DETEKMI^"ATIO^^S OF VOLATILE ACIDS. 15 



diluted. Calculate the results as sulphuric acid (H,SOj. One cubic 

 centimeter of decinormal potassium hydroxid equals 0.0049 gram of 

 sulphuric acid. 



Sulphuric acid was adopted as the term for the expression of acidit}^ 

 of fruit products because of its convenience in allowing comparison. 

 If a single organic acid were the common and dominant acid of all the 

 fruits that acid would serve the purpose better than the one chosen. 

 But such is not the case. Citric, tartaric, and malic acids are each 

 dominant in certain fruits, and the acidity of at least a large number 

 of the fruits is due to mixtures of two or more of the organic acids. 

 Besides, a part of the acidity may be due to the presence of acid salts, 

 so that an attempt to express the total acidity in terms of a single 

 organic acid characteristic of the fruit would meet with obvious diffi- 

 culties. Sulphuric acid has already been suggested and adopted by 

 a number of laboratories for similar work, and it is accepted here as 

 offering the most satisfactory basis for the expression of acidity in 

 fruits and fruit products. 



DETERMINATIOX OF VOLATILE ACIDS. 



The determination of volatile acids in fruit products may be desir- 

 able in cases where fermentation or the use of decayed fruit is sus- 

 pected. Dissolve 25 grams of substance in water, dilute to 50 cc and 

 distil in a current of steam until about 200 cc have passed over. Titrate 

 the distillate with decinormal potassium hydroxid and express the 

 results as acetic acid. Each cubic centimeter of decinormal alkali is 

 equivalent to 0.0060 gram of acetic acid. 



It should not require more than a few tenths of a cubic centimeter 

 of decinormal alkali to neutralize the volatile acid obtained from this 

 amount of fresh fruit. 



DETER3IINATION OF FREE MINERAL ACIDS. 



A. S. Mitchell'' has called attention to the presence of free sulphu- 

 ric and phosphoric acids in jellies, added to gelatinize the skins, cores, 

 and other waste material of the fruit. Phosphoric acid was found in 

 several jellies (see Table 31), one of which (659) had an acid ash and 

 another (660) an ash that was practically neutral. 



Sulphuric acid may be estimated by the Hehner method used in 

 determining free mineral acids in vinegar. To a weighed quantity of 

 the jelly add an excess of decinormal alkali, evaporate to dryness, ash, 

 and titrate the ash with decinormal acid. The difference between the 

 number of cubic centimeters of alkali added in the first place and the 

 number of cubic centimeters of acid needed to titrate the ash repre- 

 sents the equivalent of the amount of free mineral acid present. Phos- 

 phoric acid can be estimated in the ash. 



■^Communication by letter. 



