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Charles M. Beck II National Institute of Standards and Technology Chemical Science and Technology Laboratory Inorganic Analytical Research Division Gaithersburg, MD 20899
Despite the fact that instrumental analysis has rightfully assumed an overwhelmingly major role in the analytical laboratory, there remains a limited, although important, need for classical analysis. I n s t r u m e n t a l analysis is most useful for elemental determinations at minor and trace levels (about 1%all the way down to This article not subject to U S . copyright. Published 1991 American Chemical Society.
1 atom), and in this range classical analysis performs either poorly or not at all. However, instrumental analysis generally does not give high precision and accuracy at major levels (about 1%up to loo%), and in this range classical analysis does perform well. Moreover, instrumental and classical analysis complement each other and can be used in tandem to the analyst's advantage. In addition, classical analysis will always be needed to establish standards for instrumental calibration. The purpose of this article is to define and discuss the scope of classical analysis and to explore the importance of renewing the chemical com-
munity's interest in it. The histories of gravimetry and titrimetry a r e t r a c e d , including biographical sketches of some important personalities in the development of classical analysis in Europe and the United States. The significance of the events
surrounding the Karlsruhe Congress of 1860 and the importance of physical chemistry and organic reagents to the development of classical analysis are discussed. The current crisis in the United States resulting from a
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REPORT shortage of qualified classical analysts is examined. The future use of classical and instrumental analysis in tandem is examined. Because of the serious economic consequences to American industry and government that would result from the disappearance of classical analysis, a proposal is made for the renewal of education in this field. What is classical analysis? If the final step of a n analysis is a gravimetric or titrimetric determination, it is considered a classical analysis. The following test may be useful in distinguishing classical from instrumental analyses. The calculations for a classical analysis require no more than experimentally measured weights (or weights and volumes), definite chemical reactions, and atomic weights. Such a test demonstrates, for example, that a potentiometric titration is a classical analysis, because t h e potentiometer serves only to indicate the endpoint of the titration. It also demonstrates that a spectrophotometric determination is a n instrumental analysis because the calculations require a n experimentally measured transmit tance of light through a solution. There a r e two good reasons for clearly distinguishing between a classical and an instrumental analysis. First, in the hands of an experienced analyst, the relative precision of a classical analysis is about 0.1%0.2% or better, whereas the relative precision of a n instrumental analysis is about 1%-2% (although certain exceptions exist). Second, whereas almost all instrumental analyses are comparative a n d require known standards for calibration, classical analyses do not require external standards for calibration if the stoichiometries implied by the written chemical reactions reflect what is actually happening in the analytical process. For gravimetry this implies that actual precipitates correspond to written formulas, and for titrimetry that actual reactions correspond to written reactions and that indicators detect true equivalence points. We have advanced far enough beyond the 1940s to see that that decade was a watershed period marking the end of the development and general use of classical analysis and the beginning of instrumental analysis. Although the use of instrumental analysis had been growing slowly up until that time, most samples were analyzed classically. Since the 1940s instrumental analysis has assumed more importance because of the ever994 A
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increasing demands for sensitivity, speed, and economy. (For a sketch of the history of instrumental analysis, see Reference 1.) After World War 11, the existing, under -utilized instrumentation, as well as subsequently developed instrumentation, was made practical with the introduction of the photomultiplier tube, the transistor, and the microprocessor. We now have sensitivities and speeds that would have been unimaginable even a few years ago, but, as has always been the case, new demands keep pushing the definitions of “fast” and “ultratrace” to new levels. Because many, if not most, analytical chemists in the work force today were educated after the beginning of the instrumental era in the 1950s, it is useful to review the history of classical analysis. An awareness of the history of a discipline helps to provide a perspective for evaluating current conditions and needs and for anticipating future possibilities. History of gravimetry to the 1850s Gravimetry is the determination of a n element through the measurement of the weight of an insoluble product of a definite chemical reaction involving that element. Tracing the history of gravimetry amounts to tracing the early history of chemistry. Chemistry is at least as old as recorded history, but what we recognize as experimental chemistry did not emerge until the end of the six-
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teenth century. Later, with the publication of his book The Sceptical Chymist in 1661, Robert Boyle began the process of putting chemistry on a sound scientific footing. Boyle firmly dismissed Aristotle’s four elements of fire, air, water, and earth and Paracelsus’ three principles of mercury, sulfur, and salt. Instead, he advocated a comprehensive experimental approach before attempting any theoretical statements. As t h e influence of Paracelsus waned, interest in chemistry shifted from medicine to mineralogy and metallurgy. However, chemists were influenced for another century by the “phlogiston theory,” first advocated in 1681 by Johann Becher and popularized by Georg Ernst Stahl. According to this theory, when a metal burns or rusts it gives off a substance called phlogiston. When the phlogistonists observed t h a t iron gained weight when it rusted, they simply postulated that phlogiston had negative weight. As damaging as this theory was to the progress of chemistry, it did provide a general concept, and testing that concept led to a study of chemical analysis and simple chemi cal reactions. Among those making such studies were the gas chemists Joseph Black, Henry Cavendish, Carl Scheele, Daniel R u t h e r f o r d , a n d J o s e p h Priestley. Although Priestley was one of the last staunch defenders of the phlogiston theory, he initiated its downfall in 1774 when he isolated a
