96 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



For discussion of the methods of preparing standard 

 solutions and colors for accurate determinations for hy- 

 drogen ion concentrations a suitable laboratory manual 

 should be consulted. 



Determination of Acid Production. — ^What has been 

 stated above concerning determination of hydrogen ion 

 concentration will indicate the method of determining 

 whether or not acid has actually been produced by an 

 organism. It should be noted, however, that the deter- 

 mination of hydrogen ion concentration is not a determina- 

 tion of the total amount of acid produced. This can be 

 determined only by a comparative titration. It is custo- 

 mary to titrate to a definite tint a sample of the sterile 

 medium retained as a cheek, using phenolphthalein as an 

 indicator. For this purpose it is usual to place 5 ce. of the 

 sterile material mixed with 45 cc. of distilled water in a 

 porcelain evaporating dish, add a drop or two of alcoholic 

 solution of phenolphthalein and add twentieth normal 

 (N/20) alkali until a definite pink color has been estab- 

 lished. This is kept as a standard, and the amount of 

 alkali required noted. A similar titration is made using 

 the medium in which the organism to be studied has been 

 grown. This is treated in exactly the same fashion and 

 the twentieth normal alkali added until the same tint 

 has been secured as with the check. The differences be- 

 tween the amounts of alkali required for bringing to the 

 same tint of red will vary directly with the amount of acid 

 produced. This may be calculated in grams per liter if 

 the kind of acid formed is known. 



This method of acid determination is not absolutely ac- 

 curate inasmuch as there may be alkalies developed simul- 

 taneously with formation of acid. There is no simple 

 method of correcting for this possible error. 



In some cases it is desirable to determine the kind of 

 acid which has been produced. Simple chemical tests for 



