172 CULTURE MEDIA 



may be. Into one set of seven place 10 cc of N/20 NaOH 

 solution. Mark these alkali and number from 1-7. Into 

 another set of seven place 10 cc of N/100 HCl solution. 

 Mark these acid and number 1-7. Place in a test tube rack 

 in a double row. 



Beginning with No. 1 acid, add to it 0.7 cc of 0.02 per cent 

 solution of bromthjTHol blue as indicator; No. 2, 0.6 cc; No. 

 3, 0.5 cc; No. 4, 0.4 cc; No. 5, 0.3 cc; No. 6, 0.2 cc and No. 

 7, 0.1 cc. 



Beginning with No. 1 alkali add to it 0.1 cc of the indi- 

 cator; No. 2, 0.2 cc; No. 3, 0.3 cc; No. 4, 0.4 cc; No. 5, 0.5 

 cc; No. 6, 0.6 cc; No. 7, 0.7 cc. 



The tubes with corresponding numbers form pairs. Pair 

 No. 1 when looked through shows a color corresponding to 

 pH 6.4, No. 2, vR 6.6, No. 3, iM 6.8, No. 4, iM 7.0, No.5, 

 pH 7.2, No. 6, iM 7 A and No. 7, i^H 7.6. 



TITRATION. 



Measure out accurately 8 cc of distilled H2O into one of 

 the empty tubes; add to it 2 cc of medium and 0.8 cc of 

 indicator. Mix thoroughly. Compare the color with the 

 above pairs to see if the reaction comes within the range of 

 the pairs. // necessary, add from a burette N720 NaOH, 

 one drop at a time, until the color matches that of the pair 

 indicating yH 7.2. From the number of cc of N/20 used, 

 calculate the amount of N/1 NaOH necessary to standardize 

 the volume of medium being made up. Add this to the 

 whole bulk of medium, boil and filter. 



Check the filtered medium by the same method. 



The other four tubes are used in the comparator block as 

 shown in the illustration (Fig. 111). 



Meat Extract Broth. — Broth may be prepared from Liebig's 

 or Armour's meat extract by adding 5 g. of either, 10 g. pep- 

 tone and 5 g. NaCl to 1000 cc of water, boiling to dissolve, 

 then standardizing and filtering as above. 



The author after much experience finds vieat juice prefer- 

 able to meat extract for broth and other media for pathogenic 

 bacteria, and has abandoned the use of meat extracts for 

 these organisms. 



