130 Milk and Its Products 



line, titrate against one -twentieth normal hydrochloric 

 acid, using litmus as its own indicator. (Litmus is 

 blue when alkaline and red when acid.) The neutral 

 point will be recognized ^by the blue solution turning 

 reddish. If the neutral point of the solution using 

 phenol phthalein as an indicator is zero, then the 

 neutral point using litmus as an indicator would 

 require 2.5 c. c..more acid. Therefore, in order to 

 secure 1 per cent acidity to phenol phthalein, 5 c. c. 

 of the litmus solution would require 1.5 c. c. of 

 twentieth normal hydrochloric acid. As the litmus 

 solution is always alkaline, it will require normal acid; 

 the amount can be determined in the same way as in 

 titrating media. Sterilize litmus the same as water. 



Normal sodium hydroxid (NaOH) and normal hydro- 

 chloric acid (HC1). These solutions had best be bought 

 from dealers. To make twentieth normal solutions, one 

 part of the normal is added to 19 parts of distilled water. 



Sterilization. In bacteriological work, it is neces- 

 sary to have all glassware, utensils and media sterile, 

 that is, absolutely free from organic life. It must be 

 remembered that media, bottles, flasks, etc., must be kept 

 plugged, otherwise they become contaminated. There- 

 fore, if a flask or bottle of media or water is opened, it 

 must be resterilized before using again. The methods 

 of sterilizing vary for the materials to be sterilized. 



Cleaning glassware. All glassware should be thor- 

 oughly washed with some good soap or soap powder, 

 rinsed in clear water, and allowed to drain. When 

 dry, the test tubes, pipettes, flasks and graduated 

 cylinders are ready to be plugged. With the forceps, 



