Radioactivity

Frederick Soddy
1912 Annual Reports on the Progress of Chemistry  
Two new determinations of the atomic weight of radium have been made. In one1 a much larger quantity of radium was available than in previous determinations, the total weight of purified radium chloride being 1.35 grams. It was observed that a part of this preparation, which had been kept two years in a stoppered quartz tube, had absorbed oxygen, for on heating to 300° in nitrogen no water was evolved, but on fusion in hydrogen chloride it evolved chlorine and water and lost 5 per cent. in
more » ... 5 per cent. in weight. The surfwe of the quartz tube wits completely disintegrated by the action of the radiations, Quartz is an unsuitable material to use for the preservation of radium preparations. By fractional crystallisation of the material from aqueous hydrogen chloride, the value of the atomic weight found rose rapidly to 225.95, and did not further increase after numerous further fractionations. The atomic weight was determined by the aid of the methods and apparatus employed by T. W. Richards by finding the ratios RaC12/AgC1 and RaCl,/Ag. Radium chloride, dried in nitrbgen below 200° and fused in a platinum boat in hydrogen chloride at 900°, was the starting point, this compound being rendered less hygroscopic and more stable in air by fusion. The mean value obtained in both series of determinations was 225-95, individual estimations not differing from the mean by more than 0.03. Both in methods of purification and of estimation the work was in the main a repetition of Madame Curie's investigation, in which the vaTue found was 226.34 on the present values for silver and chlorine. The main points of difference were the preliminary fusion of the salt in hydrogen chloride, the precipitation of the solution with silver in the cold, and the correction for the solubility of the silver chloride by the nephelometer. These differences, taken in conjunction with the greater quantity of material employed, no doubt account for the slightly different value obtained, so that the number 226.0 may be taken t o represent the atomic weight of radium, within a very narrow margin of error, when purified and estimated by these methods. 0. Honigschmid, Moimtsh., 1912, 33, 253 ; A., ii, 523.
doi:10.1039/ar9120900289 fatcat:au6vozlczjhzffbtv34numr4aq