AGRICULTURAL CHEMISTRY — AGROTECHNY. 15 



with calcium liydrate and shaking the fat out with ether before making the 

 test. Milk serum which contains as little as 0.25 mg. of nitric acid (N2O5) 

 per liter gives a distant blue coloration. 



The test can be used quantitatively and allows the detection of as little as 

 5 per cent of added water to milk where the water contains not less than 10 

 mg. of nitric acid (X::0.,) per liter. Milk which contains much dirt will yield 

 a slight coloration with the reagent, but the presence of a little of such matter 

 does not affect the test. 



The qualitative test is conducted as follows: Two cc. of the reagent (0.085 

 gm. diphenylamin and 190 cc. dilute sulphuric acid 1: 3, shaken carefully until 

 the diphenylamin dissolves, made up to nearly 500 cc. with concentrated sul- 

 phuric acid, cooled, and filled to the mark with the same acid) is placed in 

 a test tube with 0.5 cc. of the water or milk serum (whichever is to be tested), 

 and the mixture shaken and cooled. A distinct maximum blue coloration is 

 obtained in about 1 hour if nitrates are present. 



A method for the estimation of citric acid in milk, E. Desmoulii>re (BuL 

 Sci. Pharmacol., 11 {1910), A'o. 10, pp. 56'«-^.aj ) .— The method is as follows: 



To 200 cc. of milk add 100 cc, of a 2 per cent acetic acid solution, bring the 

 mixture to the boiling point, cool, filter off 150 cc. of clear filtrate, and 

 evaporate on the water bath. The addition of from 2 to 3 gm. of sea sand and 

 frequent agitation of the mass facilitates the drying process. After drying, 

 cool the dish and its contents, add 3 cc. of dilute sulphuric acid (0.2 gm. per 

 cubic centimeter), and allow to stand for 3 hours, stirring from time to time. 

 After this period add another 3 gm. of sand to the mass and extract the citric 

 acid with ether saturated with water. Allow the ethereal layer to separate, 

 draw it off, evaporate the ether, and take up the residue with water. 



The aqueous solution thus obtained contains citric acid, acetic acid, and a 

 little phosphoric acid. The phosphoric acid and acetic acid are then determined 

 and subtracted from the total acidity, the remainder of which will represent 

 the citric acid. 



A method for the estimation of reducing sugars, S. R. Benedict {Jour. 

 Biol. Chem.. 9 {1911), No. 1, pp. 57->59).— This is a modification of a method 

 previously described (E. S. R., 19, p. 8) and another noted somewhat later," 

 which employs sodium citrate in addition to the other reagents. 



The solution has the following composition: "Copper sulphate (crystallized) 

 18 gm., sodium carbonate (crystallized) 200 gm., sodium citrate 200 gm., 

 potassium sulphocj-anate 125 gm., 5 per cent potassium ferrocyanid solution 5 

 cc, distilled water to provide a total volume of 1.000 cc. With the aid of 

 heat dissolve the citrate, carbonate, and sulphocyanate in enough water to 

 make about 800 cc. of the mixture, and filter. Dissolve the copper sulphate 

 separately in about 100 cc. of water and pour the solution slowly into the 

 other liquid, with constant stirring. Add the ferrocyanid solution, cool, and 

 dilute to exactly 1 liter. Of the various constituents, only the copper salt need 

 be weighed with exactness. Twenty-five cc. of the reagent are reduced by 0.050 

 gm. of glucose, or by 0.053 gm. of levulose." 



The directions for conducting the tests are as follows : " Measure 25 cc. of 

 the reagent into a porcelain evaporation dish (25 to 30 cm. in diameter) and 

 add 10 to 20 gm. of ci-ystallized sodium carbonate (or one-half the weight of 

 the anhydrous salt) and a very small quantity of powdered pumice stone. 

 Heat the mixture to vigorous boiling over a free flame and run in the sugar 

 solution quite rapidly until a heavy white precipitate is produced and the 

 blue color of the solution begins to diminish perceptibly. From this point the 



°Jour. Biol. Chem., 5 (1909), p. 485. 



