CHEMICAL CHANGES IN CALCIUM AESENATE. 



23 



7.0 



























































•♦ 



COz 

















©-— 



Sw^wm 



""■"■• 



.4D— 



an »«■ 



ma^mt 



""•"©' 



■•" 























5 





IV.S. 



^?-0;=r- 



5^-®— J 

























/42 



0£ 



Fig. 46.— JW (50 pounds in metal drum). 

 DISCUSSION. 



The arsenic oxid in water-soluble form and the carbon dioxid 

 increased in all of the samples except those packed in the tightest 

 containers. The close relationship between the increase in water- 

 soluble arsenic and in carbon dioxid is plainly shown by the paral- 

 lelism between the carbon dioxid and arsenic oxid curves for most 

 of the samples. 



In some cases the carbon dioxid content increases from the begin- 

 ning, but the quantity of water-soluble arsenic oxid begins to increase 

 only after some time has elapsed. This is undoubtedly due to the 

 fact that a certain quantity of free lime is necessary to repress the 

 "solubility" (really decomposition) of the calcium arsenate, and any 

 free lime in excess of this amount can carbonate without affecting 

 the soluble arsenic oxid. Once this limit is passed, however, decompo- 

 sition begins. Many of the curves also show a limiting value for each 

 of the two changes, these limits being reached at approximately the 

 same time. This is well shown in the case of subsamples stored in 

 sugar barrels, the most open containers, the greater number of 

 which reached equilibrium in from six to nine months. It might 

 naturally be assumed that this corresponds to the complete carbona- 

 tion of the free lime in the samples, the final mixture being essentially 

 tricalcium arsenate and calcium carbonate. Calculating the limit- 

 ing carbon dioxid values to their equivalent calcium oxid values and 

 subtracting these from the total calcium oxid, however, leaves calcium 

 oxid remaining in amounts from 0.2 to 4.8 per cent in excess of 

 the quantity necessary to form tricalcium arsenate with the total 

 arsenic oxid of the sample. Pure tricalcium arsenate octahydrate 

 [Ca3(As04)2.8H20], containing 42 per cent total arsenic oxid, gave 

 about 3 per cent water-soluble arsenic oxid, while the completely 

 carbonated commercial samples exceeded 1 per cent in only one case 

 (5TF), which seems to argue against this being the only calcium 

 arsenate in commercial samples. Further work will be necessary 

 before the question of the presence of basic arsenates can be defi- 

 nitely settled. 



