REACTIONS OF THE GUNNING PROCESS 371 



case of liquids, they should be previously evaporated to dryness 

 before the addition of the oxidizing mixture. At the beginning 

 of the combustion there is a violent foaming, attended with evo- 

 lution of some acid and much water, and afterwards of stronger 

 acid. This loss of acid should not be allowed to go far enough 

 to produce too great concentration of the material in the flask. 

 One of the best ways to avoid it, is to place a funnel in the 

 flask covered with a watch-glass, which will permit of the con- 

 densation and return of the escaping acid. As soon as the foam- 

 ing ceases, the flame should be so regulated as to permit of the 

 volatilized acid being condensed upon the sides of the flask. In 

 the end a colorless mass is obtained in which no metallic oxids 

 are present, and this mass can at once be diluted with water, 

 treated with alkali, and distilled. According to the nature of 

 the substance, from half an hour to an hour and a half are re- 

 quired for the complete combustion. 



Modifications of the Gunning Method. As in the case of the 

 kjeldahl method, numerous minor modifications of the gunning 

 method have been made, the most important of which relate to 

 its application to substances containing nitrates. In general the 

 same processes are employed in this case as with the kjeldahl 

 method. One of the best modifications consists in the use of the 

 mixture of salicylic and sulfuric acids, followed by the addition 

 of sodium thiosulfate or of potassium sulfate or sulfid. These 

 modifications will be given in detail under the official methods. 



321. Reactions of the Gunning Process. The various reac- 

 tions which take place during the combustion according to the 

 gunning method have been tabulated by Van Slyke. 12 



The first reaction to take place is the union of sulfuric acid 

 and potassium sulfate to form potassium acid sulfate in accord 

 ance with the following equation: 



(i) K 2 SO 4 +H 2 SO 4 =2KHSO 4 . 



When heated, the potassium acid sulfate decomposes, forming 

 potassium disulfate and water, thus : 



(2) 2KHSO 4 r=K 2 S 2 O 7 -fH 2 O. 



" Division of Chemistry, Bulletin 35, 1892 : 68. 



