DECOMPOSITION OF ALKALINE HYPOSULPHITES. 487 



with salts dried in a progressive manner, at first, in vacuo, 

 then at 150, conditions under which they undergo no altera- 

 tion. 



If, on the contrary, they are suddenly heated to 200, decom- 



rition begins under the influence of the water vapour supplied 

 the hydrates. 



When they are further heated, it is necessary to operate in an 

 atmosphere of pure and dry nitrogen, the least trace of oxygen 

 causing an oxidation and sublimation of sulphur. The decom- 

 position of the hyposulphites is shown by analysis by means of 

 iodine, which should be reduced to the half, according to the 

 theoretical formula 



4S 2 3 K 2 = 3S0 4 K 2 + K 2 S 5 . 



The first body takes I 2 , the second body only I. 



The operations were carried out in an alloy bath, the tempera- 

 tures being given by an air thermometer. With standardised 

 solutions containing a known weight of iodine, the following 

 results were obtained : 



Amount of standard 

 iodine used. 

 Div. 



S 2 3 K 2 according to theory 323 



dried in vacuo 323 



heated to 255 325 



310 ten minutes 320 



310 an hour 323 



430 for a short time 320 



470 160 



490 161 



S 2 3 Na 2 theoretical (another standardised solution) 632 



dried at 150 632 



200 634 



255 634 



331 ten minutes 633 



331 an hour 633 



358 632 



400 569 



470 375 



,, 490 ... ... 381 



It results from these analyses that the potassium and sodium 

 hyposulphites resist without alteration up to about 400. The 

 soda salt commences to alter at this temperature ; the potash 

 salt resists a little longer, up to about 430, at least if the dura- 

 tion of the heating be not prolonged too much, otherwise it 

 commences to change. At 470 the decomposition is total. It 

 is strictly theoretical in the case of the potash salt. In that of 

 the soda salt there occurs partial sublimation of sulphur, and the 

 strength found is too high by about 8 per cent, (on 50). 