gas by heating mercuric oxide in a closed system. He gave the name “dephlogisticated air” to the oxygen he collected. A few years earlier, Rutherford and Cavendish had indepen dently discovered nitrogen, which Rutherford called “phlogisticated air.” Aware of Rutherford’s a n d Priestley’s work, Antoine Laurent Lavoisier dealt the death blow to the phlogiston theory by performing quantitative experiments with mercury and air in a closed system. He correctly explained combustion and demonstrated that air was a mixture of nitrogen and oxygen. His genius, his instinctive recognition of the law of conservation of mass, and his use of the balance made him the forefather of t h e quantitative era i n chemistry. At about the same time, the Swedish chemist Torbern Bergman outlined a systematic scheme of qualitative and quantitative analysis and described all of the available known reagents. His gravimetric method for silica is recognizable as a direct forerunner of the one we use today. Strangely enough, Bergman r e mained a firm believer in the phlogiston theory all his life. Although titrimetry already was being used in France at the turn of the nineteenth century, almost all analyses were done by gravimetry. For example, Sigismund Andreas Marggraf had worked out the determination of silver as silver chloride as a substitute method for fire assay which, under certain conditions, tended to give low results for silver. (Fire assay is the ancient art of isolating precious metals from an ore. The method is based on a high-temperature liquid-liquid extraction in which the ore is fused with a mixture of lead oxide, flux, and a reducing a g e n t . Molten l e a d i s formed throughout as the mixture is heated and the precious metals pass into the lead. Upon cooling, a lead “button” forms at the bottom of the mass from which the precious metals can be extracted.) Bergman was already using gravimetric factors to calculate the amounts of iron, lead, copper, and silver in various precipitates, and others were becoming interested in the analysis of minerals and industrial materials. At the turn of the nineteenth century chemistry entered a period of great confusion. Gravimetry had been developing through the eighteenth century in an empirical manner, because the laws of chemical composition were not understood. Many chemists believed t h a t sub-
stances combined in definite propor tions. For example, they knew that a certain weight of silver always gave the same weight of silver chloride. Jeremias Benjamin Richter, a mining engineer in Silesia, also believed that there was an equivalence inherent in chemical reactions. He tried to work out the mathematical relationships and coined the word “stoichiometry” for the proportions existing among various substances. Unfortunately, he was led down many a blind path because he tried to force his data to
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fit his preconceptions. His work was a step in the right direction, however, because he had an intuitive sense of the law of definite composition. Such was not the case with the famous French chemist Claude Louis Berthollet, who wrongly postulated that the composition of a compound of two elements might vary between maximum and minimum in all proportions. Joseph Louis Proust opposed Berthollet’s view and present ed experimental evidence that metals form oxides and sulfides of definite composition. He also recognized that if a metal forms two oxides, each has a definite composition and no product of a n intermediate composition exists. With this observation he came close to discovering the law of multiple proportions,
In 1808 John Dalton published the first part of the book New System of Chemical Philosophy. He proposed the idea that matter was composed of small discrete particles. Although this concept dated back to the Greeks of 400 B.C., Dalton’s theory was much more far-reaching; it explained the law of conservation of mass as well as the laws of definite composition and multiple proportions. Dalton realized that because compounds are formed by uniting atoms of different elements w i t h different relative weights that can be expressed numerically, the composition of chemical compounds can be expressed quantitatively. He constructed a table of atomic weights, but because of his poor data he was unable to demonstrate the simple relationships that intuitively he knew must exist. Also in 1808 Joseph Louis GayLussac published a paper on the combining volumes of gases. He consistently found that the volume ratios in gaseous reactions were small whole numbers. This result seemed to contradict Dalton’s atomic theory because if one volume of C1 and one volume of H gave two volumes of HC1, then the “atoms” of C1 and H must divide-a logical impossibility if the atomic theory were true. Amedeo Avogadro reconciled the dilemma in 1811 by assuming that equal volumes of gases under the same conditions contained the same number of particles, which he called “molecules.” He reasoned that these gaseous molecules split into “halfmolecules77when they react. In effect, he supposed t h a t elemental gases contained more than one atom, but he never used the term atom. Dalton refused to accept Gay-Lussac’s law a n d t h u s could n o t a p p r e c i a t e Avogadro’s remarkable and revolu tionary insight. We know today t h a t Avogadro’s reasoning was correct, but the chemists of his day either rejected or simply ignored his hypothesis. Efforts in 1814 by Andr6 Marie Amphe and in 1826 by Jean Baptiste Dumas to revive Avogadro’s hypothesis went unnoticed. The world of chemistry was not ready for Avogadro. Sadly, it would take another 50 years of confusion about atomic weights before Stanislao Cannizzaro would success fully revive Avogadro’s hypothesis. During this period of confusion, practical analysts with little interest in theoretical matters were doing very accurate gravimetric analyses of metals, minerals, and water. Among these were Richard Kerwan, who published a n outstanding book list-
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REPORT ing all the references on water analysis since t h e time of Bergman; Martin Heinrich Klaproth, who discovered many elements in his analyses of minerals; and Louis Nicolas Vauquelin, who was widely known for his pure reagents. The dominant figure of this period was Jons Jakob Berzelius. During an extraordinarily productive decade between 1807 and 1818, he devised the system of chemical symbols and notations we use today and, through a series of elegant analyses, determined the atomic weight of many of the elements. Unfortunately, he also devised his dualistic theory, which prevented him from accepting Avogadro’s hypothesis. Nevertheless, his analytical expertise was nothing short of incredible, and the atomic weights published in his 1828 table approach today’s values, if you discount the fact that a few of the elements listed were twice their correct values. Working with the equipment of his day, much of which he improved himself, Berzelius produced a quantity of accurate work that would be a credit to a modern, wellequipped research laboratory. Although Berzelius was one of the world’s greatest analysts, it is doubtful that he ever viewed himself as such. In contrast, Karl Remegius Fresenius thought of himself as an analyst from the beginning of his remarkable career. After being a n apprentice pharmacist in Frankfurt for several years, he took courses at the university in Bonn and worked in the private laboratory of Carl Marguart, his professor of pharmacy. Working mostly alone and without instruction, Fresenius taught himself and kept good notes. Marguart was so impressed with these notes that he suggested they be published. Anleitung zur qualitativen chemischen Analyse, published in 1841, was an immediate success and was to see 16 editions under Fresenius’ authorship. The 17th edition, overseen by his son, was translated by C. A. Mitchell in 1921 as Introduction to Qualitative Chemical Analysis. Fresenius went to the University of Giessen as a lecturer, worked in Liebig’s laboratory, and obtained his Ph.D. in 1842, using his new book-already in its second edition-as his thesis. He was hired in 1845 as professor of chemistry, physics, and technology at the Agricultural College at Wiesbaden. Because the college adminis trators repeatedly refused to supply funds for a chemistry laboratory, he borrowed money from his father, a 996 A
