140 RESEARCHES UPON ATOMIC WEIGHTS. 



It has been repeatedly shown, especially in this laboratory, that most of the 

 earlier work upon atomic weights has been vitiated by neglect of certain fun- 

 damental precautions. The incomplete drjdng of solids has been responsible 

 for many of the discrepancies and errors which exist. Neglect of the solubility 

 of precipitates, together with the use of too concentrated solutions during 

 precipitation, so that perceptible inclusion and occlusion took place, undoubt- 

 edly have influenced many gravimetric processes. Volumetric processes have 

 been affected by inaccurately prepared standard solutions, as well as the diffi- 

 culty inherent in measuring exactly large volumes of solution. 



In discussing in detail the applications of the above causes of constant error 

 to the individual investigations, at the best it is possible merely to indicate 

 the nature of the difficulties; as a rule it is impossible to estimate the magnitude 

 of the error without repetition of the experimental work. Hence in this paper 

 attention is called only to points in the earlier work which have been experi- 

 mentally investigated. The uncertainty in most of the previous determina- 

 tions is emphasized by the lack of agreement in the individual analyses in each 

 series, as well as in the different series. 



The choice of method for this investigation was influenced by several con- 

 siderations. In the first place, the substance to be analyzed must be definite in 

 composition and capable of being either fused or heated to a high temperature 

 in order to insure the elimination of moisture. In the second place, in view of 

 the fact that chromium is hard to handle satisfactorily in a quantitative fashion, 

 the analytical operation should involve the determination of some other ele- 

 ment. The halogen compounds, which have been employed very successfully 

 many times, especially in this laboratory, for the determination of the atomic 

 weights of metallic elements, are less suited for use in the case of chromium on 

 account of the difficulty in the complete precipitation of the halogens by means 

 of silver nitrate. 



All things considered, the chromates of silver seemed to offer the most prom- 

 ising possibilities on account of the ease with which their silver content 

 may be determined. It is true, in order to determine the ratio of the atomic 

 weight of chromium to that of either silver or oxygen, this method neces- 

 sitates a knowledge of the exact ratio of the atomic weights of silver and oxy- 

 gen, knowledge which is still slightly uncertain. The per cent of silver in the 

 compound being known, however, analytical data may be used at any subse- 

 quent time for the calculation of the atomic weight of chromium. Furthermore, 

 since the value for the atomic weight of chromium at present accepted depends 

 very largely upon the analysis of silver chromate, a study of this salt with the 

 application of the most modern methods seemed to promise interesting results, 

 and therefore was first taken up. In a following paper is given a description of 

 the analysis of silver dichromate. 



