~ and Haag,® the only workers who have considered the question of a 
2 BULLETIN 1115, U. S. DEPARTMENT OF AGRICULTURE. 
safe to use calcium arsenate that had been stored for any appreciable 
length of time. A preliminary investigation showed that in the | 
cases of the most serious burning reported, the injury was due to the 
use of improperly made calcium arsenate. The evidence of deteriora- 
tion seemed sufficiently suggestive, however, to warrant an investiga- 
tion of the matter under accurately controlled conditions. 
RESULTS OF PREVIOUS INVESTIGATIONS. 
Robinson? showed that chemical changes occur in calcium arse- 
nate through the action of carbon dioxid in the presence of water and 
stated that under atmospheric conditions a similar but much slower 
action probably would occur. As a result of bubbling carbon) 
dioxid through suspensions of dicaletum and tricalcium arsenates 
in water, he concluded that there was an evident “solvent action 
upon the calcium arsenates.’’ However, while this writer’s results for 
tricalcium arsenate were higher, those for dicalcium arsenate were } 
lower than his figures for the solubility of the same salts in pure | 
water. | 
Patten and O’Meara‘ suggested that calcium arsenate when | 
applied to foliage would almost certainly be subject to change by | 
the carbon dioxid given off by the leaves. In pursuance of this 
idea, they determined the amount of arsenic rendered soluble when 
saturated carbon dioxid water was used in place of distilled water 
in the regular method for determining water-soluble arsenic oxid in 
calcium arsenate. The six samples used by them gave an average — 
of 0.7 per cent soluble arsenic oxid with distilled water and an aver-— 
age of 18.5 per cent soluble arsenic oxid with the carbonated water. | 
The work of the previous investigators was done in connection 
with the action of carbon dioxid on calcium arsenate when applied 
to foliage in a thin film and subjected to the effect of moisture. Reedy | 
possible change in calcium arsenate during storage, state that “dur- 
ing storage this product undergoes some change in composition which 
results in a considerable increase in the amount of water-soluble arse- 
nate.” They thought at first that moisture and carbon dioxid from 
the air caused this change, but state that ‘‘the use of air-tight con- 
tainers has not been sufficient to prevent it.’’ However, they report — 
experiments showing that exposure to laboratory air and to moist 
carbon dioxid caused an increase in the water-soluble arsenic which 
in the latter case was quite marked. As a result of their work, they 
conclude that ‘‘the decomposition of tricalcium arsenate is due to 
hydrolysis, which seems to be catalyzed by many substances that — 
3 J. Agr. Research, 13 (1918): 288; Oregon Agr. Col. Exp. Sta. Bull. 131 (1918), p. 10. 
4 Mich. Agr. Col. Exp. Sta., Quart. Bull., Nov., 1919, p. 83. 
5J. Ind. Eng. Chem., 13 (1921): 1038. 
