THE ANALYSIS OF POTASSIC BROMIDE. 



greatly diminished by the addition of an excess of either precipitant ; but 

 whether or not any is dissolved by a very dilute acid solution of argentic 

 nitrate, such as that used in washing, it is practically impossible to discover. 

 It is not unlikely, however, that about as much was dissolved by this solu- 

 tion as by the dilute hydrobromic acid used by Baxter in his admirable 

 work on the atomic weight of bromine ; l and in this solution it is easy to 

 find the amount of dissolved substance. Baxter found as a matter of fact 

 in his last seven most exact syntheses an average of 0.00004 gram of 

 argentic bromide in each of the wash-waters a figure which was added 

 to each of our weights of argentic bromide, because the other circumstances 

 of the analysis were similar in the two cases. On the average, this cor- 

 rection exactly eliminates the other ; and except for the sake of complete- 

 ness, they might both have been wholly neglected. 



Of course all the weighings were corrected to the vacuum standard, by 

 adding 0.000041 gram to every apparent gram of argentic bromide and 

 0.00029 gram to every apparent gram of potassic bromide, as calculated 

 from the figures 6.473, 2.74, and 8.30, for the densities of argentic bro- 

 mide, 2 potassic bromide, and the brass weights respectively. All the 

 determinations made are given in the table below. 



The Ratio of Argentic Bromide to Potassic Bromide. 



The extreme deviation from the mean corresponds to an error of weigh- 

 ing the potassic bromide of 0.00004 gram, a reasonable quantity. The 

 "probable error" indicates that there is but little chance that the atomic 

 weight of potassium is much below 39.113 or much above 39.114, if con- 

 stant chemical errors were successfully excluded. Stas found in a single 

 experiment the number 63.383 instead of 63.37S. 3 



With unlimited time more determinations might well have been made, 

 but the agreement of these four results is so good that further repetition 

 seemed to be not very urgent. 



iBaxter, Journ. Amer. Chem. Soc., 28, 1322 (1906). 

 2 Baxter and Hines, Amer. Chem. Journ,, 31, 220 (1904). 

 3 Stas, Untersuchungen (Trans. Aronstein 1867), page 340. 



