64 PROVISIONAL METHODS FOR ANALYSIS OF FOODS. 



8. DETERMINATION OF VOLATILE AND FIXED ACIDS. 



(a) VOLATILE ACIDS. 



Heat 15 cc of the vinegar to boiling in a flask, adding a little tannin if foaming 

 occurs; then lower the flame and pass a current of steam through the vinegar to a 

 condenser. Continue the operation until 15 cc of distillate shows no acidity upon a 

 test with sensitive litmus paper. Titrate the combined distillate with half-normal 

 sodium hydroxid, using phenolphthalein as indicator. The number of cubic centi- 

 meters of alkali required, multiplied by 0.03, gives the weight of volatile acids, 

 expressed as grams of acetic acid. 



(b) FIXED ACIDS. 



Deduct volatile acids from total acids and multiply the remainder by 0.817 for 

 sulphuric acid, or 1.117 for malic acid. Or dilute the nonvolatile residue from the 

 distillation with water until the solution appears nearly clear against a white back- 

 ground. Titrate with half-normal sodium hydroxid, using phenolphthalein as indi- 

 cator, as in case of the volatile acids. The weight of fixed acids, calculated to sul- 

 phuric or malic, is calculated by the factors given above. When 15 cc are taken, 

 multiply the number of cubic centimeters of half-normal alkali solution employed 

 by 0.163 for the percentage of fixed acids expressed in terms of sulphuric acid 

 (H 2 SO 4 ), or by 0.223 to express in terms of malic acid. 



9. DETECTION OF FREE MINERAL ACIDS. 



(a) FIRST METHOD. a 



Prepare an extract of logwood by pouring 100 cc of boiling water upon 2 grams of 

 fresh logwood chips, allowing the decoction to stand for a few hours and filtering. 

 Separate drops are spotted on a porcelain surface and dried over a water or steam 

 bath. Add to one of the spots a drop of the vinegar to be tested (after concentration, 

 if thought desirable); again evaporate to dryness. A yellow tint remains if free 

 mineral acids are absent, a red tint if present. 



(b) SECOND METHOD. 



To 5 cc of vinegar add 5 or 10 cc of water; after mixing well, add 4 or 5 drops of 

 an aqueous solution of methyl-violet (one part of methyl- violet 2B in 10,000 parts 

 of water). The occurrence of a blue or green color indicates the presence of a free 

 mineral acid. 



10. DETERMINATION OF FREE MINERAL ACIDS. 



(a) IIILGER'S METHOD. 



Neutralize 20 cc of the vinegar exactly with half normal alkali, the end reaction 

 being determined by the action of drops of the liquid upon sensitive violet litmus 

 paper. Evaporate the neutral liquid to one-tenth volume in a porcelain dish, add a 

 few drops of methyl- violet solution (that mentioned in paragraph 9), dilute with 

 3 or 4 cc of water, if needful, to secure a clear solution, bring to boiling, and titrate 

 with half normal sulphuric acid till a green or blue color begins to appear. The 

 difference, in cubic centimeters, between the seminormal alkaline and acid reagents 

 added, multiplied by the factor 0.1225, expresses the percentage of mineral acid 

 present, in terms of sulphuric acid (H 2 SO 4 ). 



(b) HEHNER'S METHOD. 



To a weighed quantity of the sample add excess of decinormal alkali, evaporate to 

 dryness, incinerate and titrate the ash with decinormal acid. The difference between 



'Ashby, Allen's Com. Org. Anal. 2d ed., vol. I, p, 



