REPORT FOR ANALYTICAL CHEMISTS calculation on the basis of the 1961 value for iodine shows that the atomic weight obtained is probably too high by about 2 parts in 100,000. The authors also converted silver iodide to silver chloride, using silver iodide specially prepared for the purpose.*10) The same ratio was vised also by Baxter and Titus, < n ) and twice by Hônigschmid and Striebel.*12) The standard deviations of the means of the four series of measurements did not in any case exceed 4 ppm. For the work of Baxter and Lundstcdt it was 1.3 ppm. The atomic weight of chlorine used in the calculations was based on its combining weight with silver, as determined in both laboratories, with identical results. The value is also identical with that calculated from recent calibrated isotopic abundance measurements for the two elements.*3'4) The results obtained from the ratio Agl/AgCl were remarkably concordant and, on recalculation, yield an average value of 107.870 for the atomic weight. This is identical with the value chosen for the 1961 Table, but the value in the table was assigned an experimental uncertainty of ±0.003 to take account of the silver nitrate ratios and of two recent mass spectrometric determinations. and of Morley*15) on its combining weight with hydrogen. Their results have been improved on only by mass spectrometry. There are also a few other interesting examples of oxygen ratios. Baxter and Hoover· 16 ) ignited carefully purified iron to its sesquioxide. The atomic weight derived from 12 experiments was 55.8456, which may be compared with the presently accepted value of 55.847 ± 0.003. Ruer and Bode*") reduced cupric oxide to the metal and found for the atomic weight of copper, 63.538. This value is in such good agreement with the results obtained by Honigschmid and Johannsen* 18 ) in comparisons of cuprous chloride with silver (63.537) and silver chloride (63.536) that the rounded value 63.54 was chosen for the 1961 Table in preference to slightly higher values resulting from mass spectrometric measure-
ments. Further work is needed to resolve the discrepancy. Baxter and Butler* 19 ) prepared iodine pentoxide, thermally decomposed it, and weighed the iodine. The value obtained for iodine is 126.8997, which differs by —0.004 percent from that now known to be correct. The good quality of the work was not recognized when it was done because other ratios, believed to be less open to suspicion, yielded values considerably higher. Suspicion was directed by Baxter and Butler mainly to a slight deviation from the normal stoichiometric composition of the oxide. D. N. Craig*20) has pointed out that if the observed ratio Ι·,0 5 /2Ι is arithmetically combined with the ratio Na2C0 3 /I 2 O g , measured byBaxter and Hale*21) three years later, a value is obtained for iodine which does not depend on the exact composi tion of the iodine pentoxide, provided the atomic ratio of iodine to oxygen was identical in the two preparations. This is a reasonable assumption because of the tendency to develop stand ardized methods of preparation in a continuing research program such as that in Baxter's laboratory. The value for iodine calculated by combining the ratios is 126.9057, which differs by only +0.001 percent from the true value. From this it is incidentally evident that sodium carbonate, if properly pre pared, is an excellent chemical standard. It appears also that, as suspected, there was at least a slight fault in the composition of iodine pentoxide as pre pared in the Harvard laboratory. Ex cept for this, Baxter and Hale's acidbase titration is a good example of a reaction well suited to exact measure ment by the simplicity of the necessary manipulations and by the ease of ob serving the equivalence point. It would be readily possible to refine the operation today by substituting iodic acid, in the form of well characterized crystals, for granular iodine pentoxide and by determining the equivalence point by differential electrometric ti tration, instead of by an acid-base indicator. ATOMIC WEIGHT OF CARBON
The atomic weight of carbon has been so well fixed by physical measure ments that it would be pointless to do any further work on it. However, car bon provides an excellent illustration of an element whose atomic weight could be determined with very high accuracy today in contrast with the difficulties encountered in earlier years. Because of the importance of the atomic weight of carbon an enormous amount of work was done on it. Manv
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