![]() Vinal and Bates discussed the possible sources of error in each method but did not indicate a preference for either method. In all this work it is evident that corrections for occlusions remain uncertain.Īlthough the iodine coulometer has received less criticism in the ensuing years, the iodine value for the faraday has not been generally accepted. Others who worked on the problem found occlusions, the amounts of which varied from 0.004 to 0.016 percent. This loss in weight amounted to a correction of only 0.004 percent in the faraday and was insufficient to explain the discrepancy between the silver and iodine values. They attributed this loss in weight to occluded foreign matter which was released on heating. Vinal and Bovard studied the occlusion problem at length and observed a loss in weight when the silver deposits were heated to slightly above 600° C. It will be shown later in this paper that no measurable separation of the isotopes of silver occurs on one, two, or three successive electrolyses therefore, the first criticism is, in fact, not pertinent. The first criticism does not appear to be valid judging from the results of the most precise determinations on the atomic weight of silver wherein in some cases the silver was reported to have been purified by repeated electrolysis. In the ensuing years the silver coulometer has been criticized on two main counts: (1) a partial separation of the isotopes of silver may occur during the deposition, and (2) the silver deposits may contain occlusions of electrolyte (silver nitrate), acid (nitric acid), and water, alone or in combination. To date this discrepancy has not been resolved conclusively. The result obtained with the iodine coulometer was 220 ppm higher than that given by the classic silver method. ![]() Since the two types of coulometers were run in series the ratio of the results given by the two methods was independent of the electrical units and the durations of the runs. In 1914 Vinal and Bates made a direct comparison of the silver and iodine coulometers in the laboratories of NBS and found that the two methods yielded different values for the faraday. ![]() However, 4 years earlier (in 1912) Washburn and Bates had proposed an improved 2 iodine coulometer for which high reproducibility and accuracy were claimed. At that time the consensus was that the silver coulometer gave highly accurate as well as reproducible results and yielded a value of the faraday accurate to within 0.001 percent (10 ppm). In 1916 Rosa and Vinal summarized the work that had been done with the silver coulometer not only in yielding a value of the faraday but also in attaining conditions wherein reproducible results could be obtained. In the original work at the National Bureau of Standards the national standards of electric current, mass, and time were used, and great emphasis was placed on the reproducibility of the method as a check on the stability of the existing standards of electromotive force (Weston cell) and electrical resistance (mercury ohm). This method has been extensively studied and under closely specified conditions 1 was used for many years in defining the international ampere. The classic method for the determination of the faraday involves the electrolytic deposition of silver on platinum from an aqueous solution of silver nitrate.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |