v DECOMPOSITION PRODUCTS 221 



water, concentrated, filtered in order to get rid of any traces of 

 barium carbonate, and neutralised with nitric acid. As only 

 9*2 g. of barium nitrate are soluble in 100 parts of water at 

 20 C., the solution of the barium salt must not be concentrated 

 more than to leave 15 c.c. of water for each 100 c.c. N/10 

 baryta used. 



The fractional precipitation of the silver salts now begins by 

 adding a few cubic centimetres of normal silver nitrate to the 

 solution of the barium salt. The precipitate is filtered off, a 

 little more silver nitrate added, and, proceeding in this way, 6 to 

 $ fractions are obtained. Each precipitate is dried between 

 filter paper, and placed in vacuum over sulphuric acid. After- 

 wards the amount of silver is determined in each of these precipi- 

 tates by carefully igniting it; the percentage of silver is 

 compared with Jensen's table for the identification of the 

 different acids. 



(6) Quantitative Estimation. 



Jensen's method for the qualitative detection of the various 

 volatile fatty acids, as described above, cannot be used for the 

 quantitative analysis of these acids. The filter and the salts 

 which are on it soak up so much of those salts which are still 

 in solution that considerable loss occurs, particularly of the 

 lower acids, which are the last to be precipitated. Owing to 

 the solubility of the silver salts of these acids, it is impossible 

 to wash them. Duclaux l has, however, worked out a method 

 which can be used with satisfactory results. It is based upon 

 the well-known fact that the volatile fatty acids in weak 

 aqueous solution distil more quickly the higher their molecular 

 weight. 



Suppose any volatile acid to be in a solution of 1 or at most 

 2 per cent., then a certain volume, say 110 c.c., is taken and 

 distilled in the ordinary way with a Liebig's condenser. The 

 distillate is collected in 10 fractions in small measuring flasks 

 holding 10 c.c. Each of the fractions is then neutralised with 

 a standard solution of alkali from a burette, and the amount 

 required for each portion is noted. 



The three following facts then come into consideration : 



1 Trait,' <l< ,1//V,Wm,%,V, Vol. III., p. 384, and Vol. IV., p. <iSf>. 



