VINEGAR. 



103 



9. Total Acids. 



Titrate a suitable amount of the sample, which has heen diluted until it 

 lappears very slightly colored, with standard alkali, using plu-nolphthalein as 

 [indicator. One cubic centimeter of tenth-normal alkali is equivalent to 0.0060 

 im of acetic acid. 



10. Volatile Acids. 



Heat 15 cc 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 with sensitive 

 [litmus paper. Titrate the combined distillate as directed for total acids. 



11. Fixed Acids. 



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

 Jfor sulphuric acid, or 1.117 for malic acid. Or, in case tannin has not been 

 added, dilute the nonvolatile residue from the distillation with recently boiled 

 [water, and titrate with standard alkali as directed for total acid. 



One cubic centimeter of tenth-normal alkali equals 0.0049 gram of sulphuric 

 acid, or 0.0067 gram of malic acid. 



12. Oxalic Acid. 



Oxalic acid may be detected and determined by precipitation with calcium 

 ilphate. 



13. Detection of Bitartrate of Potassium Allen's Method." 



Treat the residue, left from evaporation of the vinegar, with alcohol; a 

 [granular residue of tartar remains undissolved; to prove its character, pour 

 off the alcohol and dissolve the residue in a small quantity of hot water. On 

 cooling the aqueous solution and rubbing the sides of the vessel with a glass 

 rod the acid tartrate of potassium deposits in streaks. An addition of an 

 |equal bulk of alcohol makes the reaction more delicate. 



14. Free Tartaric Acid. 



Treat the alcoholic solution of the extract with an alcoholic solution of potas- 

 jsium acetate. If tartaric acid be present streaks will form on the beaker on 

 j stirring with a glass rod, or sometimes a distinct precipitate will separate. 

 Approximately quantitative results can be obtained by titration of the precipi- 

 tate with standard alkali. 



15. Detection of Free Mineral Acids, 

 (a) LOGWOOD METHOD.^ 



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

 I grams of fresh logwood chips, allowing the decoction to stand for a few hours, 

 and filtering. Place drops of the liquid on a porcelain surface and dry in a 

 I water bath. Add to one of the spots a drop of the vinegar to be tested (after 

 I concentration if desirable), and evaporate to dryness. A yellow tint remains 

 jif free mineral acids are absent, a red tint if present. 



Allen's Commercial Organic Analysis, 2d ed., 1 : 389. 



6 Ashby, Allen's Commercial Organic Analysis, 2d ed. f 1 : 393. 



