended their use to a large variety of reactions, particularly useful where very small quantities are involved. Microanalyses

It often happens, especially with the organic and biological chemist, that only very small amounts of material are available for analysis. This has made necessary the development of a n entirely new technic using samples of a few milligrams or even less than 1 milligram. Pregl, of Austria, has been particularly active in developing methods of microanalysis, and we are fortunate in having this subject presented here by two of his students, who are well known in this field. Llicroanalysis has s3 many advantages that it is being more and more viidely used. Organic Reagents

I n the application of organic reagents remarkable advances have been made. The field here is unlimited. The work of Feigl, in 17ienna,has shown the fruitful results of a systematic search for reagents nThich are specific for some particular element, just as our colleagues in medicine search for some compound which will destroy a specific bacillus. There is no doubt that much of the future advance in analytical chemistry will be in the field of organic chemistry. I have often wished that our organic friends would give us a hint regarding t h e possible use of their new compounds as analytical reagents. Such a hint might lead to very important results. Colloid and Physical Chemistry in Analysis

Colloid chemistry has contributed much to our knowledge of precipitation and the errors involved, thereby increasing the accuracy of certain processes. I t has shown us how to improve many older methods and i t has been the means of devising many new ones. Since precipitation is involved in all our gravimetric methods, a knowledge of the mechanism of i t is very important. It has already been shown that analytical chemistry is not only necessary to the other branches, but that it, in turn, draws on them for help. The application to it of physical chemistry is, as already stated, one of the outstanding events in its history. The analytical chemist who is worthy of t h e

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recently, of porous porcelain. He has electric ovens and furnaces with automatic temperature control; he can measure high and low temperatures easily and accurately. Should he not do better work? It was inevitable that the work of Stas on atomic weights, which stood unchallenged for so many years, should finally be surpassed by that of Richards, with his advantages of better theoretical knowledge and better apparatus. The field of analytical chemistry is too large t o be covered in this symposium, but the papers which follow will indicate some of the important progress that has been made.

name must be well grounded in physical chemistry. He must know why he does things a certain way, why he uses a certain indicator in titrating one substance and a different one in titrating another. He is, after all, merely applying modern theories to this branch of chemistry. Equipment of Modern Chemist

The chemist of today is more fortunate than the one of yesterday. He has improved apparatus, more resistant niaterials. He has silica ware, Pyrex, better porcelain, Bakelite. He has filters of spongy platinum, of sintered glass and, most

Microscopical Methods in Analytical Chemistry’ Clyde W. Mason CORKELL UNIVERSITY, ITHACA, N.Y .

Extension of the scope of analysis and the requirements made of it necessitates more direct interpretation in terms of properties and performance than is possible from purely chemical data. The microscope affords a rapid and vivid supplement or substitute for ordinary analytical methods, permits the use of a minute sample, and frequently reveals unsuspected but invaluable additional information. Substances may be tested for purity or identity by optical crystallographic methods or other physical means. Numerical constants, such as refractive indices, melting and transformation points, molecular weights, etc., may be determined, and significant physical factors, such as particle size or surface character, which often control chemical reactivity, may be ascertained. In the analysis of

heterogeneous mixtures, segregations or variations in composition are brought out sharply, as a guide to sampling, and localized gradations are rendered evident. Knowledge of the actual phases present, their relative fineness and distribution in the sample, and the degree of perfection with which they can be separated are valuable aids to ordinary analytical procedures. Individual particles may be separated from their surroundings, for microscopic analysis, or tested in situ. Chemical reactions may be performed in a drop of solution, in microscopic qualitative analysis, or as confirmatory observations to accompany other analytical studies, with a high degree of positiveness in proving identity. Quantitative microscopical estimations of the physical and chemical compositions of mixtures, often incapable of ordinary analysis, are discussed. ..

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N ITS fullest sense chemical analysis implies the ob-

taining of information about the nature and constitution of things. As such, analysis is the foundation of our knowledge of material objects, for its aims include determinations of properties and behavior as well as of composition. We are likely to forget that a determination of percentage composition is rarely the ultimate goal of an analysis, and that numerical data on constituents or properties are almost always translated in some manner, into comparisons of costs, behavior, purity, strength, or other aspects of the history or future performance of the sample. Such interpretations of chemical analyses are frequently indirect and inconclusive, which explains the rapid growth of analytical procedures that seek to ascertain directly the required facts. The microscope yields direct information, for it depends on our most universal and familiar method of observation, seeing. Think of all the chemical operations and tests in which constant and intensive visual observation is necessary and the worker depends for his conclusions largely on what he sees take place. Even the most elaborate and automatic apparatus will never be a complete substitute for looking at the sample, any more than a thermal diagram of the system H20 together with the “space lattice” of ice would picture a snowfall. With the microscope a t hand the phenomena, of which numerical data present only a meager and one-sided picture, become concrete instead of abstract. There is no need for the sort of inference which is practiced every day, when an unexpected precipitate is described with “I guess that’s the -coming out,” if that precipitate can be examined microscopically. 1

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How frequently the chemist gazes into his apparatus, wishing that somehow he could see what was really taking place-could be actually in the midst of the process instead of “on the outside looking in.” The microscope is the one tool of the chemist which gives him an inner view of chemical phenomena. It enables him to direct his own experience and knowledge upon minute features as well as upon those visible to the naked eye. It bridges the gap between the ordinary limit of visibility and the realm of atomic dimensions or x-ray studies. The very concreteness of microscopical observations acts as preventive against too abstract or hypothetical interpretations, and reminds the user that simple physical conditions may often serve to explain apparent differences in chemical character. When microscopical studies of properties or composition are being made, the specimen itself must be subjected to examination; and in almost every case unsuspected and highly significant additional information, which would have been missed by ordinary methods, is revealed. A plea for the use of the microscope is incomplete without a reminder that no methods are purely objective, nor any instrument wholly automatic-least of all one which plays such a multitude of roles as does the microscope in the chemical laboratory. However elaborate the stand and its equipment of lenses and accessories, the microscope is no more than a versatile tool. The most indispensable accessory of all is not an attachmen? but rests upon the shoulders of the observer; it consists of two main parts, eye and brain. So one might say that chemical microscopy is a point of view-an aggregate of related facts, principles, and