6 MASS. EXPERIMENT STATION BULLETIN 201. 



Stability. — Acid and neutral lead arsenates are practically insoluble 

 in cold water, but continuous percolation may cause decomposition. Hot 

 water is more effective, but may cause slight hydrolysis, particularly of 

 the acid salt. Dilute solutions of sodium carbonate, sodium chloride and 

 sodium sulfate have been shown (12, 13) to increase the solubility of the 

 arsenic, especially where the common acid lead arsenate is used. Stability 

 may be obtained by adding calcium hydroxide (milk of lime) to acid or 

 neutral arsenate. The former will necessarily require more base to afford 

 like protection. The basic salt sold in the West contains only about 20 

 to 22 per cent of arsenic pentoxide on a dry basis. 



Form of Gunranty. — The usual form of guaranty under which dry lead 

 arsenate is sold is substantially as follows : — 



Per Cent. 



Active ingredients: — 



Lead arsenate, not less than . . . . . . .98.00 



Total arsenic pentoxide, not less than . . . . 31.00 



To^aZ arsereic (as metallic), not less than . . . 20.20 



Inert ingredients, not more than . . . . . . . .2.00 



100.00 

 Water Soluble. 



Soluble arsenic pentoxide, not more than . . . . . . 0.75 



Soluble arsenic (as metallic) , not more than . . . . . . . 50 



As in the case of Paris green, an article is desired wliich contains a high 

 percentage of total arsenic with a low maximum percentage of soluble 

 arsenic. It is well to note further that in the above guaranty the essential 

 statements are those italicized. The other statements made simply repeat 

 this information in different form. 



The Federal standard (14) for commercial lead arsenate specifies not 

 more than 50 per cent of water, nor less than 12.50 per cent of total 

 arsenic pentoxide (equivalent to 8.15 per cent metallic arsenic), and not 

 more than 0.75 per cent of arsenic pentoxide soluble in water (equivalent 

 to 0.49 per cent metallic arsenic). 



To maintain a high standard of purity the product, whether paste or 

 powder, should be substantially free from carbonate, chloride, sulfate 

 and acid soluble matter, and should not contain more than 2.50 per cent, 

 on a dry basis, of water soluble by-products. The dry acid lead arsenate 

 sold in the East is usually guaranteed to contain 30 or 31 per cent of total 

 arsenic pentoxide (equivalent to 19.56 or 20.21 per cent metallic arsenic), 

 and not more than 0.75 or 1 per cent of arsenic pentoxide soluble in water 

 (equivalent to 0.49 or 0.65 per cent metallic arsenic). The paste is usually 

 guaranteed to contain 15 per cent arsenic pentoxide (equivalent to 9.78 

 per cent metallic arsenic), and not more than 0.75 per cent arsenic pen- 

 toxide soluble in water. 



To summarize: Lead arsenate is of low arsenic content, and that in 

 the form of pentoxide, practically insoluble in water, fairly stable under 

 New England weather conditions, and may be appHed to most plants 



