THE CHEMICAL WORLD THIS WEEK C & E N REPORTS: A C S Division of A n a l y t i c a l C h e m i s t r y , Summer Symposium
Better Procedures N e e d e d for Analysis of Less Common Metals A n a l y t i c a l chemists a r e m a k i n g increasing use of physics, radiochemistry, electronics TROY, N. Y.-Specific reagents for t h e analysis of elements in the IV and V periodic groups are, in general, unknown, and separation methods are far from satisfactory. If the separation and determination of all these elements had been studied in as great detail as has u r a n i u m within recent years, we would have much fewer problems in the analysis of t h e less common metals, said R. M. Fowler of Metals Research Laboratories. Dr. Fowler spoke at the sixth annual summer symposium of the ACS Division of Analytical Chemistry, held at Rensselaer Polytechnic Institute on June 19 and 20. T h e program was devoted to t h e analytical chemistry of the less familiar elements. Although niobium, tantalum, titanium, zirconium, a n d tungsten often occur t o gether in nature, separation procedures for these materials leave much to be desired, said Dr. Fowler. To complicate matters still further, the metallurgist m a y a d d vanadium, molybdenum, and t h e rare earths to commercial alloys. Such materials present real analytical problems. Even in t h e simple three-element combination of titanium, niobium, and tantalum, analytical difficulties are encountered, said Dr. Fowler. About 50 years ago, a considerable amount of research was done on the determination of niobium and tantalum in complex minerals. One of the leading authorities at that time concluded that no satisfactory method h a d been developed for t h e separation of niobium from titanium. While there have been impressive advances in analytical chemistry in t h e ensuing five decades, his statement is still true. Recently, various investigators have carried out separations of these materials b y fractionating the chlorides. Octachloropropane is used as the chlorinating agent. While the chlorides differ widely in their boiling points, differential chlorinations on an analytical scale are usually cumbersome and slow. Other recent work h a s been done on the thiocyanate complexes of niobium. By this procedure, niobium can b e estimated in the presence of titanium. However, tungsten, which is frequently present, can cause trouble. T h e use of cellulose-column chroma2766
tography for separating niobium and t a n t a l u m , although promising, has its shortcomings since large volumes of m e t h y l ethyl ketone-fluoride solutions must b e evaporated. However, as polyethylene laboratory equipment becomes more available, analytical chemists might become less reluctant to use hydrofluoric acid. This might help in solving separation problems i n this area since the rare earth acids are m u c h more tractable in fluoride solutions t h a n in any other media, he said. Although the rare earth elements are becoming of considerable industrial importance, the classical methods of determ i n i n g these materials in small amounts are unsatisfactory. For example, one standard procedure works well for small amounts of cerium in iron, b u t is ineffective if large amounts of magnesium are present. Researchers at Dow Chemical have published useful separations of t h e rare earths, and investigators in the Atomic E n e r g y Commission m a y have better ones classified, he said. Perhaps the long-range answer to separations in this area is not to make t h e separations at all, said Dr. Fowler. Instead, use might be made of emission spectroscopy, x-ray fluorescence, and similar methods. Instruments for this purpose are giving outstanding results with elements of high atomic number, and further improvements can be expected. Atomic Energy Program. Many of t h e 50 or more elements classified as less familiar h a v e played a vital role in the atomic e n e r g y program, said A. H. Bushey of t h e Hanford Atomic Products Operation. Two of these elements—plutonium and uranium
