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 thirty- six 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, to 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 to 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 dis- favor 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 per- manganate, 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 hydro- chloric 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 reac- tion 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 im- pregnability of this early value.

As Baxter's work on atomic weights continued. he

usefulness of its application to exact analytical work. It was evident that the compound to be taken for analysis could be recrystallized, electrolyzed, or dis- tilled 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 spectro- gram could avert worry, or give direction to new ex- periments.

Baxter gave much thought to the gases held by solids, especially porous solids, not previously fused or sub- limed 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 electroly- sis 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~ert had been intrigued bv the method. -

came to rely more and more upon spectroscopic and and on &is basis had advocated substantial downward spectroographic evidence of progressive purification of revision of the combining weights in question. But as compounds taken for analysis. He was emphatically a might have been expected, Baxter proved that quick- pioneer in developing the technic of refined qualitative silver electrodes are no royal roads to exact results. spectrum analysis, and repeatedly demonstrated the The purification and weighing of the original sample of

metallic chloride or bromide presented to him n o new tained in column and receiver, respectively, the various problem. But the configuration of the cell, the trap- fractions marched and counter-marched according to ping of the spray, the elimination of halogen as fast schedule. The volatile impurities imaginably present as it was liberated, the avoidance of oxidation of the were tabulated according to their boiling points, and amalgam or halogenation of the mercury, the estima- the procedure was planned to crack down on each of tion of residual traces of the metallic ion to be deter- them in succession, without forgetting the possibility mined and of mercury ion formed from the amalgam that mixtures of minimum or maximum vapor pressure all had to be worked out by many careful tests. An might be formed. interesting point was noted-that amalgams can be The proof by Bronsted that by slow vaporization the dried in a vacuum without loss of weight, but that if isotopes of mercury could be resolved to a measurable alcohol is used for drying, a weighable amount of extent, followed by additional instances of the sort, mercury vapor will pass off with the alcohol vapor. raised the question whether the elaborate fractionations The main argument was finally clinched by substituting, carried out as above might yield a mixture of isotopes for the weighed portion of salt, a weighed button of not corresponding exactly to that in the original sample. pure metal under sulfuric acid. This was trans- Volatile halides already pure according to Baxter's ported electrolytically into the cathode, whereupon exacting standards were further fractionated, but no Baxter accounted for the cadmium within a few parts changes in combining weights could be observed, an in a hundred thousand. That should come near to outcome, after all, in harmony with the f a d that rela- satisfying a federal income tax auditor. Correctly tively large vapor pressures, unfavorable to isotopic executed, then, the electrolytic method agreed admir- separation, always prevailed in Baxter's columns. ably with the well-established method of converting After that he slept better. metallic halide into silver halide. The accurate evaluation of the very small deviation

While Richards had already laid much stress upon coefficients of the permanent gases such as oxygen, diversification of the sources of his materials from helium, neon, and argon, from measurements a t one geographical and geological standpoints, Baxter made atmosphere and below, had already been the object of the first precise determinations of atomic weights of elaborate researches, but the results seemed to Baxter meteoric iron, cobalt, and nickel. He showed those capable of improvement. The utmost accuracy was to be identical with terrestrial sources within a few necessary if the packing effect was to be thoroughly parts in a hundred thousand, thus demonstrating the understood. In 1923 he began a memorable series of homogeneity of the universe over a wider range. Be- papers, in collaboration with Dr. Starkweather, upon fore this simple conclusion was reached, five elaborate this subject. The very real advances which resulted researches were carried out, each fortified with the ut- could only be expressed in terms of a great number of most precautions against error. Even now, he scarcely very small improvements. Notable among these was admits that the fundamental issue is settled. Alex- purification by prcferential absorptimupon the mineral ander desired new worlds to conquer, but Baxter will chabazite, already developed by 'Professor Lamb for never be content until he has obtained duly certified somewhat d ierent purposes. For the work in hand meteors from each and every galaxy of the universe chabazite possessed marked advantages in comparison and has converted them into pure fused ferrous, co- with charcoal. Gases strongly absorbed by it could be baltous, and nickelous bromides, ready for comparison weighed in concentrated form a t room temperature and with silver. low pressures. In the course of this woik i t was noticed

The halides of boron, silicon, germanium, titanium, that the product of pressure and volume of a sample of tin, and arsenic are volatile liquids, and %re readily helium in a pyrex vessel decreased unaccountab1y and hydrolyzed. These properties make them almost steadily in spite of all experimental refinements. ideal for exact gravimetric analysis. In a batallion of Baxter then proved the leakage of helium through tiny distilling flasks blown together into a single pyrex gravimetrically. He has weighed a globe con- piece of glass, given fractions of these compounds were taining it once a year for four years. On each new held stationary with refrigerating agents, or moved anniversary the residual helium has decreased by one along by baths held a t carefully regulated tempera- per cent.. tures, or sealed off when their time came to part corn- The availability, in each research upon atomic pauy from the main procession. Results, while ex- weights, of one or more very pure dry substances always cellent, might, he thought, be further improved by impelled Baxter to make determinations of density rectification in each operation. The addition to each upon them, far more accurate than would have been of the small flasks of a Hempel column with a jacket to necessary if the data had been designed solely for be held a t constant temperature was a rather large evaluation of corrections to vacuum. These data have order. Baxter found that the most complicated frac- already been invaluable in the study of the crystal tionations could be carried out in a one-piece glass lattice and cognate problems of the new physical apparatus having just two such flasks used in alterna- chemistry. It was not a long step from this work to the tion. These were ingeniously connected to by-passes, density of aqueous solutions and the attempt to inter- a mercury valve, and plenty of small reservoirs. With pret the volume changes which occur when these are the use of two dserent constant low temperatures main- 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 en- thusiasm, 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 philosophically inclined. For generations to come, Baxter's data will remain a touchstone for the appraisal of many a physico-chemical theory.