SURVEY of the W O R K o f GREGORY PAUL BAXTER* GEORGE SHANNON FORBES Harvard University, Cambridge, Massachusetts
T
ONIGHT we honor an investigator who from the outset of his career has exemplified the spirit of Theodore William Richards and who has steadily carried it forward into new lines of endeavor. Authority beyond compare, among living men, in the field of exact analysis, his publications over his thirtysix ye& of membership in this Section have enhanced *Address delivered before the Northeastern Section on the occasion of the bestowal of the Theodore William Richards Medal upon Gregory Paul Baater, April 13, 1934. Reprinted from The Nudeus of May, 1934. by permission of Dr. Awry A. Ashdown.
its prestige throughout the world. The total of his researches is imposing; as individual performances they are internally consistent, finished, conclusive; their implications are far-reaching. Dr. Greene, in The Nucleus for April, has classified them into groups. No brief discussion could begin to unfold them in their entirety. Allow me, therefore, t o survey them in the sense that I direct your attention to a few out of many landmarks upon a high plateau. Consideration of a very limited number of the problems which he has studied may t o some extent suggest the sagacity and dexterity he has brought to bear upon all the others.
Up to 1902 the determination of phosphoric acid by weighing ammonium phosphomolybdate was in disfavor among careful analysts. The composition of the precipitate seemed to them to vary in a capricious fashion. Baxter contended that phosphate should be poured into molybdic acid, because the weight of the precipitate would be less affected by the occlusion of the molybdate than by the elimination of phosphate uncombined with molybdic acid which would occur if the operation were conducted in reverse fashion. He showed how the total of ammonium molybdate and molybdic acid occluded by these precipitates, and their ratio, too, are dependent upon analytical conditions; in other words, reproducible. Then, from a plot of composition as abscissas against excess of molybdic acid as ordinates, under conditions not too inflexibly defined, the percentage of phosphorus pentoxide in any new precipitate could be interpolated a t once, and with surprising accuracy. In addition, he found that only two ammonium ions were present per molecule unless the precipitate was digested with ammonium nitrate, while various other combinations of ammonium, hydrogen, and potassium ions could be established in analogous experiments without greatly affecting the general properties of the precipitate. Ferrous iron must often be determined in the presence of hydrochloric acid. Permanganate, which serves as its own indicator, would be preferable to dichromate if i t did not give too high results. Some of the permanganate, in such cases, unfortunately, goes to oxidize chloride iron. Before 1905 i t was supposed that a substantial excess of manganous ion must initially be present to avoid error, but Baxter showed correct results obtainable even in rather concentrated hydrochloric acid provided that the temperature exceeded eighty degrees, and that three-tenths of one per cent. of the permanganate was subtracted. In the case of oxalic acid present with hydrochloric acid, mere elevation of temperature eliminated error. Finally, the side reaction a t low temperature proved to be the formation and volatilization of hypochlorous acid, nosof chlorine as had been generally supposed. In 1904 the belief was still current that the eminent Belgian chemist Stas was infallible in the field of atomic weights. But in that year Baxter supplied definite and convincing proof that Stas's value for the atomic weight of iodine was a t least a tenth of a unit too low. This was the first large error to be detected in Stas's work, and the publication in question preceded the even more deflationary paper of Richards and Wells regarding Stas's errors anent sodium and. chlorine. Baxter's subsequent papers on iodine, ingenious and painstaking as they were, only served to demonstrate the impregnability of this early value. As Baxter's work on atomic weights continued. he came to rely more and more upon spectroscopic and spectroographic evidence of progressive purification of compounds taken for analysis. He was emphatically a pioneer in developing the technic of refined qualitative spectrum analysis, and repeatedly demonstrated the
usefulness of its application to exact analytical work. It was evident that the compound to be taken for analysis could be recrystallized, electrolyzed, or distilled in such varied fashion that, few would demand spectroscopic evidence of its purity in addition. The same cannot be said of many precipitates and other reaction products. Even if these can be washed thoroughly, the slightest tendency to hold impurities in solid solution will be fatal. A single good spectrogram could avert worry, or give direction to new experiments. Baxter gave much thought to the gases held by solids, especially porous solids, not previously fused or sublimed in a vacuum. At least one modern authority recognizes him as a pioneer in this important field. One ingenious test was made by heating a sample of pure silver with one of pure iodine in an evacuated quartz tube. When these had been converted into fused silver iodide, the pressure of the residual gases was measured and found to be negligible from the gravimetric standpoint. The determination of atomic weight by the electrolysis of a weighed sample of a zinc or cadmium or tin salt into a mercury cathode looked so easy that certain of the relativelv inex~erthad been intrigued bv the method. and on &is basis had advocated substantial downward revision of the combining weights in question. But as might have been expected, Baxter proved that quicksilver electrodes are no royal roads to exact results. The purification and weighing of the original sample of
