110 PROVISIONAL METHODS FOR ANALYSIS OF FOODS. 



sulphid. The nitrocom pound becomes converted into ammonium metadiamidoben- 

 zoic acid, which possesses a red color. This reaction takes place immediately, and 

 is seen at the surface of the liquid without stirring. 



(3) Third method. 



Evaporate to dryness at low temperature. If benzoic acid is present in great 

 quantity it will crystallize out in shining leaflets, with characteristic odor. These 

 will melt at 120 C. 



5. DETECTION AND DETERMINATION OF BORIC ACID AND BORATES. 



(a) FIRST QUALITATIVE METHOD. a 



Render decidedly alkaline with lime water about 25 grams of the sample and 

 evaporate to dryness on a water bath. Ignite the residue to destroy organic matter. 

 Add about 15 cc of water and hydrochloric acid, drop by drop, to acid reaction. 

 Then add about 1 cc of concentrated hydrochloric acid. Moisten a piece of delicate 

 turmeric paper with the solution; if borax or boric acid is present, the paper on 

 drying will acquire a peculiar red color, which is changed by ammonia to a dark 

 blue-green, but is restored by acid. This color is almost unmistakable, but it is 

 best for one not familiar with it to conduct a blank. 



A preliminary test may be made by immersing a strip of turmeric paper in about 

 100 cc of liquid foods, to which about 7 cc of concentrated hydrochloric acid has 

 been added. Solid and pasty foods may be heated with enough water to make them 

 thoroughly fluid, hydrochloric acid added in about the proportion of 1 to 15, and 

 tested in the same manner. 



(b) SECOND QUALITATIVE METHOD. b 



Add an equal volume of fresh saturated turmeric tincture and a drop of hydro- 

 chloric acid, and heat a few seconds. If the sample is a liquid, evaporate with the 

 turmeric and heat with a drop of dilute hydrochloric acid for a few seconds; then if 

 borax or boric acid is present a pink or dark red color will appear, depending upon 

 the amount of boric acid present. Cool and add a drop of ammonium hydroxid, 

 when a dark blue-green will appear. 



(c) QUANTITATIVE METHOD. 



Render 100 grams of the sample decidedly alkaline with sodium hydroxid, and 

 evaporate to dryness in a platinum dish. Ignite the residue cautiously, heat with 

 about 20 cc of water, and add hydrochloric acid, drop by drop, until all is dissolved. 

 Transfer to 100-cc flask, the bulk not being allowed to go over 50 cc or 60 cc. Add 

 0.5 gram of calcium chlorid and a few drops of phenolphthalein, then a 10 per cent 

 solution of caustic soda until a permanent slight pink color is produced, and finally 

 25 cc of lime water. Make the volume up to 100 cc. Mix well and filter through a 

 dry I liter. To 50 cc of the filtrate add normal sulphuric acid till the pink color dis- 

 appears, then methyl orange, and continue the addition of the acid until the yellow 

 is just changed to pink. Then add fifth-normal caustic soda till the liquid assumes 

 the yellow tinge, excess of soda being avoided. Boil to expel carbon dioxid. Cool 

 the solution, add a little phenolphthalein, and an equal volume of glycerine. Titrate 

 with standardized sodium hydroxid until a permanent pink color is produced. 



One" cubic centimeter of fifth-normal soda solution is equal to 0.0124 gram crys- 

 tallized boric acid. 



U. S, Dept. of Agr., Div. of Chem., Bui. 51, p. 134. 

 > U. S. Dept. of Agr., Div. of Chem., Bui. 51, p. 113. 

 c Thomson's method Button's Volumetric Analysis, page 100. 



