282 Journal of Agricultural Research voi. xiii. No. s 



Recent work on these two salts is very limited. In 1844 Rammels- 

 berg ^ prepared the pure salt corresponding to the theoretical composi- 

 tion CaHAs04.2H20, containing two molecules of water of crystallization 

 which was further substantiated at about the same date by the work 

 of Klaproth ^ and Dufet.^ Others, at even an earlier date, obtained 

 different results, indicating variation in amounts of water of crystalliza- 

 tion and constitution. Although citations are made to possible methods 

 of preparing the tricalcium arsenate, no data containing actual figures 

 of analysis were found. Furthermore, numerous analyses of commercial 

 samples, both the so-called chemically pure salts, intended for reagents 

 in chemical work, and those sold by manufacturers for spraying purposes 

 proved to be mixtures of various calcium salts, and no two were alike in 

 composition. This point will be further commented upon later. 



Previous investigators suggest the preparation of calcium hydrogen 

 arsenate by using calcium chlorid (CaClj) and sodium hydrogen arsenate 

 (Na2HAs04) , the reaction being in accordance with the following equation : 



CaClj -I- Na^HAsO^-^CaHAsO, + 2NaCl 



These salts may, however, also react as represented in the following 

 equation, giving the tricalcium arsenate : 



3CaCl2 + 2Na2HAsO,^Ca3(AsO,)2 +4NaCl + 2HCI 



It is obvious, therefore, that both the arsenates may be formed in 

 greater or less amounts when prepared in the above manner. This 

 may account for the variable composition of the commercial arsenates 

 noted above, and emphasizes more strongly the necessity of specific 

 control of conditions in such a manner that one or the other salt will 

 be produced. 



PREPARATION OF CALCIUM HYDROGEN ARSENATE AND TRICAIvCIUM 



ARSENATE 



In order to make a complete study of those physical and chemical 

 properties of the calcium arsenates that would have an immediate 

 bearing upon their value as a spray material, the preparation of these 

 salts in very pure form is essential. 



Numerous methods of preparation were undertaken, but in most 

 cases mixtures were obtained of varying composition. After trial experi- 

 ments the following method was found to be satisfactory : 



Solutions of calcium chlorid and sodium hydrogen arsenate were 

 made, and very slightly acidified with either acetic, hydrochloric, or 

 nitric acid. After filtration, to obtain absolutely clear solutions, the 

 cold calcimn-chlorid solution was gradually poured into the sodium- 



i Gmelin, Leopold, and Kraut, K. J. handbuch der anorganischen chemie. Aufl. 7, Bd. 3, 

 Abt. 2, p. 569. Heidelberg, 1908. Cites Rammelsberg, Klaproth, and Dufet. 



