452 LEON S. MEDALIA 



described, with pipettes. The tubes were then stoppered with 

 corks previously dipped in hot melted paraffin. This proce- 

 dure yields tightly sealed paraffined tubes of "standard color" 

 solutions. 



Titration of culture media 



For the titration of a culture medium 2 three test tubes, similar 

 to those used for the "color standards," are filled, each with 2 

 cc. of the filtered medium and 8 cc. of distilled water. To one 

 of these 0.8 cc. of the 0.02 per cent of the brom thymol blue indi- 

 cator is added and compared with the "color standards" tubes. 

 The other two tubes are used to offset the color of the medium 

 in the "comparator" block described below. The fluid to be 

 titrated is placed in the center row of the "comparator" block 

 having two test tubes of distilled water behind it. On either 

 side are placed the two pairs of test tubes nearest to the pH de- 

 sired (pairs 4 and 5 pH 7 and pH 7.2). In front of each pair 

 is placed the tube containing 2 cc. of the medium and 8 cc. of 

 distilled water to offset the color of the medium. It is abso- 

 lutely necessary to compensate for the color of the medium in this 

 way or the result will be misleading, fo NaOH is carefully run 

 in the tube that is being titrated until it matches pair no. 4 = 

 pH 7. Twenty-five times the amount of -& NaOH used, will rep- 

 resent the amount of normal XaOH necessary to neutralize 1 

 liter of the medium. 



/1000 cc. 1 \ 



X — =25 

 V 2 cc. 20 / 



If medium is too alkaline add ■,* HC1 until the color matches 

 and calculate the amount of ? HC1 to be added to one liter of 

 medium in the same way. The same factor is used whatever 

 the pH value selected for the reaction of the medium. The 

 medium should be retitrated after the addition of the alkali or 

 the acid as the case may be. The use of the "comparator block" 



2 Hot fluid agar or gelatin media, before final filtration, can be titrated as 

 described. The 2 cc. of the agar or gelatin when diluted with 8 cc. of water, 

 remain fluid for a much longer time than is necessary to carry out the titration. 



