A STORY of INDIUM SIDNEY J. FRENCH Colgate University, Hamilton, New York
I
T IS AN INTERESTING fact that out of four ele- Its identification as well as its name came from the mentary textbooks of chemistry recently examined beautiful indigo-blue line which it so prominently only one carried a reference to the element iudium. displays in its spectrum. Its commercial development This one reference stated in part, "One family of the came as the result of the knowledge that it should be a [third] group includes aluminum together with the very valuable stabilizer of non-ferrous metals much as rare elements gallium and iudium . . . Aluminum is the chromium is a stabilizer of the ferrous metals. only one of these metals that need be further discussed in Mr. WiUiam S. M u m y has been the dynamic force this text." Events move so rapidly in the field of behind this commercial drama of indium and to him science that today's truth is tomorrow's lie, for indium must be accorded all the honor due a pioneer. True, is no longer a "very rare" element. The writer has many other explorers have contributed much but he and before him a five-gram sample of this beautifully soft his co-workers alone have carried the fight through all lustrous metal which was recently cut with a penknife of its phases to commercial success. Mr. Murray was from a hundred-gram ingot. Ten years ago, or even born in western Pennsylvania in 1887. His college five, this small sample would not have been available a t education was obtained a t Colgate University where he any price. studied chemistry under Professors Joseph Frank McIn 1924, an order was placed with a prominent New Gregory and Roy B. Smith. Professor McGregory York chemical supply company for a substantial once said of him, "He couldn't be kept within the conquantity of indium. After several months of corre- fines of any course so we just gave him a laboratory desk spondence with foreign sources, one gram of the metal and didn't bother him." This method of training was located and purchased, it being all that was then proved eminently satisfactory for native curiosity and available in the world. I t was valued a t ten dollars a will to investigate grew unstunted. gram, the price being considerably higher than that of In 1924, Mr. Murray and his colleagues had reached platinum. In 1932, Mr. William S. Murray displayed the conclusion, after a series of investigations with before the Rotary Club of Utica, New York, an ingot of various metals, that indium should have valuable indium weighing more than three thousand grams and properties in stabilizing non-ferrous metals. When the valued a t more than twenty thousand dollars. Today, &ort to purchase considerable quantities failed so this same quantity of indium can be purchased for completely, the little group was forced to decide either less than three thousand dollars. The potential supply to abandon the project or to start out on a long, difficult of indium has risen from a few grams to more than two search for an element which was considered by most million grams in ten years while the price has fallen ten- authorities to be non-existent in commercial quantities. fold. Back of this rise and fall is a romantically in- They chose the harder path and the search began. I t teresting story of hardship, courage, and chemical became a hunt which dragged out into years bringing acumen. forth many failures and disheartening results. More I t is indeed a coincidence that aluminum and indium than once, trivial incidents turned defeat into success. should belong to the same chemical family, for in many The first step was to locate, if it existed, an ore respects the stories of ,their development parallel one which contamed paying quantities of indium. In this another. As aluminum burst forth on the world in search, the spectroscope became the "piece de resiscommercial quantities at the turn of the century to take tance." Sample ores of zinc, lead, silver, and gold its place with the abundant metals, so indium blossoms were begged, bought, and borrowed. Each in turn forth a third of a century later to take its place with appeared before the spectroscope and each in turn the nobler metals. In each case, about seventy years announced to the anxious observers that indium was have elapsed between the isolation of the metal and its not present. True, they saw the blue line of indium commercial development. Both metals were made many times but always faintly. The thrill that comes commercially possible in answer to real needs. Just as to scientists seldom in a lifetime must have stirred this aluminum had a struggle to find a place for itself in the little group of pioneers when, after the study of huncommercial world because its previous high price had dreds of negative samples, they saw inscribed before precluded experimentation with it, so indium faces that their eyes the intense, unwavering blue line announcing same struggle today but with fine prospects of soon that indium was present and in paying quantities. finding its place in the sun. Unlike aluminum, which Fortunately, the sample was labeled, showing the area was known to be present in the earth's crust one hun- from which the ore had come. The first step seemed dred years before it was actually isolated by Oersted solved. and by Wohler, indium was both discovered and isoA train dropped Mr. Murray in the heart of a midlated in the same year (1863) by Reich and Richter. western state. There he set up his spectroscope in a 270
