652 AGRICULTURAL ANALYSIS 



548. Discussion of Methods of Analysis of Formaldehyde Solu- 

 tions. Method I has been carefully tested and has been found 

 to give excellent results on strong solutions of formaldehyde. 

 Method II, the original hydrogen peroxid method, gives results 

 that are slightly below the truth. The reasons for the superior- 

 ity of Method I over Method II has been explained by Haywood 

 and Smith. 38 Method III has been carefully tested and has been 

 found to give good results. It can, however, only be used on 

 dilute solutions of formaldehyde, or on concentrated solutions, 

 which are diluted to a large extent. Since the error of analysis 

 would be greatly multiplied by examining concentrated solutions 

 of formaldehyde by this method it seems best to restrict it to 

 solutions that are dilute in the beginning. 



549. Statement of Insecticide Analyses. Two typical insecti- 

 cides may be considered in presenting a scheme for expressing 

 the results of analyses, namely, paris green and london purple. 

 In making complete analyses of paris green the following con- 

 stituents should be determined, namely, moisture, sand, sulfur 

 trioxid, copper, total arsenic, soluble arsenic, and acetic acid. 



It is best to report the sulfur trioxid in the form of sodium 

 sulfate, since from the method of manufacture of paris green it 

 is almost certain that the sulfur trioxid present exists in this 

 form in the green. The copper should be reported as cupric 

 oxid (CuO), and the total arsenic as arsenic trioxid or arsenious 

 oxid. The acetic acid present should not be reported as acetic 

 acid as is usually done, but as acetic anhydrid, since the report- 

 ing of the copper as CuO has left the acetic acid as acetic 

 anhydrid. 



In reporting soluble arsenic it should be given as arsenious oxid 

 or arsenic trioxid and should be determined in two ways; (i) 

 by the Avery-Beans method to approximately determine the actual 

 free arsenious oxid present, and (2) by the prolonged water 

 soluble method to determine to some extent the stability of the 

 green. 



In examining samples of london purple the following deter- 

 minations should be made, moisture, insoluble in hydrochloric 

 M Journal of the American Chemical Society, 1905, 27 : 1183. 



