RALPH E. OESPER University of Cincinnati, Cincinnati, Ohio
T H E readers of THIS JOURNAL, are well acquainted with the interesting and scholarly papers on a wide variety of historical topics that have come from the pen of Wilhelm Prandtl. He is a worthy scion of a family that has lived in Munich since the beginning of the eighteenth century. His father, Antonin Prandtl (1842-1909), was the originator of the centrifugal cream separator, but he has never been given credit for this invention. His device had the defect of not being continuous, and though it attracted inquiries, even from America, it was never developed commerically.' Disappointed, the young inventor then turned his entire attention to his profession, i. e., brewing, and here he made a real success. In 1875 he was made diiector of a large brewery in Hamburg and did so well that in 1884 he and his brother Carl (1838-1927), who also had acquired an outstanding reputation in the science and art of brewing, bought a small brewery in Munich. They operated this very successfully until the introduction of electricity and refrigeration machinery gave the large breweries an overwhelming advantage which the Prandtl brothers could not overcome because their financial resources were too small. Wilhelm (Antonin Alexander) Prandtl, born a t Hamburg on March 22,1878, has lived in Munich practically all of his life. After graduation from the humanistic Gymnasium he studied chemistry under Adolf v. Baeyer a t the University of Munich. The doctorate (1901) was taken under the directicn of K. A. Hofmann; the dissertation dealt with rare earths. After two years in industry, Dr. Prandtl returned t o his alma mater and from 1903 to 1910 was assistant to Th. Paul in the laboratory of applied chemistry. J p 1906 Prandtl qualified as Privatdozent, specializing in complex compounds, particularly those of vanadium. When Hofmann was called to Berlin in 1910, Prandtl became his successor. He occupied this chair of inorganic chemistry until 1937, when the Nazis forced his retirement because he refused to bow the knee. In 1946 he was reinstated, and now as professor emeritus he devotes his energies almost entirely to writing and teaching in the field of the history of chemistry. Although Dr. Prandtl has worked on polyacids and also has to his credit the first preparation of trichloronitrosomethane, phosgene oxime, and other derivatives
of carbonic acid, the preponderant bulk of his long research career was devoted to the rare earths. From 1911 to 1937 he applied his experimental talents to the difficult and laborious tasks of separating these materials, a comnlex chemical ~roblemthat has eneaeed the minds of fairly small number of specialists. I n 1938 he summed up his results in a paperZwhich begins:
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In more than 25 years of work, I have succeeded in separating from the crude mixtures of the rare earths, which occur in the minerals, all of their members individually and have obtained them in a more or less pure state. During the course of these studies, I have become acquainted with the most important procedures which were recommended for the separation of the rare earths, and I have likewise tried new methods. However, I have also wandered into by-paths and taken wrong turnings, and so consumed much time and labor, which I could save if I now had another chance to carry out the separation of the rare earths. Though I do not plan to do this, and furthermore, it is no longer possible, nevertheless, I wish to give the benefit of my experiences to those who, sfterme, Choose to take on this line of work which demands so much self-sacrifioe.. . . I t has long been customary to use fractional crysts.1lization and fractional basic precipitation to separate mixtures of these earths. In the production of every individual earth, both methods lead-disregarding a few exceptions-to the objective only after many hundreds or thousands of repetitions. A series of crystallizations can be carried out with the expenditure of much less time, work and reagents than an equal number of basic precipitations. Consequently,.the latter is applied only when the individusl basic precip~tation accomplishes a much greater separatihg effoct than the individual crystallization. This applies, for example, in the precipitation of the very weakly basic ceric hydroxide or soandium hydroxide, or in the separation of the strongly basic lanthanum from the less bssic cerite eerths.. . . Recently electrolytic reductionhas, been added to the old familiar methods of basic precipitation aad fractional crystallisation. I t is applicable for the separation of the earth elements (samarium, europium, and ytterbium) whioh can be reduced to the divalent state, and in this condition are mare readily separated from their trivalent attendants. . . . By oxidation, i. e., conversion into a higher valence state, it has been ~ossibleno to now.. to .Dreei~itate . onlv cerium. hut not the two dther elements, praseodymium and terbium, which e m go over into higher oxides by taki;ig up oxygen.
These procedures were modified and given new applications by Prandtl. For example, he made fractional precipitation by means of ammonia water a much more delicate tool by including in the reaction mixture ammonium nitrate and a metal salt, such as cadmium nitrate, which hinds ammonia. The concentration and ~ P R ~ N A,, D ~Dinglera , Polylech. J . , 174, 149 (1864). hence the overall reaction rate of the ammonia are thus See also W. PRANDTL, "Antonin Prandtl und die Erfindung der regulated, this way praseodym~um, neodymium, Milchentrahmung durch Zentrifugieren," Munich, 1938. The samaduml etc'p are precipitated? leaving lanthanum in first continuous separator was constructed in 1875 by Alexander Prrtndtl(1840-1896), older brother of Antonin. Alexander's son. solution. He produced a quantitative precipitation of Ludwig, acquired an international reputation in technical physics, aerodynamics, etc. 398
'PRANDTL, W.. 2. anorg. u. allgem. Chem., 238, 321 (1938).
AUGUST, 1949
cerium by a somewhat similar device whenhe introduced the use of cohaltic trinitrato-triammine [Co(NO& (NH&]. This holds its ammonia tenaciously, but when added to a cerous solution the redox reaction produces a ceric salt and also a cobaltous ammine which releases its ammonia a t the desirable low rate. Europium and ytterbium can be obtained best by electrolytic reduction followed by precipitation as the difficultly soluble earth (11) sulfate. The list of reagents with whose separative action he became familiar included bromate, ferricyanide, oxalate, fluoride, nitrate, etc. The original papers must be consulted for details. The net result of this quarter century of tedious, careful, patient, devoted labor was the preparation of all the rare earths in a high state of purity. The single exception was holmium oxide, and apparently no one has succeeded in this most difficult task. The excellence of Prandtl's preparations is evidenced by the fact that they sewed as reliable starting materials for the atomic weight determinations by his Munich colleague, Otto Honigschmid.3 They likewise were used for mapping 3
THE JOURNAL. 17, 562 (1940).
399
the arc spectra of the rare earth elements a t the Vatican O b s e ~ a t o r ya, ~study which gives an impressive survey of Prandtl's life work. Prandtl's publications in the field of history are characterized not only by erudition and comprehensive knowledge of the field, hut also by exactitude and attention to detail. His latest volume, "Davy and Berzelius," whom he considers the most eminent chemists of their time, appeared in 1948, and his discussion of "Liebig and His German Contemporaries" is in press. Professor Prandtl is an ardent but critical collector. He has valuable collections of stamps, coins, medals, and plaques. His library of early scientific hooks is exceptionally fine and he is an authority in this field. Fortunately, his collections were not lost during the bomb'mg of his apartment, which is not far from the Deutsches Museum. When his home is repaired, he plans to hold there a seminar on the history of science. -
' GATTERER, A,, AND J. JUNKES,"The Atlas of Spectra. of the
Rare Earths," Vatican City, 1945. See also W. PRANDTL mn K. SCHEINER, 2.amrg. U. allgem. Chern., 220, 107 (1934).