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metallic chloride or bromide presented to him n o new problem. But the configuration of the cell, the trapping of the spray, the elimination of halogen as fast as it was liberated, the avoidance of oxidation of the amalgam or halogenation of the mercury, the estimation of residual traces of the metallic ion to be determined and of mercury ion formed from the amalgam all had to be worked out by many careful tests. An interesting point was noted-that amalgams can be dried in a vacuum without loss of weight, but that if alcohol is used for drying, a weighable amount of mercury vapor will pass off with the alcohol vapor. The main argument was finally clinched by substituting, for the weighed portion of salt, a weighed button of pure metal under sulfuric acid. This was transported electrolytically into the cathode, whereupon Baxter accounted for the cadmium within a few parts in a hundred thousand. That should come near to satisfying a federal income tax auditor. Correctly executed, then, the electrolytic method agreed admirably with the well-established method of converting metallic halide into silver halide. While Richards had already laid much stress upon diversification of the sources of his materials from geographical and geological standpoints, Baxter made the first precise determinations of atomic weights of meteoric iron, cobalt, and nickel. He showed those to be identical with terrestrial sources within a few parts in a hundred thousand, thus demonstrating the homogeneity of the universe over a wider range. Before this simple conclusion was reached, five elaborate researches were carried out, each fortified with the utmost precautions against error. Even now, he scarcely admits that the fundamental issue is settled. Alexander desired new worlds to conquer, but Baxter will never be content until he has obtained duly certified meteors from each and every galaxy of the universe and has converted them into pure fused ferrous, cobaltous, and nickelous bromides, ready for comparison with silver. The halides of boron, silicon, germanium, titanium, tin, and arsenic are volatile liquids, and %re readily hydrolyzed. These properties make them almost ideal for exact gravimetric analysis. In a batallion of tiny distilling flasks blown together into a single piece of glass, given fractions of these compounds were held stationary with refrigerating agents, or moved along by baths held a t carefully regulated temperatures, or sealed off when their time came to part cornpauy from the main procession. Results, while excellent, might, he thought, be further improved by rectification in each operation. The addition to each of the small flasks of a Hempel column with a jacket to be held a t constant temperature was a rather large order. Baxter found that the most complicated fractionations could be carried out in a one-piece glass apparatus having just two such flasks used in alternation. These were ingeniously connected to by-passes, a mercury valve, and plenty of small reservoirs. With the use of two dserent constant low temperatures main-
tained in column and receiver, respectively, the various fractions marched and counter-marched according to schedule. The volatile impurities imaginably present were tabulated according to their boiling points, and the procedure was planned to crack down on each of them in succession, without forgetting the possibility that mixtures of minimum or maximum vapor pressure might be formed. The proof by Bronsted that by slow vaporization the isotopes of mercury could be resolved to a measurable extent, followed by additional instances of the sort, raised the question whether the elaborate fractionations carried out as above might yield a mixture of isotopes not correspondingexactly to that in the original sample. Volatile halides already pure according to Baxter's exacting standards were further fractionated, but no changes in combining weights could be observed, an outcome, after all, in harmony with the f a d that relatively large vapor pressures, unfavorable to isotopic separation, always prevailed in Baxter's columns. After that he slept better. The accurate evaluation of the very small deviation coefficients of the permanent gases such as oxygen, helium, neon, and argon, from measurements a t one atmosphere and below, had already been the object of elaborate researches, but the results seemed to Baxter capable of improvement. The utmost accuracy was necessary if the packing effect was to be thoroughly understood. In 1923 he began a memorable series of papers, in collaboration with Dr. Starkweather, upon this subject. The very real advances which resulted could only be expressed in terms of a great number of very small improvements. Notable among these was purification by prcferential absorptimupon the mineral chabazite, already developed by 'Professor Lamb for somewhat d i e r e n t purposes. For the work in hand chabazite possessed marked advantages in comparison with charcoal. Gases strongly absorbed by it could be weighed in concentrated form a t room temperature and low pressures. In the course of this woik i t was noticed that the product of pressure and volume of a sample of helium in a pyrex vessel decreased unaccountab1y and steadily in spite of all experimental refinements. Baxter then proved the leakage of helium through pyrex gravimetrically. He has weighed a globe containing it once a year for four years. On each new anniversary the residual helium has decreased by one per cent.. The availability, in each research upon atomic weights, of one or more very pure dry substances always impelled Baxter to make determinations of density upon them, far more accurate than would have been necessary if the data had been designed solely for evaluation of corrections to vacuum. These data have already been invaluable in the study of the crystal lattice and cognate problems of the new physical chemistry. It was not a long step from this work to the density of aqueous solutions and the attempt to interpret the volume changes which occur when these are formed from their constituents. As early as 1911,
and ever since, for that matter, Baxter has concentrated his attention upon the separate factors such as the change in volume when liquid water is converted to water of hydration, instead of discussing the apparent mold volume of the solute in solution as did so many others. Over the years he has accumulated a vast array of precise data which has already yielded a wealth of tantalizing suggestions. It looks now as if the &ort to develop a comprehensive theory of the liquid state
will prove to be the next big physico-chemical enthusiasm, and we can confidently predict that Baxter's values will be one of the most solid pillars which will some day support it. For each figure to the right of the decimal point from which uncertainty is banished, the less demoralizing will the use of the little word "is" appear to the philosophicallyinclined. For generations to come, Baxter's data will remain a touchstone for the appraisal of many a physico-chemical theory.
