8 THE ANALYSIS OF POTASSIC CHLORIDE. 



chlorine, bromine, or iodine were obtained. The second group alone is 

 concerned with the present work. 



The early work of Berzelius, Pelouze, and Marignac need scarcely be 

 considered here. 1 It dealt for the most part with the precipitation of 

 argentic chloride from a solution of potassic chloride and gave very 

 widely discrepant results. Even the later work of Marignac on chloride, 

 bromide, and iodide of silver was of doubtful value, especially the last; 

 and accordingly until the present research was undertaken, the atomic 

 weight of potassium rested chiefly on the analyses of potassic chloride 

 made by Stas, by Richards and Archibald, and by Archibald alone, and 

 upon the analysis of potassic bromide made by Stas. 



As in the case of sodium, Stas 2 made several series of determinations 

 of the amount of silver needed to precipitate a known amount of potassic 

 chloride. One of these series was made in 1865, a later one in 1881, and 

 yet another shortly before his death. The earliest series, although in 

 some respects more careful and thorough than the later ones, was greatly 

 at fault, because in it he overlooked the solubility of argentic chloride. 

 Accordingly of this work only the later need be considered. The mean 

 of seventeen analyses indicated that 100 parts of silver were equivalent 

 to 69.122 parts of potassic chloride, corresponding to an atomic weight 

 for potassium of 39.130, if silver is taken as 107.93 and chlorine as 35.473. 

 Later, Richards and Archibald 3 found as a side issue of a research upon 

 the atomic weight of caesium that 100 parts of silver correspond to 69.115 

 parts of potassic chloride, and also that 100 parts of argentic chloride 

 correspond to 52.022 parts of potassic chloride ; and still more recently, 

 Archibald 4 found results not very different. With the values given above 

 for silver and chlorine these results indicate a value for potassium of 

 39.123 and 39.128, respectively, in the case of Richards and Archibald, 

 and 39.122 and 39.135, respectively, in Archibald's research. 



In the case of the bromide Stas found in an unusually varying series 

 of experiments that 100 parts of silver needed 110.346 parts of potassic 

 bromide for complete precipitation, a result not very far from that of the 

 less precise early work of Marignac. If Baxter's value for bromine is 

 accepted (79.953), 5 this corresponds to an atomic weight for potassium 

 of 39.143, noticeably higher than the results from the chloride. At that 

 time the discrepancy did not appear, because the atomic weight of chlorine 



1 For a brief discussion of this work Clark's Recalculation of the Atomic Weights 

 (1897) p. 41, may be consulted. 



2 Stas, Mem. Acad. Roy. Belg., 43 (1880) ; also Oeuvres Posthumes, edited by W. 

 Spring. See Clarke's Recalculation (1897), p. 42. 



3 Richards and Archibald (1903), Proc. Am. Acad., 38, 443 (1903). Zeit. anorg. 

 Chem., 34, 353, 1903. 



Archibald, Trans. Roy. Soc. Canada [2], 10, III. 47 (1904). 



5 Baxter, Journ. Am. Chem. Soc., 28, 1322 (1906). 



