LONDON PURPLE 633. 



about reading the end point when the standard thiosulfate solu- 

 tion is added to use up the iodin set free. The end point is 

 not reached when the solution is a dirty white, but only when the 

 white is entirely unsoiled by any darker coloring. The potas- 

 sium cyanid method gives very fair results when carefully car- 

 ried out, but there is a tendency to get figures slightly above the 

 truth. 



A full discussion of the method of determining sodium-acetate- 

 soluble arsenious oxid and Method I of determining water-soluble 

 arsenious oxid will be found in the Proceedings of the i8th An- 

 nual Convention of the Association of Official Agricultural Chem- 

 ists. 9 To what is said there of water-soluble arsenious oxid, 

 it is only necessary to add that future work will probably show 

 that the time of extraction with water, i. e., ten days, can prob- 

 ably be considerably reduced, say to about five days, without in 

 any way decreasing the value of the method. From the work 

 performed by Colby it would appear that Method II gives re- 

 sults considerably below the truth, in that all free arsenious oxid 

 is not dissolved in 24 hours. 10 Haywood has never tested Meth- 

 od III for soluble arsenious oxid, nor has he seen it used by 

 any one but Colby, its originator. It would appear that this last 

 method would give just about the same results as would be ob- 

 tained by extracting one part of paris green with 1000 parts of 

 water for three to five days. If such is the case, it would be 

 simpler to extract all at once instead of by three operations. 



531. Green Arsenoid. The same methods of analysis are used 

 in examining this compound as are used for paris green, except 

 that the sum of moisture, sand, sodium sulfate, total arsen- 

 ious oxid and total copper subtracted from 100 represents col- 

 oring matter instead of acetic acid. 



532. London Purple. London purple is a by-product obtained 

 in the manufacture of certain of the anilin dyes. It is com- 

 posed principally of calcium arsenate and calcium arsenite to- 

 gether with an organic dye residue. Both the calcium arsenate 

 and arsenite are soluble in water to a limited extent, but in ad- 

 dition to this london purple may contain a certain amount of 



9 Bureau of Chemistry, Bulletin 67, 1902 : 98. 



10 California Agricultural Experiment Station, Bulletin 151, 1903 : 18. 



