42 PROTECTION OF PLA^|TS, 1920-21 



pound PbHAs04. This compound is generally highly satisfactory and non- 

 injurious to foliage. But there are some cases where foliage injuries, sometimes 

 very serious, result. The explanation of these is to be found in the presence of 

 impurities in the water used. Di-lead arsenate in pure water decomposes in the 

 manner shown in the following equation : 



3PbHAs0i=Pb3(As04)2 +H3ASO4. 



That is, there is a certain concentration of free arsenic acid, but this con- 

 centration in pure cold water does not exceed a few milligrams per litre of water, 

 an amount entirelj^ negligible. 



If some impurities are in the water, the amount of water soluble arsenic 

 may be increased tremendously. Thus ordinary calcium carbonate reacts in a 

 manner which may be indicated in the following equation : 



5 PbHAs04 + 2CaC03 = Pb4 (PbOH) (As04)3 +2CaHAs04+2C02 + H20. 



The di-calcium arsenate thus produced is fairly soluble in water. This 

 reaction may even be produced from the calcium carbonate adhering to the 

 sides of a spray tank which has at some time contained lime. 



If common salt is contained in the water a reaction follows which may be 

 represented as follows : 



5PbHAs04-KNaCl =Pb4 (PbCl) (ASO4) 3+NaH2As04+H3As04. 



Here again, large amounts of water soluble arsenic are liberated. Similar 

 reactions occur with other water impurities, and consequently where PbHAs04 

 is used on tender foliage (I am speaking of its use unmixed with any fungicide) 

 a fairly pure water is necessary. With impure waters the basic lead arsenate 

 is much preferable, and this has been recognized for many years. 



It is also very noteworthy that while PbIIAs04 is unreacted upon by the 

 carbon dioxide of the air, mixtures of PbHAs04 and lime are reacted upon pre- 

 cisely to the same extent as is calcium arsenate. It follows, therefore, that it is 

 unnecessary to use the expensive PbHAs04 in mixtures containing excesses of 

 lime, for calcium arsenate fulfills the same purpose. 



Calcium arsenate 



The exact composition of commercial calcium arsenate is not known. Sever- 

 al calcium arsenates are known, most notably di-calcium arsenate CaHAs04 

 and Tri-calcium arsenate Ca3(As04)2. Both these compounds are too soluble 

 for spraying purposes. The commercial compound contains a higher propor- 

 tion of calcium and is very insoluble. 



