262 



E. H. 



Ashley 



— Dithionic 



Acid and the Dithionates. 













Table II. 









Decomposition of barium dithionat€ 



by boiling with sulphuric acid. 









I value 





I value 















of 





of 





Errors 



Errors 







s 2 o 5 



S 2 5 



I 



Na 2 S 2 3 



S 2 5 



in 



in 







;aken. 



taken, taken. 



required. 



found. 



I. 



S 2 5 . Time. 



No. 



grm. 



grm. 



grm. 



grm. 



grm. 



grm. 



grm. min. 



1 



1039 



•2310 



•5759 



•3435 



•1045 



+ •0014 



+ •0006 



20 



2 



1046 



•2326 



5708 



•3372 



•1051 



+ •0010 



+ •0005 



28 



3 



1039 



•2311 



•5740 



•3435 



•1037 



— '0006 



— •0002 



34 



4 



1033 



•2297 



5701 



•3387 



•1041 



+ •0017 



+ •0008 



45 



5 



1721 



•3827 



5712 



•1876 



•1726 



+ •0009 



+ •0005 



35 



6 



1719 



•3820 



5702 



•1894 



•1713 



— •0012 



— •0006 



50 



7 



1726 



•3838 



•5734 



•1898 



•1726 



— •0002 



•oooo 



12 



8 



1724 



•3832 



•5727 



•1885 



•1728 



+ •0010 



+ -0004 



10 



9 



1721 



•3826 



•3727 



•1886 



•1728 



+ •0015 



+ •0007 



10 



10 



0692 



•1539 



3130 



•1599 



•0689 



— •0008 



^•0003 



12 



11 



0350 



•0777 



3109 



•2323 



•0354 



+ •0009 



+ •0004 



4 



12 



2061 



•4582 



6205 



•1632 



•2057 



— •0009 



— •0004 



15 



13 



2402 



•5340 



6215 



•0862 



•2408 



+ •0013 



+ •0006 



15 



These results show that dithionic acid may be determined 

 by boiling with sulphuric acid and estimating the sulphur 

 dioxide liberated, while when hydrochloric acid is used the 

 results are far from satisfactory. 



There are three reasons why sulphuric acid should work 

 better than hydrochloric acid in this process : 



First, when sulphuric acid is added to the solution of barium 

 dithionate, barium sulphate is precipitated and dithionic acid 

 is left in free condition, this reaction proceeding at once to 

 completion because the barium sulphate formed is removed 

 from the system. It would seem that when hydrochloric acid 

 is used the dithionic acid is completely liberated only by a 

 gradual change in the conditions of equilibrium. 



Second, when the solution containing sulphuric acid is boiled, 

 the water is driven off, the concentration of the solution 

 increases and the high temperature of the fuming point of 

 sulphuric acid is reached. Under such conditions the decom- 

 position of the dithionic acid is rapid and complete, the time 

 being dependent upon the original dilution of the solution. 

 In one case, No. 11, the operation was ended in four minutes. 



Third, no appreciable amount of acid distils over from the 

 Yoit flask into the receiver containing the iodine to interfere 

 with the back titration with sodium thiosulphate, the only acid- 

 present being that produced by the oxidation of the sulphur 

 dioxide. Under these conditions the starch indicator acts 

 sharply, which is not the case when hydrochloric is used. 



