Estimation of Molyhdic Acid. 



159 



Before making the test experiments, 

 a stream of the purified carbon dioxide 

 was passed through the whole appa- 

 ratus for some minutes and the stop- 

 cock in the funnel was closed. 



Ammonium moljbdate, from 0*1 to 

 0*4 grm., dissolved in lO'^'^ of boiled 

 water, was put in the stoppered funnel 

 and almost all of it allowed to run into 

 the first flask. It was necessary that a 

 few drops be left in the funnel or the 

 liquid to follow would carry down 

 bubbles of air. This precaution is 

 necessary to the attainment of good 

 results. Ten cubic centimeters of 

 boiled water containing 0'5 grm. of potassium iodide were 

 introduced similarly and finally 20^^^ of strong hydrochloric acid 

 (sp. gr. 1*20). Before the acid was allowed to run in wholly, 

 the stoppered funnel was again filled with carbon dioxide and 

 finally left connected with the generator so that carbon dioxide 

 might be passed into the apparatus at any time. 



The liquid in the first flask, which at that time held the 

 ammonium molybdate, 0-5 grm. of potassium iodide, 20'^'^ boiled 

 water and 20'''' of hydrochloric acid (sp. gr. 1*20), was boiled 

 until the volume decreased to 25*'^ Under these conditions 

 we do not find it necessary to digest the mixture before boil- 

 ing. The iodine in the second and third flasks was titrated 

 with standard thiosulphate. 



The ammonium molybdate which we used in all the experi- 

 ments was prepared by twice recrystallizing presumably pare 

 molybdate. This salt was proved to contain 81*55 per cent of 

 molybdic acid by the fact that a given weight of the molybdate 

 was found to liberate 81 '55 per cent of the amount of iodine 

 set free by the same weight of pure molybdic anhydride dis- 

 solved in sodium hydroxide and treated similarly. 



We have found it to be a matter of importance that the 

 potassium iodide present shall never exceed the theoretical 

 requirement by more than half a gram ; otherwise the reduc- 

 ing action on the molybdic acid will be forced too far. When 

 the amount of molybdic acid present is less than 0'3 grm. it is 

 safer to restrict the potassium iodide used to half a gram. 



M0O3 as ammonium 



bdate taken. 



KI. 



M0O3 found. 



Error. 



grm. 



grm. 



grm. 



grm. 



•2585 



0^5 



•2580 



•0005 — 



•2995 



0-5 



•2991 



•0004 — 



•2524 



0^5 



•2513 



•oou- 



•2446 



0-5 



•2457 



•0011-f 



•2903 



0^5 



•2914 



•OOll-h 



•2V98 



05 



•2808 



•0010-f 



•2656 



0^5 



•2663 



•0007 + 



