ACTION OF SOAP UPON LEAD ARSENATES ‘ 
By R. M. Pinckney 
Assistant Chemist, Montana Agricultural Experiment Station 
Soap is sometimes added to the water used for applying lead arsenate 
as an insecticide. Few authorities advise its use on apple trees or tender 
vegetation, but it is occasionally advised for use upon hardy and smooth¬ 
leaved crops, such as cabbage and sugar beets. It is said that the use of 
soap with arsenates is increasing. 
THE ADVANTAGES 
Several advantages are to be gained by the use of soap. The lead 
arsenate remains in suspension longer in soap solution than in pure water, 
hence is more easily and evenly applied, especially when hand-operated 
sprayers are used. The soap also helps to spread the arsenical. J. R. 
Parker (j) ^ found that soap retarded the settling of lead arsenate and 
stated that it also improved the spreading upon smooth-leaved plants. 
THE DISADVANTAGES 
The disadvantage to be feared is that of burning the leaves of the crop 
sprayed by arsenic dissolved by the action of the soap upon the lead 
arsenate. 
Tartar and Bundy (6) in 1913 reported that fruit trees were injured 
by spraying with soap and lead arsenate and showed that the use of the 
soap increased the quantity of soluble arsenic in the liquid. They also 
noted that acid arsenate was much more soluble in soap solution than 
neutral arsenate. Headden (j) had already pointed out the danger in 
using water containing alkali salts for arsenical sprays. It was therefore 
natural that some presumed that the solubility of lead arsenate in soap^- 
solution was due to free sodium carbonate in the soap, and that the dam¬ 
age could be avoided by using only neutral soaps. However, it has been 
commonly known to chemists for a long time that lead readily forms 
insoluble soaps. Therefore it is to be expected that neutral soaps might 
undergo double decomposition with the lead arsenate, forming lead soap 
and alkali arsenates which would be soluble. This possible reaction may 
be represented thus: 
Sodium soap-h lead arsenate = sodium arsenate + lead soap. As the 
sodium compounds are soluble and the lead compounds are insoluble, it is 
not to be expected that the reaction will go to completion in either direc¬ 
tion. This double decomposition, however, is not the only imaginable 
reaction which might take place between the soap and lead arsenate; and, 
in fact, others have been reported by some investigators and will be dis¬ 
cussed later. So far as observed by the writer, no one has heretofore 
reported whether or not soaps of different fatty acids behaved differently 
toward lead arsenates. 
1 Accepted for publication May 29, 1922. 
* Reference is made by number (italic) to “ I/iterature cited, ” p. 95. 
Journal of Agricultural Research, 
Washington, D. C. 
abd 
(87) 
Vol. XXIV, No. 1 
Apr. 7,1923 
Key No. Mont.-9 
