318 Andrews and Farr — Determination of Arsenic. 



expedient, which fortunately proved to be unnecessary, since 

 sufficient protection is afforded by closing the solution in a 

 bottle with a well-ground glass stopper, provided the temper- 

 ature is not raised higher than 35° or 40°. The latter should 

 not be lower than 25°, since the reaction then becomes unde- 

 sirably slow. 



In the absence of tartaric acid, the arsenic does not usually 

 visibly deposit on the glass walls, but when the precipitation 

 of tin compounds is prevented, a small part of the arsenic 

 sometimes forms an extremely thin film, adherent to the glass. 

 The existence of such a film should be assumed, even when 

 none is visible. Experiment has demonstrated that a minute 

 truly, but titratable amount of arsenic may be present in this 

 way on the seemingly quite clean surface of the vessel in 

 which the reduction has taken place, 



The process finally assumed the following form. The mate- 

 rial to be examined, if in the form of a solution more bulky 

 than 20 cc . is neutralized and boiled down to 10 or 20 cc and 

 transferred to a white bottle of 80 to 100 cc capacity, provided 

 with a very well-ground stopper. To the liquid is added two 

 and a half times its volume of the tin reagent, prepared by 

 dissolving twenty grams of stannous chloride crystals and 

 forty grams of tartaric acid in one liter of forty per cent hydro- 

 chloric acid.* The stopper is inserted and the flask set aside 

 to stand in a warm place till the precipitated arsenic has sub- 

 sided, leaving the supernatant liquid water- whitef and clear. 

 This usually takes about two or three hours, more or less, if 

 the temperature is nearly 40°. An asbestos filter is prepared 

 in the usual manner, either in a Gooch crucible or in a ISTeu- 

 man filter tube. The precipitated arsenic is transferred to the 

 filter with the aid of a small amount of concentrated hydro- 

 chloric acid, rigorously chlorine-free. The flask is rinsed 

 repeatedly with small amounts of water which are passed 

 through the filter, which is completely washed by aid of suc- 

 tion without allowing it to be exposed to the air more than 

 necessary, except at the last draining, a precaution which may 

 not be requisite, but which seems indicated. The proper 

 amount of centinormal or of decinormal iodine solution is now 

 measured into the flask by a pipette as indicated by the equa- 

 tion, 



As + 51 + VNaHC0 3 = Na 2 HAs0 4 + 5NaI + 7C0 2 + 3ll 2 0, 

 allowing for an excess of ten to one hundred per cent above 



* This reagent will remain colorless if the constituents are arsenic-free. 

 It should be kept in bottles holding not more than 200 co and closed in such a 

 manner as to give assurance of protection of the contents against oxidation. 



f Of course this does not apply when nickel chloride or other colored salts 

 are present. 