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—are large-scale products. T h e existence of fission materials aad other radioactive isotopes and t h e importance of neutron absorption have brought many unfamiliar elements into prominence. T h e high levels of radioactivity and the attendant radiation hazards of many of these materials have imposed severe limitations on their analysis. This has placed a considerable premium on n e w techniques that are rapid a n d can b e carried out remotely. T h e result, said Dr. Bushey, h a s been t h e establishment of strong analytical research and development programs within the atomic energy project and the employment of highly capable analytical chemists in this field. In recent years, radiochemistry h a s been introduced as a major analytical technique. By the first of 1953, t h e Atomic Energy Commission h a d made over 32,000 shipments of radioactive isotopes to various institutions a n d laboratories. The greatest use of these materials has been in the fields of biology a n d organic chemistry, where tracer techniques provide a unique m e t h o d for establishing chemical and physiological mechanisms. With a shortlived radioisotope such as sodium-24, it is possible to detect as little as 10~19 gram, h e said. The need for determining a wide variety of elements, t h e emphasis on microanalysis, t h e need for remote handling, and the many requirements for isotopic analysis have led directly and naturally to the employment of physical techniques for analytical use, h e said. The conventional wet methods are slow and, in most cases, require macro samples. I n addition, they are subject to m a n y interferences that in the past have often been neglected or ignored. On t h e other hand, analytical techniques based on physical properties are rapid and eliminate t h e need for slow, laborious chemical separations. To carry out his work effectively and to rehite it to its ultimate use, t h e analytical cnemist has to b e well versed in engineering, physical chemistry, metallurgy, and other fields. T o this list of specialties, Dr. Bushey said, must now be added, with greater emphasis, physics, radiochemistry, a n d electronics if h e is to take full advantage of the new developments and the latest trends in pure and applied research. Determination of Uranium. T h e importance of u r a n i u m as a fissionable material has stimulated considerable interest in t h e determination of this element, said C. J. Rodden of the A E C New Brunswick Laboratory. Analyses have been developed for materials ranging all t h e way from 10" 11 gram per gram of sample to practically 1 0 0 % pure uranium. Within t h e past few years, new gravimetric, volumetric, colorimetric, fluorometric, and radiochemical methods have been introA N D
E N G I N E E R I N G
N E W S
C - L . L u k e (left) a n d M . E . Campbell, Bell Telephone L a b s , discuss t h e determination o£ impurities in semiconductors with M. L. Selker of Clevite-Brush D e v e l o p m e n t Co. duced, a s well as n e w x-ray absorption and coulometric procedures. Almost all recent advances in methods o£ uranium separation have involved t h e u s e of organic solvents. T h e greatest prog r e s s in solvent extraction, said Dr. Rodden, h a s come from England, where studies on t t i e use of paper strips and packed cellu lose columns have resulted in improved time-saving procedures. In t h e paper strip method, dissolved uranyl nitrate moves w i t h t h e solvent front as diffusion pro c e e d s t h r o u g h the absorbent paper. Most οΈ the other components remain stationary 03: move only slowly in comparison to t h e uranium. T h e uranium can then be meas u r e d with a polarograph, colorimeter, or fHiorimeter. The main advances in volumetric pro cedures have been in t h e method used in reducing the uranium. T h e determina t i o n of uranium in ores b y a counting method serves mainly as a means of deter mining the range. Generally, this is fol lowed b y a colorimeter or fluorometric analysis. By far t h e most sensitive pro cedure is fluorometric. " W e are continu i n g to modify our fîuorimeters, ,, said Dr. Rodden, "and hope to develop a reliable flattery-operated unit that can b e adapted for field use. We are also considering t h e possibility of designing an automatic fluorimeter t o monitor plant effluents." Impurities in S e m i c o n d u c t o r s . As an outgrowth of work on such materials as txansistors, it has been necessary to d e velop quantitative chemical methods for t i i e determination of metal impurities in semiconducting materials. T h e methods a i e designed for use in research, in t h e quality control of raw materials, and in various manufacturing operations, accordi n g to C . L. Luke of Bell Telephone L a b o ratories. An analytical program at the Bell Labs Has called for the development of methods For determining copper, t h e Group V metals (arsenic, antimony, and phosphorus ) , and the G r o u p III metals ( alu-
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3 1, N O .