prosperous attorney, bought a house, and remodeled it into a private analytical laboratory. He opened the doors in 1848 with five students and one assistant, Emil Erlenmeyer. Only 30 years old, Fresenius was already a n experienced analyst. Short ly after assuming his duties at Wiesbaden, he published his second book, Anleitung zur quantitativen chemischen Analyse, which ran to six editions and was translated in 1904 by A. I. Cohn as Quantitative Chemical Analysis. Fresenius’ small houseAaboratory would grow to become the worldrenowned Fresenius Institute. By 1855 his laboratory had 60 students, all of whom were eligible to receive university credit for the time they spent there. In addition to providing instruction, the institute rapidly became known throughout government, industry, and academia as an analytical laboratory. In 1862 Fresenius founded Zeitschrift f u r analytische Chemie, the first journal entirely devoted to analytical chemistry. His influence throughout Europe and the entire world was enormous. History of titrimetry to the 1850s Titrimetry is the determination of an element through the measurement of the weight of a chemical necessary to just complete a definite chemical reaction in a solution containing that element. The weight of the chemical is usually obtained indirectly by measuring the volume of a standard solution of that chemical, although
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for very accurate work the amount of standard solution needed can be measured by weighing it. Industry’s need for rapid methods for determining acids, alkalis, carbonates, and hypochlorites provided the driving force for the development of titrimetry. Early development was confined almost solely to France, where it was crudely practiced in the eighteenth century. Endpoints were determined by the “clear point,” the cessation of effervescence, or the use of a few plant -extract indicators. Fransois Antoine Henri Descroizilles devised a method for determini n g t h e hypochlorite s t r e n g t h of bleaching solution used in the textile industry. He first added a measured amount of dilute sulfuric acid containing a n indigo indicator to a graduated cylinder and then slowly added t h e hypochlorite solution (whose strength was being tested) until the color changed from blue to pale green. He then read the volume from the graduations on the side of the cylinder. Descroizilles later devised a method for determining the alkaline strength of potash. He used a graduated buret filled with dilute sulfuric acid and controlled the flow by covering a small air hole in the top with his finger. Gay- Lussac’s contributions to ti trimetry and the widespread use of his methods helped establish titrimetry as part of classical analysis. He improved Descroizilles’ method for potash and made several improve-
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ments in the hypochlorite method, notably the introduction of the redox indicator. As an analyst, Gay-Lussac is most famous for the silver assay method named after him. The French government, which was losing money because of errors in the fire assay of silver, asked Gay-Lussac in 1829 to devise a rapid and simple method with a relative error of 0.05%. He made a 100-mL pipet that would deliver repeatedly with very high precision, and he prepared a standard chloride solution that he adjusted so that the delivery of the pipet was equivalent to 1.0000 g of Ag. He then dissolved a bullion sample expected to contain just over 1 g of Ag and added one pipet full of the standard solution. After vigorous agitation, the silver chloride precipitate was allowed to flocculate and settle to the bottom of the flask. Using a 1:lO dilution of the standard chloride solution, he continued the titration in 1-mL increments until no more turbidity was produced in the supernatant. He achieved a relative accuracy and precision better than the requested 0.05%,and to this day no improvements on this assay method have been made except for the use of a potentiometric endpoint for deeply colored solutions or alloys of silver containing tin. From 1835 to about 1855 many different titrimetric methods were developed but not widely used. As a result of Gay-Lussac’s work, however, titrimetry became known outside of France, especially in Germany and England. I t was not yet possible to establish a general system of titrimetry because the concentrations of standard solutions had no chemical basis; there were no unique atomic weights and stoichiometry was not understood. Houton de la BillardiGre discovered the usefulness of iodine in titrimetry in 1826, and in 1853 Robert Wilhelm Bunsen published an excellent paper on iodimetry describing the determination of more than 20 elements. In that same year, Karl Leonhard Heinrich made a great advance by recommending the use of sodium thiosulfate for titrating iodine, Nev ertheless, the famous analysts of the period remained contemptuous of titrimetry. Berzelius never used it, and Fresenius recommended that it not be used for important analyses. Karl Friedrich Mohr did much to overcome the difficulties of titrimetry. He studied under the direction of Heinrich Rose and Leopold Gmelin, Bunsen’s predecessor at Heidelberg.
Mohr took over his father’s pharmacy in 1830 and in his spare time experimented with various titrimetric methods. Because he remained outside academia, he was never regarded as a scientist in Germany, but his contributions to t i t r i m e t r y were many. He introduced the use of potassium chromate as an internal indicator for chloride determination (Mohr method), oxalic acid as a primary standard for alkalimetry, fer rous ammonium sulfate (Mohr’s salt) as a standard for oxidizing agents, and the idea of back-titration. As for laboratory equipment, he invented the cork borer, the Leibig condenser, the Mohr pinch - cock, the pinch - cock buret, and calibrated pipets. After h i s book Lehrbuch d e r chemischanalytischen Titrirmethode was pub lished in two parts in 1855 and 1856, titrimetric analysis became widely known in Europe. The publication of Mohr’s books marked the end of the early history of titrimetry, although in 1883 Johan Gustaf C. T. Kjeldahl developed his well- known method for the determination of nitrogen. He discovered that sulfuric acid could be used to digest organic materials, converting f r e e a m m o n i a a n d “organically bound” nitrogen to ammonium sulfate. After adding excess strong caus tic, he distilled the liberated ammonia into a known excess of standard sulfuric acid and back- titrated with s t a n d a r d sodium hydroxide. The method has remained unchanged, except for the use of various catalysts to aid in the digestion and Lauos Winkler‘s discovery that the distilled ammonia can be absorbed into a boric acid solution and titrated directly with standard acid. Winkler contributed more than 200 original papers on gravimetry and titrimetry and is probably best remembered for his titrimetric method for the determination of dissolved oxygen in water (published in 1888). The Karlsruhe Congress of 1860 By the late 1850s classical analysis had come about as far as it could without a consistent table of atomic weights. Atomic weight tables varied among different countries and some times among different laboratories within the same country. To resolve this problem, F r i e d r i c h August Kekul6-with the help of Charles Adolphe Wurtz and Carl Weltzieninvited delegates to a n international congress at Karlsruhe, Germany, in 1860. About 140 of the world’s leading chemists attended. Cannizzaro, a young professor at
