224 MILK AND DAIRY PRODUCTS PART 



which is thus obtained lies midway between the curves for the 

 two acids. Where one acid is present in larger quantities than 

 the other the curve for the mixed acids lies nearer to the curve 

 for the predominating acid. To simplify these calculations 

 Duclaux has drawn up a series of tables (see Table X in the 

 Appendix) for different mixtures of fatty acids from C 2 to C 5 . 



It is evident that in mixtures of two acids the beginning 

 and end of the curve must closely approach the curves for the 

 single acids, so that it is enough to take the numbers for the 

 3rd, 4th, 5th, 6th. and 7th fraction. When these data have 

 been obtained, it is not difficult to determine the respective 

 quantities of each of the acids present. With the help of 

 table A (p. 222), it is first found in what proportions the two 

 acids are present in the 10 fractions, and so the quantity of 

 lime-water which would have been required to neutralise the 

 mixture of acid originally present in the flask can be calculated. 

 When this* has been ascertained, and the strength of the lime- 

 water and the equivalent proportions of the two acids in the 

 mixture being known, it is not difficult to calculate the 

 absolute quantity of each of the two acids present in the 

 mixture. 



The practical carrying out of the method does not present 

 any difficulties. First of all, the volume of milk is measured, 

 and then without filtering, but after making acid with H 2 SO 4 , 

 it is distilled with steam (p. 220). The steam is allowed to 

 pass into the milk before the flask is heated, and so burning is 

 prevented. When a sufficient quantity of distillate has been 

 collected it is neutralised with lime water, the quantity 

 required being noted, and the solution evaporal ed to less than 

 100 cc. Afterwards, as much of a 10 per cent, solution of 

 tartaric acid as is equivalent to the lime-water is added, and 

 after cooling, the mixture is diluted to exactly 110 c.c. The 

 amount of volatile fatty acid in this solution must not exceed 

 2 per cent. After 24 hours, the liquid is decanted from the 

 crystals of calcium tartarate into a distillation flask, a few drops 

 of concentrated sulphuric acid added to completely liberate the 

 volatile fatty acid, and the distillation carried out in the usual 

 way, fractions of 10 c.c. being collected. Some pieces of 

 pumice-stone must be put into the liquid before distillation. 

 The measuring flasks holding 10 c.c. are used as receivers. 



