37 2 AGRICULTURAL ANALYSIS 



The potassium disulfate at a higher temperature decomposes, 

 forming normal potassium sulfate and sulfur trioxid, thus: 



(3) K 2 S 2 7 =K 2 S0 4 +S0 3 . 



At a sufficiently high temperature the two preceding reactions 

 may take place in one, thus: 



2KHSO 4 =K 2 SO 4 +H 2 O+SO 3 . 



At the temperature at which these reactions take place, the 

 water that is set free does not recombine with the sulfur trioxid 

 nor with the sulfuric acid that is present in excess, but is ex- 

 pelled from the mixture ; hence the mixture becomes more con- 

 centrated during the digestion. The sulfur trioxid set free acts 

 upon the organic matter in the powerful manner peculiar to it, 

 and the potassium sulfate, formed in the last reaction above, unites 

 with another molecule of sulfuric acid, and the same round of re- 

 actions is repeated continuously so long as there is an excess of 

 free sulfuric acid present in the mixture. As the liquid becomes 

 more concentrated with the continuation of the digestion, the 

 boiling-point increases so that the effect is the same as heating 

 under pressure. The danger of too great concentration and risk 

 of consequent loss of nitrogen is avoided by using increased pro- 

 portions of sulfuric acid. 



As compared with the kjeldahl, the gunning method presents 

 the following advantages : 



1 i ) The gunning method requires fewer reagents. As no form 

 of mercury is used, no potassium sulfid is needed, and there is no 

 risk of loss from the presence of mercurammonium compounds. 



(2) The solution to which caustic soda is added is clear, so 

 that in neutralizing it is an easy matter to avoid great excess of 

 alkali, and so, in most cases, to avoid foaming and bumping in 

 distillation. 



(3) In the blank determinations less nitrogen is found in the 

 reagents used in the gunning method. In only one case was 

 more nitrogen reported in a blank by this method than in the 

 other methods; in all the others the amount was considerably 

 less. 



322. The Official Gunning Method. 13 In a digestion flask hold- 

 ' 1S Bureau of Chemistry, Bulletin 107, 1907 : 7. 



