34 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



was demonstrated. The reason why he obtained consistent results 

 was simply because he used exactly the same method in each analysis. 



Obviously, then, a solid salt is not to be dropped into a precipi- 

 tating solution indeed, such a proceeding is so repugnant to the 

 instinct of a precise quantitative analyst that it is surprising Stas should 

 have been guilty of it. But how dilute must the precipitating sodic 

 chloride be in the present case in order to avoid the danger ? Careful 

 experiments showed that with fifth normal solutions, as in the case of 

 silver, such slight occlusion as existed was quickly removed by gentle 

 agitation, and after a short time the mixture showed a constant equi- 

 librium point. Hence in our final experiments the solution was always 

 as dilute as this. In previous similar work in this laboratory, the 

 solutions of halide were rarely stronger than fifth normal, and never 

 much stronger, so that the error from this source must have been 

 very small. 



It might be supposed that when the sodic chloride and argentic 

 nitrate are too dilute to be carried down with the precipitate, sodic 

 nitrate is not likely to be carried down in important amounts. For if 

 both silver and chloride are fifth normal, the resulting sodic nitrate 

 must be tenth normal, a dilution at which even silver nitrate is not 

 sensibly occluded. Nevertheless, as a matter of fact, the final experi- 

 ments yielded quantitative evidence of the occlusion of a trace of 

 this salt, amounting to 0.005 per cent of the weight of the precipitate. 

 This is probably because the precipitate was obliged to stand for some 

 time in the solution, and it is doubtful if this source of error could be 

 wholly avoided. 



From these experiments it appears that the degree of dilution at 

 which occlusion ceases to be seriously troublesome is about the point 

 at which most physical chemists agree that the realm of dilute solu- 

 tions begins. Hence it would seem that in these cases of adsorptive 

 occlusion the undissociated salt is that which is occluded, in confir- 

 mation of other work.* 



Obviously the occlusion of argentic nitrate, or of sodic chloride, 

 would affect the results of the analysis, no matter whether silver or 

 argentic chloride served as the weighed standard wherewith to com- 

 pare the weighed sodic chloride, although more seriously in the former 

 case than in the latter. On the other hand, the occlusion of sodic 

 nitrate could only affect the result when the argentic chloride was 

 weighed ; its occlusion would have no effect on the amount of silver 

 required to precipitate the chlorine. The possible effect of these dis- 

 turbing influences will be alluded to again during the recital of the 

 quantitative operations. 



*Richards, MacCaffery, and Bisbee, Proc. Am. Acad., 36, 377 (iQOi). 