the University of Genoa, who for some time had realized the value of Avogadro’s hypothesis in resolving the problems surrounding atomic weights, addressed the Karlsruhe Congress with great passion and pedagogical skill. He showed how Avogadro’s hypothesis could be used to establish the molecular weight of a gas and demonstrated that by com-
paring the vapor densities of a series of gaseous compounds of a particular element, the molecular weight and atomic weight of that element could be determined. Although Cannizzaro pleaded with his colleagues for the adoption of atomic weights based on Avogadro’s hypothesis, he was unable to sway congress members in his favor. The congress adjourned, having reached no agreement on atomic weights, but it was in adjourning that the key event of the congress occurred. As the delegates departed, Cannizzaro’s colleague, Angelo Pavesi from t h e University of Pavia, passed out reprints of Cannizzaro’s 1858 publication, Sunto di un corso di
Filosofia chimica (Sketch of a Course of Chemical Philosophy). This paper set forth clearly what Cannizzaro had been teaching his students at Genoa. Although his paper had gone largely unnoticed in the literature, it did not
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REPORT go unnoticed in the hands of the delegates. Julius Lothar Meyer read it twice on his homeward journey. Everything became clear to him, as if scales fell from his eyes. Dmitrii Ivanovitch Mendelbev later said that even though no a g r e e m e n t w a s reached by the congress, the truth of the law of Avogadro as advocated by Cannizzaro soon convinced everyone. Without the insight that both Meyer and Mendelhev gained from Canniz-
of the century. From about the middle of the century, Germany was the place to go for graduate education in chemistry. Justus Liebig, who established a teaching laboratory at the University of Giessen, generated remarkable enthusiasm and camarade rie among his students, and his laboratory became a model for other graduate programs and teaching lab oratories throughout Germany. William Francis Hillebrand was born in Honolulu seven years before
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zaro’s paper, it is doubtful that they independently would have gone on to work out the periodic table in 1869. It is hard to overestimate the impact of the events surrounding the Karlsruhe Congress. The confusion over atomic weights almost completely disappeared in just a few years, and analysts were able to write down correct stoichiometric formulas for t h e i r precipitates. Gravimetry was placed on solid, although still somewhat empirical, ground. Titrimetry also benefited from this great advance, but it (and, to a lesser extent, gravimetry) lacked a firm scientific basis until the development of physical chemistry at the end of the nineteenth century. Gravimetry comes to the United States In the mid-nineteenth century almost all new developments in chemistry were taking place in Europe. Graduate schools did not develop in the United States until near the end 998 A
the Karlsruhe Congress, and his stor y is perhaps the best example of how classical analysis, particularly gravimetry, was brought to the Unit ed States. His father, a German-born medical doctor, established a practice in Hawaii because its climate would help him to recover from a serious illness. Because his father intended to return to Germany with the entire family, Hillebrand was sent to Corne11 in 1870 to prepare for study at a German university. The family moved to Bonn in the summer of 1872, and Hillebrand had to choose a career. He had no interest in medicine and felt he lacked the mental qualifications for law or engineering. His father suggested chemistry. Remembering with pleasure his study of the basics of chemistry back in Honolulu, Hillebrand decided to give chemistry a try. He entered the University of Heidelberg in 1872 and studied under the guidance of Bunsen and Gustav Kirchhoff, earning his Ph.D. in 1875. He stayed on
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at Heidelberg for another year of research and then spent three semesters with Rudolph Fittig a t Strasbourg. Realizing t h a t organic chemistry did not appeal to him, he decided to finish his studies a t the mining academy a t Freiberg to supplement t h e training i n mineral analysis, which he received under the direction of Bunsen. He had made up his mind to become a n analytical chemist. He returned to the United States in 1878 and, failing to find immediate work in the East, made his way in 1879 to Leadville, CO, where he became the third partner in a small assaying firm. Samuel F. Emmons, an occasional customer who was in charge of the Rocky Mountain Division of t h e newly formed United States Geological Survey (USGS), offered Hillebrand a job as a chemist. Hillebrand considered Emmons’ offer the opportunity of a lifetime and quickly accepted. He remained in the Denver laboratory of the USGS until 1885, when he was transferred to the Washington USGS laboratory to work under the direction of the chief chemist, Frank W. Clarke. Hillebrand’s careful work set a new standard of excellence in the analysis of rocks and ores. During his 29 years a t the USGS, he made more than 400 complete analyses of silicate rocks. The significance of his work lay in the perfection of a separation scheme for materials as complex as carbonate and silicate rocks. Even in the late nineteenth century, the problem wasn’t so much the lack of a suitable method for the final determination of an element as it was the lack of suitable separation methods preceding the final determination-a problem that still exists. I n 1897 Hillebrand wrote a 50page introduction to USGS Bulletin No. 148 on the methods of analysis of silicate rocks, and this was quickly translated into German. In 1900 the introduction was revised, enlarged, and printed as a n independent document, Bulletin No. 176. The next edition appeared in 1907 as Bulletin No. 305. I t included carbonate rocks and was also translated into German. The series culminated in 1919 with the well-known Bulletin No. 700, a book of 285 pages. Hillebrand was unquestionably one of the world’s leading analysts when he was called to be chief chemist a t the National Bureau of Standards (NBS) in 1909. Administrative duties in his new position limited his time at the bench, but the steady stream of books and papers contin-
