20 INSECTICIDES AND FUNGICIDES. 



Total arsenic oxid. Fifty cc of the hydrochloric acid solution (rep- 

 resenting 0.3333 gram), spoken of above, is heated to 80 C. on the 

 water bath and then taken off, and 50 cc of hydrochloric acid and 3 

 grams of potassium iodid added. The mixture is allowed to stand for 

 at least 15 minutes, the " ic " arsenic thus being reduced to ' ' ous " arsenic 

 by the action of the potassium iodid in acid solution, iodin being set 

 free. The solution is then rinsed out in a large beaker, diluted, and 

 tenth-normal sodium thiosulphate added, drop by drop, to get rid of 

 the iodin. The end -point here is rather difficult to read on account of 

 the very dark color of the solution, but with a little practice one can 

 determine it very easily by proceeding as follows: 



The sodium thiosulphate is run in a little at a time and occasionally 

 a drop of the solution is added to a drop of starch paste. This will, 

 of course, give a blue color with the starch, which becomes fainter 

 and fainter as the iodin is used up. Finally, when a drop of the solu- 

 tion only gives the slightest blue color with the starch, a little starch 

 paste is added directly to the whole solution, and the blue color dissi- 

 pated with a few drops of thiosulphate. With a little practice one 

 can in this way get the exact end-point every time. The solution is 

 immediately made alkaline with solid sodium carbonate. It is again 

 made slightly acid with hydrochloric acid, taking care that all of the 

 solid particles of the sodium carbonate on the bottom are neutralized 

 by the acid, and finally made alkaline with sodium bicarbonate. Starch 

 paste and tenth- normal iodin are now added until the blue color appears. 

 This end-point is easily read if the beaker is placed on a white surface 

 between the eye and the light and iodin run in until a distinct purple 

 color appears. The figure thus obtained gives the total amount of 

 arsenic in the solution as arsenious oxid. Subtracting the figure 

 obtained in the determination of arsenious oxid above from this we 

 have the amount of standard iodin corresponding to arsenic oxid in 

 50 cc of the original liquid. 



Calcium oxid. A portion of London Purple is dissolved in hydro- 

 chloric acid (an aliquot portion of the 300 cc used in the determination 

 of arsenious and arsenic oxids will do) and hydrogen sulphid passed 

 through. The precipitate is well washed, the filtrate is evaporated to 

 small bulk and transferred to a 200 cc flask, when it is treated with 

 ammonia (to precipitate the iron, etc.,) and made to the mark. A 

 100 cc portion of this is filtered off and treated with ammonium oxalate 

 in the usual way. 



Soluble arseniom oxid. Extract 1 gram of the sample for 10 days 

 with 500 cc of carbon-dioxid free water at room temperature, shaking 

 occasionally each day. Pass through a dry filter and use 100 cc for 

 analysis. Add sodium bicarbonate and starch paste and titrate to the 

 appearance of a blue color. This will give a number of cc of standard 

 iodin corresponding to the arsenious oxid in 0.2 gram of the sample. 



