368 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 54 



In the case of the iron salt tlie method of working was this : The 

 pure, artificial FeWO^ was fused with sodium carbonate, the resulting 

 sodium tungstate was extracted by water, and the thoroughly washed 

 residual ferric oxide was dissolved in hydrochloric acid. This solution 

 was then reduced by zinc, and titrated for iron with potassium perman- 

 ganate. Corrections were applied for the drop in excess of permanganate 

 needed to produce distinct reddening, and for the iron contained in the 

 zinc. 11.956 grammes of the latter metal contained iron corresponding to 

 0.6 cc. of the standard solution. The permanganate was standardized by 

 comparison with pure ammonium-ferrous sulphate, Am2Fe( 804)2-61120, 

 so that, in point of fact, Zettnow establishes directly only the ratio 

 between that salt and the ferrous tungstate. From Zettnow's four 

 experiments in standardizing I find that 1 cc. of his solution corresponds 

 to 0.0365457 gramme of the double sulphate, with a probable error of 

 ±.0000012. 



Three sets of titrations were made. In the first a quantity of ferrous 

 tungstate was treated according to the process given above; the iron 

 solution was diluted to 500 cc, and four titrations made upon 100 cc. at 

 a time. The second set was like the first, except that three titrations 

 were made with 100 cc. each, and a fourth upon 150 cc. In the third 

 set the iron solution was diluted to 300 cc, and only two titrations upon 

 100 cc each were made. In sets one and two thirty grammes of zinc 

 were used for the reduction of each, Avhile in number three but twenty 

 grammes were taken. Zettnow's figures, as given by him, are quite com- 

 plicated; therefore I have reduced them to a common standard. After 

 applying all corrections the following quantities of tungstate, in grammes, 

 correspond to 1 cc. of permanganate solution : 



■First set 



.028301 

 .028291 

 .028311 

 .028301 

 .028367 



.028367 





.028367 



.028438 .^^.^^ ^^^ 



.028438 J 



Mean, .0283549, ± .0000115 



Hence W = 184.41. 



With the silver tungstate, Ag^WO^, Zettnow employed two methods. 

 In two experiments the substance was decomposed by nitric acid, and 



