CHEMICAL CHANGES IN CALCIUM ARSENATE. 23 
DP HTS 
NV: 
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S 
P4 
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 
[Ca,(AsO,),.8H,O], 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 
(BW), 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. 
