SODIUM OE POTASSIUM : EDWAED DIVERS. 67 



sulphurous acid, as was shown by its bleaching a minute quan- 

 tity of iodine solution. That it showed also inactivity for a 

 time upon the iodide and starch reagent was due in part 

 certainly to the presence of the same impurity. The solution 

 did, however, begin to act upon the reagent sooner than this 

 in a blank test began to grow blue. But this was no proof that 

 hyponitrous acid passes spontaneously into nitrous acid. For, 

 first, there is the possibility of nitrous acid having been present 

 through not quite complete sulphonation and hydrolysis in the 

 production of the hyponitrite : this nitrous acid would indeed 

 have been converted into nitric oxide by the sulphurous acid 

 retained by the silver salt, but when all this was gone, the 

 nitric oxide would have become nitrous acid again by oxidation. 

 Secondly, it is almost a certainty that the oxidation by the air 

 of the sulphurous acid will have induced oxidation of some 

 hyponitrous acid, in accordance with the observations of Mohr, 

 M. Traube, van t'Hoff and Jorissen, Engler and AVild, Bach, etc. 



Quaniitaiive estimation of hyponitrotis acid. — Hyponitrous 

 acid can be estimated accurately both gravimetrically (Zorn) 

 and volumetrically (Thum). Solutions of the free acid or of its 

 alkali salts in water or of its other salts in very dilute and 

 cold nitric acid are mixed with excess of silver nitrate and 

 all free acid just neutralised with sodium carbonate or with 

 ammonia. The washed precipitate is either dried and weighed 

 as such, or w^eighed as metal, or as chloride. 



Volumetrically the acid can be estimated, after it has been 

 got into solution in the free state and unmixed with any other 

 acid, by adding a good excess of solution of potassium perman- 

 ganate to it, leaving it for a quarter of an hour, then adding 

 sulphuric acid, letting stand for another quarter of an hour, 