garage and began a systematic analysis of the ores of the region. But as ore after ore was examined with negative results, hopes began to fall. Finally the conclusion was reached that the sample was not correctly labeled for no ores showing more than a trace of indium could be found in the region. Had it not been for the memory of one man the search must have ended here. This man, having known the area intimately for many years, recalled that a t some previous time several shipments of ore from other areas had been mixed and sent out with the local ores. He recalled further that the foreign ore bore a marked resemblance to the sample. The search was off to a new start. This time it landed the investigators a t the mouth of an abandoned prospect mine. The mine was explored and samples studied. Indium was present and in paying quantities. The local area proved to contain very definite and persistent quantities of indium ore. To date, some thirty-five thousand tons of the ore have been blocked out and analyzed to show an average yield of 1.93 ounces of indium per ton. The total extent and depth of this particular vein are still unknown. Next came the technical task of determining a suitable method for relining the ore. The literature on extraction and purification was scanty and conflicting. Known methods were wasteful and adapted to securing only small amounts of the metal. To assist in this technical work Mr. Daniel Gray was called in. Mr. Gray is a graduate of James Milliken University and has been deeply interested in chemistrv since the aee s of -ten. The problems called for boldness a n d imagination. Mr. Gray possessed both. acid. From this solution, the indium was either plated The ore was a very complex out by metallic displacement or thrown out by neutralone containing sulfides of lead, ization of the acid. The impure indium was dissolved, zinc, iron, copper, silver, and purified, and again plated out. The process as now gold as well as indium. An used produces indium of a purity higher than ninetyexperimental plant was set up. nine per cent. Mr. Murray and Mr. Gray now hold The ore was ground and treated basic patents in several countries for the refining of by the flotation process to indium. The field was such a pioneer one that the separate the concentrates from French patents were granted without question and the gangue. Concentrates of little of a conflicting nature was found in the American zinc and lead-silver were ob- patent records. tained. It was found that the There still remained one difficultproblem to solve. indium was present in the zinc If the metal were to be used to stabilize non-ferrous concentrate. In the refining metals, a suitable plating bath must be devised. The process first used, the zinc con- investigations of Thiele, Dennis and Gear, Mather, centrate was roasted in the Kollock and Smith, Westbrook, and others on this presence of sodium chloride subject were carefully studied and it was found that since it was found that indium none of the suggested methods was ideal. The double chloride fumed a t the roasting cyanide bath usually used for such purposes promised temperature. The fumes were to be satisfactory provided a suitable stabilizing vehicle caught in a Cottrell precipita- in the nature of a very weak organic acid could be found tor, dissolved, and the indium to insure a firmly adhering, fine-grained deposit. A plated out. This method was large number of such vehicles were tried and of them it soon replaced by a simpler one was found that glycine (amino acetic acid) was the most not involving the use of a satisfactory. This compound was, however, rare and Cottrell. T h e concentrate expensive. Efforts to produce glycine cheaply met was roasted and the soluble with little success. Aaain urorrress was barred and the portion dissolved in sulfuric successful commercial ilatidgorindium might have been
DR. McGneomu ( ~ ~ r rArN)D MR. M U R R A Y Between these men is a one-gram sample of indium (the size of the original sample obtained ten years ago from Germany) and a larger sample (the first produced by Mr. Murray in this country).
