TWENTY-FIRST ANNUAL MEETING. 209 



The Preparation of Lead Ortho-arsenate. 



While this is essentially the work of a chemist and 

 hardly to be undertaken by the average fruit grower, yet 

 with fair laboratory facilities, it is not so very difficult, 

 and hence a brief statement of the method is included 

 here. The method used by us, both in making and test- 

 ing our ortho-arsenate for experimental use was essen- 

 tially as described by Volck in the article referred to in 

 footnote 1. With slight modifications, his method is as 

 follows : 



Solutions of lead acetate or lead nitrate and solutions 

 of arsenic acid or of ammonium, sodium or potassium ar- 

 senate containing the correct weights of the oxids (1 part 

 of AS2O3 to 2.9 parts PbO) are poured together. The 

 water should be sufficient to dilute the precipitate and 

 keep it from forming too thick a mass. To this mixture 

 ammonia is added to strong alkalinity, and care is taken to 

 keep the solution alkalin throughout the reaction, as upon 

 this point depends the real success of the process. The 

 mixture is then heated gently for an hour or more, allowed 

 to settle, and the clear liquid tested for arsenic oxid, by the 

 ammonia test described below. If a precipitate forms, a 

 little more lead solution is added and the procedure re- 

 peated until no precipitate appears. 



It is well to wash the finished product, preferably by 

 decantation, and again apply the ammonia test. If any arsen- 

 ic oxid appears in the filtrate, more lead solution should 

 be added. Finally wash to the complete removal of water- 

 soluble salts. In accordance with well-known principles 

 of chemistry, the retention of the arsenic oxid will not be 

 complete until there is an excess of the precipitant, that is, 

 lead oxid. For this reason, true ortho samples will show 

 less than the theoretical percentage of arsenic oxid (25.59). 

 With commercial samples, where allowance has to be 

 made for impurities, this percentage is still further re- 

 duced. 