ued. He greatly contributed to the increase of the standard samples program at NBS, expanding it from a few cast irons to more than 60 materials. (Today, the Standard Reference Materials Program at the National Institute of Standards and Technology [NIST, formerly NBS] has more than 1200 standard reference materials available, covering a broad range of materials that are certified for chemical composition and/or physical property.) Hillebrand had a talent for gathering around him capable people, and his success in bringing Gustaf E. F. Lundell from Cornel1 in 1917 is a good example. In 1923 he and Lundell began coauthorship of Applied Inorganic Analysis. Hillebrand died in 1925 before the work was completed, but by 1929 Lundell had finished the book, which became known as the “analyst’s bible.” Together with James I. Hoffman, Lundell wrote a companion volume in 1938, Outlines of Methods of ChemicalAnalysis, which be came another classic. (Hoffman and Harry A. Bright brought out a second edition of Applied Inorganic Analysis in 1953.) Lundell was appointed chief chemist at NBS in 1937, and under his leadership the scope and renown of its efforts in chemical analysis grew. He had a talent for picking the right person for a particular assignment, and he had an almost uncanny ability to sense whether work on a project that had hit a snag should be stopped or continued. Often his timely encouragement resulted in the solution of a seemingly hopeless problem. In 1933 Lundell published his wellknown paper, “The Chemical Analysis of Things as They Are” (2). The continuing relevance of his insights is remarkable, and this paper should be required reading for all analytical chemists. Physical chemistry applied to classical analysis Once t h e confusion over atomic weights was settled, the stage was set for the maturation of gravimetry. Although many refinements would result from the application of physical chemistry to gravimetry, putting it to practical use did not require an understanding of the rate of precipitate formation, growth of crystalline precipitates, adsorption and occlusion of impurities, and the aging of precipitates. Such was not the case w i t h t i t r i m e t r y . Correct atomic weights certainly were as necessary for titrimetry a s for gravimetry. However, i n p u t from t h e newly
emerging physical chemistry would be essential for titrimetry to reach maturity. Physical chemistry did not emerge until the last third of the nineteenth century. A great breakthrough was made between 1864 and 1879 when the Norwegian brothers-in-law Cat0 M. Guldberg and Peter Waage formulated the law of mass action. In 1884 Jacobus Henricus van’t Hoff made a n elegant derivation of the law of mass action based on thermodynamics. His clear and inspiring
t r e a t m e n t of chemical dynamics brought the entire subject of reaction kinetics and equilibria before the chemical world. Certain anomalies that occurred when the laws of physical chemistry were applied to solutions of electrolytes were explained in 1887 when the revolutionary theory of electrolytic dissociation was published by Svante August Arrhenius. It was Wilhelm Ostwald who recognized the importance of the work done by van’t Hoff and Arrhenius. Through his books, research, and personal contacts, Ostwald was influential in spreading the ideas of the new physical chemistry. With van’t Hoff he founded the journal Zeitschrift jkr physikalische Chemie in 1887, and he championed the cause of physical chemistry as a science in
its own right. In essence, he “organized” physical chemistry at the end of the nineteenth century. In 1894 Ostwald published Die wissenschaftichen Grundlagen der analytischen Chemie (The Scientific Foundations of Analytical Chemistry) and thereby began to put classical analysis on a scientific basis. In the preface he noted that although the technique of chemical analysis stood at a very high level, its scientific treatment was almost completely neglected. Ostwald’s book was a significant start toward correcting this deficiency. He discussed precipitation in detail, including the increase in particle size of s t a n d i n g crystalline precipitates-a process that became known as “Ostwald ripening.” The most important part of the book dealt with chemical separations. By combining the law of mass action and Arrhenius’ theory of electrolytic dissociation, he introduced the concepts of dissociation constants and solubility product constants. Although Ostwald’s little book broke new ground and was soon recognized as a classic, it suffered some significant omissions. For example, t h e phase rule of Josiah Willard Gibbs was not mentioned. Perhaps Ostwald was simply unaware of Gibbs’ work because it had been published in a rather obscure journal. However, it is difficult to understand why he made no mention of the work of Walter Nernst, who originated the famous Nernst equation while working in Ostwald’s own laboratory in 1889. Nernst explored many analytical applications of his equation, and he is rightly considered the father of modern electroanalytical chemistry. Titrimetry comes to the United States As was the case with gravimetry, titrimetry developed in Europe and t h e n came to t h e United States, chiefly through the efforts of Izaak Maritus Kolthoff, who came to the University of Utrecht in Holland in 1911. Kolthoff ’s lack of certain prerequisites demanded by the chemis try department was a blessing in disguise because it brought him under the tutelage of Nicolaas Schoorl in the school of pharmacy. Schoorl had studied under the direction of van’t Hoff and was aware of the recent advances in physical chemistry. He immediately recognized Kolthoff’s talent and encouraged him to carry out independent research. Kolthoff had trouble understanding the proper selection of indicators for acid-base titrations and there-
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REPORT fore launched his own investigation. He had already acquired a used copy of Ostwald’s 1894 classic, and he was further encouraged by the 1909 paper of t h e Danish physiological chemist S.P.L. Sorensen, who introduced the concept of pH. Kolthoff was further inspired in 1913 by the work of Joel Hildebrand, who used the hydrogen reference electrode in electrometric titrations. Kolthoff borrowed pH measuring equipment, but within a year he had devised his own potentiometric apparatus. In 1915 he published his first paper on the titration of phosphoric acid. Kolthoff quickly realized the importance of physical chemistry to a n alytical chemistry, but in those early years, sometimes t h e going was rough. Not recognizing the signifi cance of his early work, chemistry department faculty members sometimes criticized him. Some went so far as to try to block him from publishing and lecturing. Despite such criticism, Kolthoff forged ahead and in 1918 presented his Ph.D. thesis on the “Fundamentals of Iodimetry,” a topic he returned to again and again over the years. In 1914 the Danish physical chemist Niels J. Bjerrum published a book showing how to calculate both the shape of neutralization curves and the titration errors in visual endpoint determinations. Bjerrum’s work inspired Kolthoff to develop a theoretical interpretation of all the methods of titrimetry. (This work resulted in his famous two-volume book, first published in German in 1926 and translated i n 1928 and 1929 a s Volumetric Analysis by N. Howell Furman of Princeton University. A three-volume revision of the work appeared between 1942 and 1957.) In 1924 Kolthoff conducted a lecture tour of Canada and the United States, where he met both Furman and Hobart H. Willard of the Univer sity of Michigan. Willard had pioneered precipitation from homoge neous solution and also investigated the analytical uses of perchloric acid. Furman made outstanding contributions in potentiometry, electrodeposition, and polarography, and together with Willard worked on the analytical applications of ceric salts. I n addition to becoming good friends, these men became “The Big Three” in graduate education in analytical chemistry in this country. In 1927 Kolthoff accepted a professorship a t the University of Minnesota a n d continued h i s m o n u m e n t a l achievements, which touched almost 1000 A
