ARSENIC. 213 



ARSENIC. 



For the determination of the atomic weight of arsenic three compounds 

 have been studied the chloride, the trioxide, and sodium pyroarsenate. 

 The bromide may also be considered, since it was analyzed by Wallace 

 in order to establish the atomic weight of bromine. His series, in the 

 light of more recent knowledge, may properly be inverted, and applied 

 to the determination of arsenic. 



In 1826 Berzelius * heated arsenic trioxide with sulphur in such a way 

 that only S0 2 could escape. 2.203 grammes of As 2 3 , thus treated, gave 

 a loss of 1.069 of S0 2 . Hence As = 74.460. 



In 1845 Pelouzef applied his method of titration with known quan- 

 tities of pure silver to the analysis of the trichloride of arsenic, AsCl 3 . 

 Using the old Berzelian atomic weights, and putting Ag = 1349.01 and 

 Cl = 443.2, he found in three experiments for As the values 937.9, 937.1, 

 and 937.4. Hence 100 parts of silver balance the following quantities 



of AsCl s : 



56.029 



56.009 

 56.016 



Mean, 56.018, .004 



Later, the same method was employed by Dumas, J whose weighings, 

 reduced to the foregoing standard, give the following results : 



4.298 grm. AsCl 3 = 7.673 grm. Ag. Ratio, 56.015 



5.535 " 9.880 " " 56.022 



7.660 " 13.686 " " 55-97 



4-680 " 8.358 " " 55-993 



Mean, 56.000, -_h .008 



The two series of Pelouze and Dumas, combined, give a general mean 

 -of 56.014, .0035, as the amount of AsCl 3 equivalent to 100 parts of 

 silver. Hence As = 74.450, .019, a value closely agreeing with that 

 deduced from the single experiment of Berzelius. 



The same process of titration with silver was applied by Wallace to 

 the analysis of arsenic tribromide, AsBr 3 . This compound was repeatedly 

 distilled to ensure purity, and was well crystallized. His weighings 

 .show that the quantities of bromide given in the third column are pro- 

 portional to 100 parts of silver : 



8.3246 grm. AsBr 3 = 8.58 grm. Ag. 97.023 



4.4368 " 4-573 " 97.022 



5.098 " 5.257 " 96.970 



Mean, 97.005, .012 



* Poggend. Annalen, 8, i. 

 fCompt. Rend., 20, 1047. 

 I Ann. Chim. Phys. (3), 55, 174. 1859. 

 I Phil. Mag. (4), 18, 270. 