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minum, gallium, indium, and b o r o n ) in germanium dioxide and in germanium and silicon metals. T o date, methods have been completed for die determination of 0.1 to 1 parts per million of arsenic, phosphorus, antimony, aluminum, a n d copper in germanium and germanium dioxide. In addition, a method has b e e n worked out for t h e determination of 1 to 10 parts per million of arsenic in silicon metal. In trace analysis, it is almost invariably necessary to isolate the impurity in question before attempting its determination, said Mr. Luke. Fortunately, germanium can b e easily removed b y distillation as chloride without loss of most of the metal impurities mentioned. Arsenic and boron must be separated by other means, however. Because of the small a m o u n t of metal impurities involved, spectrophotometry is used for its high sensitivity. By the choice of the most sensitive photometric methods, by the u s e of as large a sample and as long a light path as possible, a n d by the proper purification of reagents, especially high sensitivity can be obtained. Beryllium. Beryllium is a very toxic element that is of considerable biological importance in the submicrogram range, said T. Y. Toribara of t h e University of Rochester. Because colorimetric, fluorometric, and spectrographic methods used to determine small quantities of beryllium are subject t o interference by other elements, it has b e e n necessary to develop a separation scheme to isolate t h e beryllium before measurement. T h e separation of beryllium from bone has proved t h e most difficult because of the large amount of calcium phosphate. By the use of precipitation, ion exchange resins, and organic complexers, it has been possible to isolate completely t h e smallest measurable quantity of beryllium. T h e efficiency of separation in each step was determined with t h e radioisotope beryllium-7. T h e most sensitive chemical method for measuring beryllium, based on the fluorescence developed b y the inter-
JULY
6,
1953
action of morin and beryllium at a p H over 1 1 , h a s b e e n exrploredL, h e said. Analysis for C e r i u m . Orr-e aspect of t h e determination of cerium t h a t has b e e n given little attention i n the past has been t h e measurement of small quantities and concentrations of tri valent cerium in t h e presence of large amounts of tetravalent cerium. T h e usual m e t h o d s involve t h e direct potentiometric titration of oxidizing agents in carbonate rcieduirxi a n d t h e estimation of trivalent cerium by difference. However, these methods fail w h e n t h e ratio of eerie to ceroixs salts is very large, said D . N . H u m e of MIT. It has been found that, at a rotating platinum microelectrode, t h o electrochemical behavior of eerie and cerous ions in strong carbonate medium allows a direct amperometric t i t r a t i o n of c e r o u s ion with ferricyanide in the presence of much larger quantities of cerif interest. T h e resulting spectra a r e therm p h o t o g r a p h e d and evaluated b y conventional techniques. T h e m e t h o d permits deterxnination of impurities t o 10 parts p e r million ± : 3 0 % . Selenium a n d Tellurium. A spectrophotometrie method for t h e m e a s u r e m e n t of trace quantities o f selenium a n d tellurium was reported b y S. Έ . W i b e r l e y of Rensselaer Polytechnic Institute. T h e free elements are precipitated firom solution b y reduction and are t h e n dissolved in con centrated sulfuric a c i d . TThe absorbance of t h e solution is m e a s u r e d at 350 m/χ for selenium and a t 520 π ι μ fox tellurium. T h e critical variables a r e t h e t e m p e r a ture at which t h e e l e m e n t s a r e dissolved, t h e l e n g t h of time t r i e solxition is heated, t h e extent of absorption of water vapor by t h e concentrated a c i d solution, a n d t h e relative concentrations o € tlie selenium a n d tellurium. By t r i e use of a one-centi meter cell, t h e practical lower limit of t h e procedure is 50 micrograms of tellu rium or 200 micrograms o f selenium per 5 0 m l . of sulfuric acid solution. T h e m e t h o d h a s b e e n u s e d successfully for t h e determination of s m a l l percentages of tel lurium in m a g n e s i u m alloys.
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