LONDON PURPLES. 



Soluble arsenic oxid. An aliquot portion of the liquid from the 

 method above, for soluble arsenious oxid (say 200 ce) is transferred to 

 a flask, made slightly alkaline with NaOH, and evaporated to about 

 25 cc on a hot plate. The flask is then removed and allowed to cool 

 to about 80 C. , and an equal volume of concentrated hydrochloric 

 acid and 3 grams of potassium iodid added. It is allowed to stand 15 

 minutes, the iodin set free, exactly used up with tenth-normal thio- 

 sulphate, using starch if necessary, and the solution neutralized with 

 sodium carbonate. It is again made slightly acid with hydrochloric 

 acid, taking care that all lumps of sodium carbonate are acted on, then 

 made alkaline with an excess of sodium bicarbonate, and titrated with 

 iodin, using starch as an indicator. From this figure is subtracted the 

 number of cubic centimeters of standard iodin corresponding to the 

 soluble arsenious oxid in 200 cc of the filtrate, and the remainder is 

 calculated as arsenic oxid. 



Soluble calcium oxid. One, hundred cc of the liquid from the method 

 above for soluble arsenious oxid is treated with hydrochloric acid and 

 hydrogen sulphid passed through. The solution is filtered, ammonia 

 added, and the solution filtered again. Calcium is determined in the 

 final filtrate by means of ammonium oxalate in the ordinary manner. 



ANALYSES AND DISCUSSION. 



Table III, following, gives the composition of 11 samples of London 

 Purple: 



TABLE TIL Composition of London purples. 



The columns in Table III representing moisture and sand show 

 that these two substances vary greatly in different samples of London 

 purples. The former varies from 1.87 to 4.90 per cent and the latter 

 from 1.36 to 9.74 per cent. It would be natural to expect some vari- 

 ation in an article like London Purple, which is obtained as a by- 

 product in the dye industry. No fault can be found on this score with 

 any of the samples anal y/ed, with the exception of sample 19843, which 

 contains so much sand as to suggest that perhaps this substance has 

 been added to gain weight, or that, at least, the sample has been very 

 carelessly handled. 



