THE PREPARATION OF CULTURE MEDIA 



141 



and the use of pure substances by a reliable worker, this may, with 

 reasonable safety, be omitted. The principle of making the dilutions 

 is that carefully measured amounts of molecular solutions of acids 

 and alkalis are mixed in series so that each successive tube shall con- 

 tain a definite hydrogen ion concentration. These tubes are the 

 standard. Clark and Lubs, in the first of the series of papers noted above, 

 cite tables of mixtures for such purposes, with a range of P H extending, 

 from 1 to 10. For the exact composition of all of these mixtures, 

 the reader is referred to the original paper of Clark and Lubs, page 25. 

 For the routine of the ordinary laboratory bacteriology, only the 

 fourth section of this table is necessary. This is as follows: 



KH 2 PO 4 NaOH 



The method of procedure in using these solutions is as follows: 

 From the solutions made above a colorimetric scale is prepared. All 

 glassware must be very carefully cleansed, and it should be remem- 

 bered that some of the cheaper glassware obtained in laboratories 

 at the present time often seems to give off considerable amounts of 

 alkali. Whatever method of cleaning is used, the final thorough 

 rinsing must be done thoroughly with redistilled water. 10 c.c. each of 

 the respective series of standard mixtures is placed into each test 

 tube, and to it 10 drops of the indicator are added. For the range 

 from 6.8 to 8.4, which is sufficient for all ordinary pathogenic work, 

 the indicator used is phenol-sulfon-phthalein, or phenol red in con- 

 centration of 0.02 per cent aqueous solution. If ranges from 6 to 7.6 

 are desired, brom-thymol blue in a concentration of 0.04 per cent 

 may be used, and when ranges just above 8 are desired, cresol red 

 in concentration of 0.02 per cent is recommended. 