every area of analytical chemistry. Between 1932 and 1960 these three men supervised 120 Ph.D.s in analytical chemistry. Their educational progeny are into the third and fourth generations and number in the thousands. Kolthoff also has influenced many thousands of undergraduate students through his well-known
Textbook of Quantitative Inorganic Analysis, coauthored with Ernest B. Sandell in 1936. A fourth edition appeared i n 1969 with Sandell, Edward J. Meehan, and Stanley Bruckenstein as co-authors and now that it is out of print, it is particularly treasured by those who own a copy. Organic reagents applied to classical analysls Classical analysis does not suffer from a lack of good analytical methods for the final gravimetric or titrimetric determination of an element, provided t h a t t h e element occurs alone. But because this is never the case, interfering elements must be removed. The elaborate scheme perfected by Hillebrand for the analysis of rocks is a n example of a general separation scheme built on the experience of generations of analysts. Based almost entirely on precipitation separations using inorganic reagents, it stands as one of the monumental achievements of classical analytical chemistry. Beginning in the last century and extending up to the 1940s, various organic reagents were introduced that could be used as precipitants in gravimetric determinations. They were extremely useful because they were selective for a very few elements and could be used to determine one of those elements when others were absent or had been removed. Most of these reagents were chelating agents such as dimethylglyoxime and a-benzoin oxime, used for the determination of nickel and molybdenum, respectively. In addition to being used as precipitants, these and other organic reagents were extremely useful when coupled with immiscible solvent extraction. This procedure can be used for the separation of aluminum in the widely used t i t a n i u m alloy with nominal composition 6% Al, 4% V, and 0.2% Fe. Addition of cupferron to an acid solution of the alloy produces the insoluble cupferrates of titanium, vanadium, and iron. Extraction with chloroform leaves aluminum in the aqueous phase ready for determination by titrimetry or gravimetry. In the 1930s it was discovered that certain amino polycarboxylic acids
ANALYTICAL CHEMISTRY, VOL. 63, NO. 20, OCTOBER 15, 1991
formed stable, soluble complexes with many metals, and in the 1940s Gerold Schwarzenbach began a theoretical examination of the complexes. This work led to the development of the titration of calcium and magnesium with EDTA and the introduction of metallochromic indicators. Schwarzenbach’s work made an extraordinary contribution to titrimetry, and thousands of papers have appeared on EDTA-related methods. Ion-exchange resins are perhaps the most significant contribution to classical analysis resulting from the use of organic reagents. Ion exchange had been observed in certain natural materials in the middle of the nineteenth century, but these materials were unstable in acidic and alkaline solutions. In 1934 two British chemists, B. A. Adams and E. L. Holmes, discovered the cation-exchange properties of a synthetic organic polymer made from formaldehyde and tannic acid. Later, cross-linked polystyrene resins with ionizable acidic or basic functional groups were synthesized, and these could exchange with cations or anions. Great advances in the understanding and use of ion-exchange resins occurred during World War I1 because of the work done on the Manhattan Project. In 1942 George E. Boyd and his group used ion-exchange resins to separate plutonium from uranium. Later Boyd and his associates used ion exchange to separate the elements produced in fission prior to chemical analysis. Meanwhile, Frank H. Spedding and his group a t Ames, IA, performed largescale separations of naturally occurring rare-earth elements. Because of the secrecy surrounding the Manhattan Project, none of this work was announced until after the war. Since t h a t time, many ionexchange separation procedures have been developed and there is extensive literature on the subject. The equipment for ion exchange is relatively simple but, unlike immiscible solvent extraction, ion- exchange separations are time consuming. W h e n one considers t h e i r usefulness, however, they are worth the investment of time. For example, manganese, iron, cobalt, nickel, copper, and zinc in strong hydrochloric acid can be completely separated on a n anion-exchange column using successive elutions with hydrochloric acid of different concentrations ( 3 ) . This ion-exchange system has greatly simplified the assay for zinc in zinc ore concentrates. After dissolving the ore concentrate in acid and volatiliz-
ing arsenic, antimony, and tin a s their bromides, the solution is placed on an anion-exchange column where all remaining interfering elements (except cadmium) can be separated by elution with 0.5 M HC1. Zinc and cadmium are eluted with 0.5 M HCl, and the zinc is determined by EDTA t i t r i m e t r y after t h e cadmium is masked as its iodide complex. The immense power of ion exchange is further illustrated by the complete separation of the common rock- forming elements (except silicon and phosphorus) using a single cation-exchange column (4). After acid dissolution of the rock sample and volatilization of SiF,, the solution is placed on a cation-exchange column and, by using a series of eluants, vanadium, sodium, potassium, titanium, zirconium, iron, manganese, magnesium, calcium, and aluminum are eluted in order. Although the procedure is slow, taking two 8 - h days, the separations are complete and the recoveries compare well with those of traditional classical separation methods. The present Although classical analysis will never be used to the extent that it was in the past, it will continue to be essential. However, the current situation in the United States is critical; a recent review article (5)states that
The last two years have shown a continued acceleration of the trend away from classical ‘wet chemical’ techniques in favor of high-speed, low overhead instrumentation. With the exception of a few techniques based on fundamental approaches, these high-speed instruments are dependent upon comparative methodology that requires calibration and validation with traceable standard reference materials. The loss of experience and knowledge in the area of classical analytical chemistry upon which so many of the extant reference materials are based remains a crisis of growing proportions. Nowhere is this trend greater than in the U.S. where the classical wet chemical laboratory is vanishing from the steel industry.
