236 



Journal of Applied Microscopy. 



indicator 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 obtained with phenol- 

 phthalein does not correspond with the neutral point obtained with litmus. Con- 

 sequently it has been found necessary to deduct twenty or twenty-five cubic cen- 

 timeters, 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 approximately, in another batch of the same or of 

 other media. Inasmuch as litmus has always been employed for neutralizing 

 purposes, it is well to adhere to this indicator, especially if the object sought for 

 can be attained with ease and exactness. 



The following method, as devised by the author, determines, with reference 

 to litmus, the neutral point of any medium, whether gelatin, bouillon, 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 

 alkalinity (or acidity) to a given medium. 



Two solutions of sodium hydrate are necessary : (1) One that will contain 

 forty grams of this base in one liter of water. This is known as normal sodium 

 hydrate. To prepare this solution so that it will have exactly the strength, 



requires some experience in chemical work. For 

 practical purposes it is sufficient to dissolve forty 

 grams of sodium hydrate in distilled water and 

 to dilute this solution to one liter. This will give 

 an approximately normal solution. ('2) One that 

 will contain four grams of the base in one liter of 

 water. This is known as a decinormal solution. 

 It is prepared by taking 100 cc. of the normal 

 solution and diluting it to one liter. It is evident 

 that this solution has one-tenth of the strength of 

 the former. In other words, 10 cc. of the No. 2 

 solution has the same strength as 1 cc. of the 

 No. 1 solution. 



The two solutions are placed in burettes of 

 o(> cc. capacity, graduated in one-tenth cubic 

 centimeter. In a laboratory where the solutions 

 are to be used frequently, it is desirable to con- 

 nect the burettes with bottles which contain a 

 stock of each solution. 



Titration of the Gelatin. — By means of a 

 pipette measure out 10 cc. of the gelatin solution 

 into each of four large test-tubes and label these 

 1 , 2, 8, and 4. 



Kig. 2. 



To tube 1 add 2.5 cc. of the deci-normal solution. 

 To tube 2 add 2.8 cc. of the deci-normal solution. 

 To tube 8 add ;>.() of the deci-normal solution. 

 To tube 4 add 8.;} of the deci-normal solution. 



