THE CALCIUM ARvSBNATES 



By R. H. Robinson 

 Acting Chemist, Oregon Agricultural Experiment Station 



INTRODUCTION 



Investigations during the past years on the preparation of the calcium 

 arsenates, together with a study of their chemical and physical prop- 

 erties, indicate the possibility of an economic substitute for the arsenates 

 of lead as an insecticide. During the past two decades the value of the 

 lead arsenates as a stomachic insecticide has been demonstrated. Re- 

 cently, owing perhaps to conditions brought on by the world war, the 

 cost of this necessary spray material has advanced in price to such an 

 extent that there is a possibility of the curtailing of its use, to the detri- 

 ment of our orchards and the quality of crops produced. 



Heretofore numerous field experiments have been made throughout 

 various sections of the United States to ascertain the practicability of 

 using calcium arsenate as a spray. In most cases these trial experi- 

 ments have failed, owing to excessive burning of the foliage. As with 

 most arsenicals, the cause of the burn is due to the action of arsenic 

 upon the foliage. The high water-soluble content of commercial sam- 

 ples of calcium arsenate indicates the possibility that therein lies the 

 cause of the intensive burning of foliage. This difficulty was encoun- 

 tered when the commercial lead arsenates were first used, but a study 

 of the properties of laboratory-prepared salts enabled manufacturers to 

 produce a high-grade insoluble lead arsenate that gave orchardists no 

 trouble and caused very little burning. Consequently, it was thought 

 that a more complete knowledge of the composition of the calcium arsen- 

 ates and a study of the methods of preparation and physical and chemical 

 properties would give an insight relative to the practicability of their 

 use as a substitute for the arsenates of lead. 



EXPERIMENTAL WORK 



A review of the literature relative to compounds of calcium and 

 arsenic shows that numerous preparations are discussed from a theo- 

 retical standpoint, but few have actually been obtained in the laboratory. 

 The calcium arsenates that are of vital interest to us from an insecticidal 

 standpoint must necessarily have such physical and chemical properties 

 that indicate a fairly stable salt. Preliminary experiments convinced 

 us that two salts, the tricalcium arsenate [Ca3(As04)2] and the calcium 

 hydrogen arsenate (CaHAsO^) appeared to be the only favorable ones. 



Journal of Agricultural Research, Vol. XIII, No. 5 



Washington, D. C. Apr. 29, 1918 



roz Key No. Oreg.-3 



(281) 



