32 THE BACTERIOLOGICAL EXAMINATION OF MILK 



Solutions Nos. 2 and 3 must be accurately made up, and must 

 correspond with the normal solutions referred to below. Solutions 

 of sodium hydrate are prone to deterioration from the absorp- 

 tion of carbon dioxide and the consequent formation of sodium 

 carbonate. To prevent as much as possible this change it is well 

 to place in the bottle containing the stock solution a small 

 amount of calcium hydroxid, while the air entering the burettes 

 or the supply bottles should be made to pass through a " U " tube 

 containing caustic soda to extract from it the carbon dioxide. 



The medium to be tested is brought to the prescribed volume 

 by the addition of distilled water to replace that lost by boiling, 

 and after being thoroughly stirred, 5 c.c. are transferred to a six- 

 inch porcelain evaporating dish ; to this 45 c.c. of distilled water 

 are added, and the 50 c.c. of fluid are boiled for three minutes 

 over a flame. One c.c. of the solution of phenolphthalein (No. i) 

 is then added and by titration with the required reagent (No. 2 

 or 3) the reaction is determined. In the majority of cases the 

 reaction will be found to be acid, so that the -gV normal sodium 

 hydrate is the reagent most frequently required. This deter- 

 mination should be made not less than three times and the 

 average of the results obtained taken as the degree of reaction. 



One of the most difficult things to determine in this process 

 is exactly when the neutral point is reached as shown by the colour 

 developed, and to be able in every instance to obtain the same 

 shade of colour. In regard to this, it may be remarked that in 

 bright daylight the first change that can be seen on the addition 

 of alkali is a very faint darkening of the fluid, which on the addition 

 of more alkali develops into what may be described as an Italian 

 pink. A still further addition of alkali suddenly develops a clear 

 and bright pink colour, and this is the reaction always to be 

 obtained. All titrations should be made quickly and in the hot 

 solutions, to avoid complications arising from the presence of 

 carbon dioxide. 



The next step in the process is to add to the bulk of the 

 medium the calculated amount of reagent, either alkali or acid as 

 may be determined. For the purpose of neutralisation a normal 

 solution of sodium hydrate or of HCl is used, and after being 

 thoroughly stirred the fluid thus neutralised is again tested in the! 

 same manner as at first to ensure the proper reaction of the mediumj 

 being attained. When neutralisation is to be effected by the' 

 addition of alkali, it not infrequently happens that after the calculated 

 amount of normal solution of sodium hydrate has been added, the 



