154 BACTERIOLOGY. 



The neutralization is usually accomplished by the cautious addi- 

 tion of a saturated solution of sodium carbonate. After each addi- 

 tion of 1 to 2 c.c. of the alkali, the liquid is well stirred, and a drop is 

 taken out by means of a glass rod and touchedito a blue litmus paper. 

 If the reaction is acid, this paper will turn red. The addition of al- 

 kali is continued until the blue paper retains its color, and the red 

 litmus paper turns slightly blue. 



In the hands of the beginner, this procedure not infrequently 

 fails, because of the diflBculty in judging the end-reaction. Moreover, 

 even the practised eye cannot establish the same degree of alkalin- 

 ity in two separate preparations. For these reasons, some workers 

 prefer to titrate the solution with an alkaliof known strength, using 

 phenol-phthalein as an indicator. The latter is a most delicate indi- 

 cator when mere aqueous solutions of acid and alkali are to be tested. 

 In the presence of organic matter, ammonium salts and carbonic acid 

 it ceases to be a sharp indicator. Moreover, the neutral point, as ob- 

 tained with phenol-phthalein, does not correspond with the neutral 

 point obtained with litmus. Indeed, gelatin neutralized thus, is in- 

 tensely alkaline to litmus. Consequently, it has been found necessary 

 to deduct 20 or 25 c.c. from the total amount of alkali necessary to 

 neutralize a liter of the medium. The amount thus subtracted is so 

 arbitrary that the resulting reaction cannot be duplicated, except ap- 

 proximately, in another batch of the same or of other media. 



The following method determines, with reference to lit- 

 mus, the neutral point of any medium, whether gelatin, b&uil- 

 lon, or agar to a degree of exactitude that leaves nothing to 

 be desired. The beginner, with no previous knowledge of 

 quantitative analysis, can impart any desired degree of al- 

 kalinity (or acidity) to a given medium. 



In order to neutralize the gelatin solution, it is neces- 

 sary to prepare two solutions of sodium hydrate. 



1. — One that will contain 40 g. of this base in one liter. 

 This is known as normal (N) sodium hydrate. To prepare 

 this solution so that it will have exactly this strength 

 requires some experience in methods of quantitative 

 analysis. ' 



1 For the preparation of exact normal and deci-normal solutions 

 the student is referred to the author's Laboratory Work in Physiolog- 

 ical Chemistry, 2nd ed., pp. 220-239. 



