384 Allen and Crenshaw — Stokes Method for the 



lows : A similar quantity of water to that used in the stand- 

 ardization tests was treated in the same way with sulphur 

 dioxide and carbon dioxide successively. To the cooled solu- 

 tion was then added O86 g. ferric alum, thus giving practically 

 the same conditions as those which obtain in the standard- 

 ization itself. The quantity of the permanganate solution 

 required in the blanks varied from 012 g. to 0*14 g. of per- 

 manganate solution, while 0*86 g. of ferric alum alone, with the 

 usual amount of water, required always about 0*04 g. or one 

 drop of permanganate solution. In the titration of an iron 

 solution obtained in the Stokes process, the latter blank was 

 subtracted, since here the ferric iron was reduced by the disul- 

 phide of iron and not by sulphur dioxide. 



(b) Oxidation of either ferrous solution or finely divided 

 sidphide by atmospheric oxygen. — The oxygen of the air, if 

 not prevented, may affect the results in several particulars. 

 First, when the finely ground sulphide is transferred from the 

 desiccator to the reaction flask. This operation requires only 

 about two minutes. The oxidation of 1-2 rag. of the sulphide 

 would have an appreciable effect, but oxidation during this 

 operation must be negligible, for a moist mixture of sulphides 

 of similar weight to that used in the oxidation experiments 

 exposed in a Goodi crucible to the air for 20 minutes gave an 

 extract with distilled water which reduced only 0*1 g. perman- 

 ganate solution. 



The purification of the sulphide itself has been sufficiently 

 insisted on by Stokes. He boiled it with hydrochloric acid, 

 washed it successively with hydrochloric acid and boiled water 

 cooled in carbon dioxide, in an atmosphere of carbon dioxide 

 gas, and finally dried it in a vacuum desiccator. The apparatus 

 which we employed in the washing has been described in a 

 previous paper.* Whether necessary or not, we have dried the 

 product more carefully than Stokes did. The washed sulphide 

 in the Gooch crucible was removed from the above mentioned 

 apparatus and put directly into a vacuum desiccator previously 

 filled with carbon dioxide. The desiccator was then evacuated 

 by a May-Nelson pump connected with driers which contained 

 lime and phosphorus pentoxide. Thus any air in the desicca- 

 tor was practically all removed in a couple of minutes. The 

 sulphide was dried for about one hour, during which time the 

 pump was kept running. It would seem impossible that any 

 oxidation could have occurred during this process. 



It may be said further that we assured ourselves by direct 

 experiment with a titrated ferrous solution that no oxidation 

 occurs when it is filtered in the described way. It has been 



* Allen, Crenshaw and Johnston, loc. cit. 



