ANALYSIS OF LONDON PURPLE 637 



the figure representing the amount of soluble arsenious oxid, 

 and calculate the remainder as arsenic oxid. 



Calcium Oxid. Dissolve a portion of london purple in hy- 

 drochloric acid by the aid of heat, filter, wash the residue with 

 hot water, and pass hydrogen sulfid through the filtrate. Wash 

 the precipitate so obtained in the filter with hot water till clean. 

 Evaporate the filtrate to a small bulk, transfer to a 200 cubic cen- 

 timeter flask, treat with ammonium hydroxid to precipitate iron 

 and make to the mark. Filter the solution through a dry paper 

 and determine the calcium in an aliquot portion of the filtrate 

 by means of ammonium oxalate. 



Sand. 15 Dry the residue remaining from the hydrochloric acid 

 extraction in the previous method, transfer to a crucible, burn 

 paper and contents, and finally weigh as sand. 



533. Discussion of Methods of Analysis of London Purple. 

 Method I for total arsenious and arsenic oxid have been care- 

 fully tested by Haywood and excellent results have been ob- 

 tained. The Association of Official Agricultural Chemists has 

 also tested Method I, sometimes obtaining good results and 

 sometimes obtaining widely divergent results. It is undoubted- 

 ly true that it is extremely difficult to read the two end points 

 in Method I for total arsenic oxid, i. e., the point where the 

 thiosulfate uses up all the iodin and the point where all arseni- 

 ous oxid is oxidized to arsenic oxid by standard iodin, however 

 by closely following the directions it is easy to obtain closely 

 agreeing results. 



Method II for total arsenious and arsenic oxid have also been 

 tested by Haywood and results were obtained that closely agreed 

 with the results obtained by Method I. The Association of 

 Official Agricultural Chemists has also tested Method II, but 

 with unsatisfactory results up to the present time, the tendency 

 being to obtain low results on arsenious oxid and high results 

 on arsenic oxid. Method II is a decided improvement on Meth- 

 od I, as it greatly lessens the difficulty of reading the two end 

 points mentioned above and gives equally good results. 



The methods for determining soluble arsenious and arsenic 

 15 Bureau of Chemistry, Bulletin 68, 1902 : 20. 



