Andrews and Farr — Determination of Arsenic. 317 



firm the statement quoted, provided, naturally, that a layer of 

 sufficient thickness be examined and that the liquid is other- 

 wise absolutely colorless. 



The arsenic as thrown down in the BettendorfT test, 

 although finely divided, settles in two or three hours and can 

 be filtered on the expiration of that time through an asbestos 

 filter without exhibiting any tendency to run through, and can 

 be washed with concentrated hydrochloric acid and with water, 

 without undergoing oxidation. As pointed out by Betten- 

 dorff, it contains more or less tin. Bettendorff found 95*86 to 

 98*56 per cent of arsenic. The amount of tin, present as basic 

 chlorides, varies, however, over a much wider range, than these 

 figures would indicate, depending on the amount of oxidation 

 undergone by the stannous chloride, the temperature, etc. The 

 precipitate dissolves in decinormal, centinormal, or millinormal 

 iodine solution, made slightly alkaline by addition of suitable 

 amounts of sodium bicarbonate or phosphate. The readiness 

 with which this solution occurs depends mainly on the amount 

 of stannic acid or of basic stannic chlorides carried down with 

 the precipitate. If the degree of this contamination is con- 

 siderable, the precipitate must be shaken for several hours with 

 the liquid in order to effect complete solution ; but if the 

 amount be smaller or vanishing, solution takes place in less than 

 a minute. This observation led us to prevent co-precipitation 

 of basic tin compounds by adding tartaric acid to the reaction 

 mixture of hydrochloric acid, stannous chloride and arsenite. 

 There seems to be no inclination for the precipitate to entrain 

 stannous compounds. 



Bettendorff found the reaction to be incomplete, even after 

 the lapse of a long time, with hydrochloric acid of 23 per cent 

 HC1, while with such of 25 per cent it was complete " after 

 some minutes." This concentration represents the minimum. 

 It is better to operate with a concentration of nearly 30 per 

 cent, which presents no difficulties when the ordinary reagent 

 acid of 40 per cent is used. That is to say, 25 cc of the latter 

 acid should be employed as a minimum to each 10 cc of the 

 solution to be examined.* The most obvious cause of errors 

 lies in the possibility of loss by evaporation of the highly vol- 

 atile arsenious chloride during the reduction process. This we 

 guarded against in our earliest experiments by sealing up the 

 reaction mixture in glass tubes, an effective but inconvenient 



* This rests of course on the assumption that the substance to be exam- 

 ined contains nothing to neutralize the hydrochloric acid. Otherwise a far 

 greater amount of acid may become necessary. Thus when tartar emetic is 

 to be examined for arsenic, the salt is dissolved directly in the 40 per cent 

 acid, and enough of the latter must be taken to furnish 25 to 30 per cent of 

 absolute acid, after that consumed in converting the potassium and the anti- 

 mony into chlorides has been duly allowed for. 