As an illustration of the current need for classical analysis in industry, consider my own experiences over the past seven years. While working for a large corporation engaged in advanced ceramic research,
workers at our laboratory were asked to assay each of the elements (except oxygen) in the following compounds: Sic, Tic, WC, BN, AlN, Si,N,, TiN, A1203,SiO,, TiO,, and Y,O,. In most cases the required relative precision and accuracy was 0.5%or better, and in many cases mixtures of two or more of the compounds were submitted for analysis. While working for a major forging company, we were asked to assay aluminum-, titanium-, and nickel-based alloys for Al, V, Mo, Cr, Co, Zn, and Sn in the 5%60% range with a required relative precision and accuracy of 0.2% o r better. All of these requests finally arrived a t the classical analyst’s bench because instrumental methods had failed to provide the necessary precision and accuracy. Without exception each of the above assays was of considerable economic importance to the industries involved. Even though such critical needs exist, only a minimal amount of classical analysis is being taught a t colleges and universities. Although a few exceptional schools may offer more, most cover only one or two gravimetric determinations a n d three or four titrimetric determinations. To some degree, this is understandable. Colleges and universities responded to the explosive growth of instrumental analysis by changing their curricula. Having more material to cover in four years, professors had to sacrifice something, and traditional qualitative and quantitative analysis were among the casualties. The analytical chemistry community allowed training in classical analysis to slip away until, by about the mid- 1970s, industrial, government, and private laboratories were having trouble finding qualified clas sical analysts. As the shortage intensified, laboratories began providing on-the-job training, but eventually their experienced classical analysts who were doing the training retired. The final phase in the decline came when young would-be classical analysts tried to learn from the old texts and reference works, only to find most of them out of print. The unavailability of these books remains a serious problem for anyone wanting to study classical analysis. Wishing for them to be reprinted would be to ignore the economic realities of publishers. However, some way must be found to preserve and make more widely available the information they contain. Attempts to revise these old reference books would be a disaster, because the methods they contain are of great value and often
can provide the basis for elegant reconstructions when combined with instrumental techniques. Using classical and instrumental analyses in tandem Classical analysts have been combining classical and instrumental methods of analysis for many years in individual situations. However, two papers by Silve Kallmann that appeared in the mid- 1980s advocated a conscious, systematic approach to using classical and instrumental analysis in concert (6, 7). Often, older classical analyses can be reconstructed and revitalized by using them together with instrumental determinations. Sometimes entirely new methods, some of which are very elegant, are possible. Kallmann suggests that the classical analyst continually consider such possibilities as a part of the overall analytical approach to methods development. There are a t least two opportunities for combining instrumental determinations with gravimetry. The first is checking the purity of precipitates. Despite the fact that every effort is made to choose optimum conditions for precipitation, m a n y precipitates are significantly contaminated with coprecipitated elements. Such an impure precipitate can be redissolved and the contaminant(s) determined by instrumental analysis. The second area is checking the filtrate from a gravimetric determination for solubility product effects. No gravimetric precipitate is completely insoluble, but the filtrate can be examined by instrumental analysis for the small amount of unprecipitated element. For example, the classical method for silica can be simplified greatly by isolating the precipitated silica from a single dehydration and then determining the remainder in the filtrate instrumentally. Kallmann points out other examples of how a method that may have been rejected because of a slightly soluble precipitate can now be reconstructed. There are also opportunities for combining instrumental determina tions with titrimetry. In the zinc assay of a zinc ore concentrate cited earlier, the small amount of cadmium present in all zinc ores is eluted together with the zinc. The cadmium can be separated from the zinc, but that requires either using a second ion-exchange column or masking the cadmium through its iodide complex. The easiest way to solve this problem is to titrate the zinc and cadmium together and determine the cadmium
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REPORT separately by atomic absorption spectroscopy. The foregoing examples will not pass the test for a classical analysis proposed at the beginning of this a r ticle, but the purpose of the test is to exclude methods with relative precisions poorer than about 0.1%-0.2%. The use of an instrumental determination together with a classical de termination does not degrade the overall precision as long as the instrumental method is used to determine a very small part of the total. In addition, the accuracy of gravimetric determinations is increased by reducing the positive bias of coprecipitation and the negative bias of solubility product effects. Revitalizing education for classical analysts We have seen that classical analysis has a long and distinguished history. The last 100 years have witnessed a strange reversal in classical analysis. The analysts a t the end of the nineteenth century, for the most part, had excellent technique even though their scientific understanding was very limited. Through the middle of the twentieth century, classical analysis had both good technique and good scientific understanding. Today’s chemists have excellent understanding but almost no practical experience in classical analysis. This extraordinary turnaround must be reversed or there will be severe economic consequences for American industry. The following advertisement appeared in the catalog of a wellknown analytical laboratory (8).
For over two y e a r s . . . (we have) been providing sanctuary for members of an endangered species. Now, after proper care and nurturing, we are ready to re-introduce our specimens of this vanishing breed, so that they can provide services for the public. This creature to which we refer is the Classical Analytical Chemist. He is the primary source of accurate analysis and is, therefore, extremely valuable to our industry.
It is doubtful that colleges and universities ever again will offer sufficient training to resupply the work force with well-trained classical analysts. Because of the fiercely competitive nature of research grant funding, few professors of classical analysis could long survive in a modern American chemistry department. How ironic that our situation bears 1002 A
some similarity to t h a t of midnineteenth century Europe, which had few laboratories where students could practice analysis. By the middle of the nineteenth century European colleges and universities were eager to equip analytical teaching laboratories because classical analysis was on the cutting edge of fundamental research. Elements remained to be discovered, and new minerals and rocks needed to be characterized. Teaching laboratories for classical analysis are needed today, but for different reasons. Industry will continue to depend on the classical analyst to decide if raw materials meet specifications and if finished products should be released for sale. Classical analysis is essential in the development of new materials and in the production of the reference materials on which almost all instrumental methods depend. For years many laboratories have been looking for people to replace their retired classical analysts. Some analytical laboratories have given up the search. I speculate that many hundreds, and perhaps as many as several thousand, well-trained clas sical analysts may be needed. If there were a recognized school where qualified classical analysts were trained, recruiters would probably line up a t the door at graduation time. Fresenius’ laboratory was started with private funds in a converted house with one assistant and five students. In five years he had 30 students, and in eight years he had 60 students and a worldwide reputation. The same thing could happen today. With a few capable and dedicated teachers, a school with an excellent reputation could be established in less than a decade. Industry or a joint venture by industry and government would have to bear the cost, but further delay in addressing the problem would be more costly. Actually, such a venture would not require a huge investment. The building and equipment require ments for such a teaching laboratory would be relatively modest, but those in charge of the administration and instruction would need to be people of great ability, experience, and commitment. They would also have to understand the correct pedagogical order. The traditional educational sequence of studying qualitative and quantitative analysis before physical chemistry must be avoided. U n d e r s t a n d i n g t h e t h e o r y of gravimetry and titrimetry requires a knowledge of physical, inorganic, and organic chemistry, and all of these
ANALYTICAL CHEMISTRY, VOL. 63, NO. 20, OCTOBER 15, 1991
must be considered prerequisites. The science, art, and “state of m i n d necessary for a good classical analyst would have to be firmly imparted to each student. Students would have to practice until they mastered t h e techniques of common classical analytical operations and would have to analyze materials used in industry and government to gain proper experience. From such a school could flow a small but steady stream of qualified classical analysts to serve in our industrial, government, and private laboratories. The graduates of this school would have the mental and manual skills necessary to do classical analyses. They would be able to work according to Kolthoff‘s maxim, “theory guides, experiment decides.” Perhaps such a school exists. If not, now is the time to establish one.