day on the indium monetary standard. Its use for stabilizing silver in automobile headlight reflectors to prevent the rapid fall in headlight efficiency bas been suggested. Its long liquid range from 155°C. to about 1450°C., together with its high coefficient of expansion suggests its use in pyrometers for measuring high temperatures. Little is accurately known regarding the salts of indium. The metal has a valence of three and is active enough to dissolve in strong acids. A number of interesting and relatively stable salts can be formed. So far as is now known, salts of indium are not toxic to the human system. They show promise of considerable usefulness in the field of medicine. In fact, the successful treatment of African sleeping sickness with indium salts has already been reported. A complete and valuable bibliography on indium has recently been prepared under the direction of Dr. John B. Ekeley of the University of Colorado. This infant metal has now passed through the first two stages common in the life history of new metals, discovery and isolation, and commercial development, and has entered the third stage, commercial exploitation. It is no longer a "very rare" element known only as a name or number and gazed upon only by a very few favored scientists. It will probably become as well known by name and appearance to the layman in the next decade as chromium has in the last, due largely to the exacting efforts and enthusiastic energy of Mr. Murray, Mr. Gray, and their colleagues.
retarded many years had it not been for the active imagination of Mr. Gray. In casting about in his mind for cheaper materials, he suddenly had the happy thought that common sugar would do the trick as well as glycine. Sugar was tried and proved to be entirely satisfactory. How often in the history of scientific achievement have the simpler methods been among the last to be tried. Gray's stabilization of the indium SOME COMMONLY ACCEPTED FACTS CONCERNING I N D I U M plating bath has made indium plating a commercial Atomic weight 114.8 49 fact. The plate obtained in the bath is uniform, soft, Atomic number 115 and gray. When the plated metal is subjected to heat Isotopes 5.76? potential, volts (gas) treatment, the plate diffuses into the metal giving a Ionizing 1.40 A.U. Atomic radius (Wyckoff) hard stabilized surface resistant to oxidation and tarnish. Ionic radius ( I n t + + ) .95? The final chapter on indium will be written during the Melting point 155°C. 1450°C.? next decade. The supply has increased immeasurably Boiling point 0.381 volt and the price has fallen tenfold during the past decade. Electrode potential In t I n + + +f 3.12.5 Coefficient of cubical expansion X 10' cm. The scientific public is rapidly becoming indium con- Atomic volumc 15.1 scious. The field of usefulness of the metal is still Density 7.3 limited by the fact that indium has not heretofore been Crystal lattice Face centered? Between Cd and TI? available for experimental purposes. So rare has the Position in electromotive series 3 and 1 (and 2?) metal been, that two very prominent chemists when Valences Color Silver-white recently shown pure samples of it failed to identify it, Hardness Range of lead though one recognized i t to be an element. In Septem- Ductility Very ductile and malleable ber, 1033, Mr. Murray presented a paper on indium and 8.37 X ohms a t displayed samples of the metal before the Industrial Resistance 0*C.? and Engineering Division of the American Chemical The metal alloys well with a number of non-ferrous metals. Society a t Chicago. Keen interest was shown in this The metal dissolves in mineral acids but not in alkalies. new metal. Two oxides of formulas In20sand In,04 are recognized. The properties of the metal indicate some of its Sulfides, InS (brown) and In& (yellow), are known, the latter being soluble in dilute mineral acids. probable fields of usefulness. In addition to its value as Trihalides, the dichloride, InCb, and double halides, such as a stabilizer of non-ferrous metals, it should form many KJnCla, are known. InCls is volatile a t 600°C. valuable and important alloys with the nobler metals The element forms sulfates, Inl(SOJs, and alums (though not as readily as aluminum), also double sulfates of the type as well as with some of the more active and abundant M1+SO~,M~+++(S04)r8H10. metals. The relative lightness and firmness of its hydroxide is somewhat amphoteric and the salts moderalloys with silver suggest its use in dental work in place The ately hydrolyzed in solution. of the heavier and more expensive gold. It is not even In solubility, the salts resemble those of aluminum but are less beyond the bounds of reason to imagine ourselves some hydrolyzed.
