TUNGSTEN. 261 



corresponds to 0.0365457 gramme of the double sulphate, with a prob- 

 lable 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 

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

 la 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 

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

 [were used for the reduction of each, while 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 : 



.028301 1 

 .028291 

 .028311 

 .028301 j 

 .028367 



First set. 



Second set. 



.028367 



.028367 _, 



.028438 I 

 .028438 J 



Mean, .0283549, .0000115 



With the silver tungstate, Ag 2 W0 4 , Zettnow employed two methods. 

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

 the silver thus taken into solution was titrated with standard sodium 

 chloride. In three others the tungstate was treated directly with com- 

 mon salt, and the residual silver chloride collected and weighed. Here 

 again, on account of some complexity in Zettnow's figures, I am com- 

 pelled to reduce his data to a common standard. To 100 parts of AgCl 

 the following quantities of Ag 2 WO 4 correspond : 



By First Method. 

 161.665 

 161.603 



Mean, 161.634, .021 



By Second Method. 

 161.687 

 161.651 

 161.613 



Mean, 161.650, .014 

 General mean from both series, 161.645, .012 