For many helpful discussions and suggestions, the author is indebted to James R. DeVoe, John C. Travis, Michael S. Epstein, Richard I. Martinez, Jerry D. Messman, John R. Moody, Marc L. Salit, Johanna M. B. Smeller, and Thomas W. Vetter of NIST; Karen D. Norlin, WymanGordon Co.; Donald L. Dugger and William J. Rourke, GTE Laboratories, Inc.; Leo W. and Marjorie P. Ollila, Luvak, Inc.; and James I. Shultz. ASTM.
References (1) Laitinen, H. A. J. Res. Nut. Bur. Stand. 1988. 93. 175-85. (2) Lundell, G.E.F. Ind. Eng. Chem. Anal. Ed. 1933,5,221-25. (3) Kraus, K. A.; Moore, G. E. J. Am. Chem. SOC.1953, 75, 1460-62. (4) Strelow, F.W.E.; Liebenberg, C. J.; Victor. A. H. Anal. Chem. 1974. 46. 1409-14. (5) Dulski, T. R. Anal. Chem. 1991, 63, 65R. (6) Kallmann, S. Anal. Chem. 1984, 56, 1020 A-1028 A. (7) Kallmann, S. In Chemical Analysis of I
,
Metals; Coyle, F. T., Ed.; ASTM: Philadelphia, 1987; pp. 128-33. (8) Brammer Standards Company, Inc. Catalog Supplement, 1988; p. 3.
Suggested readlng Belcher, R. Analyst, 1978, 103, 29-36. Some Fundamentals of Analytical Chemistry; Byme, F. P., Ed.; ASTM: Philadelphia, 1974.
Cannizzaro, S. Sketch of a Course of Chemical Philosophy; Livingstone: Edinburgh (Alembic Club Reprint #18),1969. Chirnside, R. C. Analyst, 1961, 85, 314-24. deMilt, C. J. Chem. Educ. 1951,28,421-25. Findlay, A. A Hundred Years of Chemistry, 3rd ed.; revised by Williams, T. I.; Duckworth: London, 1965. Hamilton, L. F.; Simpson, S. G. Calculations of Analytical Chemistry, 7th ed.; McGraw Hill: New York, 1969. Hillebrand, W. F. /. Ind. Eng. Chem. 1917, 9, 170-77.
Hillebrand, W. F.; Lundell, G.E.F.; Bright, H. A,; Hoffman, J. I. Applied Inorganzc Analysis, 2nd ed.; Wiley: New York, 1953.
Ihde, A. J. The Development of Modern Chemistry; Harper & Row: New York, 1964.
Kodama, K. Methods of Quantitative Inorganic Analysis; Interscience: New York, 1963.
Kolthoff, I. M.; Stenger, V. A.; Belcher, R.; Matsuyama, G. VoZumetric Analysis; Interscience: New York, 1942, 1947, 1957 (3 vols.). Kolthoff, I. M.; Sandell, E. €3.; Meehan, E. J.; Bruckenstein, S. Quantitative Chemical Analysis, 4th ed.; Macmillan: New York, 1969. Kolthoff, I. M. Anal. Chem. 1973, 45, 24 A-37 A. A History of Analytical Chemistry; Laitinen, H. A.; Ewing, G. W., Eds.; The Division of Analytical Chemistry of the American Chemical Society: Washington, DC,
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Lundell, G.E.F.; Hoffman, J. I.; Bright, H. A. Chemical Analysis of Iron and Steel; Wiley: New York, 1931. Lundell, G.E.F.; Hoffman, J. I. Outlines of Methods of Chemical Analysis; Wiley: New York, 1938. Meinke, W. W. Anal. Chem. 1970, 42, 26 A-38 A. Partington, J. R. A History of Chemistry; Macmillan: New York, 1961, 1962, 1964 (3 vols.). Standard Methods of Chemical Analysis, The Elements, 6th ed.; Furman, N. H., Ed.; Krieger: New York, 1975. Szabadvary, F. History of Analytical Chemistry; Pergamon Press: New York, 1966. Szabadvary, F.; Robinson, A. I n Comprehensive Analytical Chemistry; Svehla, G., Ed.; Elsevier: New York, 1980; Vol. 10. Treadwell, F. P.; Hall, W. T. Analytical Chemistry, 9 t h ed.; Wiley: New York, 1942; Vol. 2. Washington, H. S. The Chemical Analysis of Rocks, 4th ed.; Wiley: New York, 1930. Wilson, H. N. Analyst, 1960, 85, 540-50. Wilson, H. N. An Approach to Chemical Analysis; Pergamon: New York, 1966.
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me Joutnal of Oganic Chemistry solicits manuscripts that address topics at the interface of organic chemistry and biology. hile such manuscripts should address fundamental
W Charles M.Beck 11 is a research chemist in the Inorganic Analytical Research Division at the National Institute of Standards and Technology.He received his B.S. degree from Worcester Polytechnic Institute in 1963 and has worked for more than 20 years as an analytical chemist in government, private, and industrial analytical laboratories. Before joining NIST in 1 9 9 0 , he was senior chemist a t Wyman-Gordon Co., Eastern Division, Worcester, MA. His research interests are in the areas of sample preparation, chemical separations, classical elemental determinations, certification of standard reference materials for chemical composition, and the history of analytical chemistry.
problems in organic chemistry (structure, mechanism, synthesis), we encourage submission of manuscripts in which these problems are solved with the use of techniques not traditionally associated with organic chemistry (enzyme kinetics, enzyme isolation and purification, identification of active site residues, etc.). The Journal hopes to foster integrated publications in which the chemical aspects are not separated from the biological aspects. For manuscript format, see J Org. Chem, 1990,55 (I), 7A-1OA. Send manuscripts to: C. H. Heathcock, Editor-in-Chief,The Journal of Organic Chemistry, Department of Chemistry, University of California, Berkeley, CA 94720 For subscription information American Chemical Society Sales and Distribution 1155 Sixteenth Street, N.W., Washington, D.C. 20036 (202)872-4363 Toll Free, 1-800-227-5558 ANALYTICAL CHEMISTRY, VOL. 63, NO. 20, OCTOBER 15,1991
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