HYDROGEN IONS, ETC., OF BACTERIOLOGICAL MEDIA 569 



APPENDIX 



METHOD FOR THE TITRATION OF MEDIA 



Equipment 



A set of colorimetric hydrogen ion standards of the following ranges 

 and containing the indicators mentioned. 



pH 5.0 - 5.8 (methyl red) 



pH 5.8 - 6.8 (brom cresol purple) 



pH 6.8 -8.0 (phenol red) 

 A comparator block. 



Drop bottles or pipette bottles containing the indicators mentioned. 

 Solutions of N/20 NaOH and N/20 HCl. 

 Two finely graduated burettes. 

 A very accurate 1 cc. or graduated 2 cc. pipette. 

 Tubes of uniform internal diameter similar to those containing the 

 standards. Ordinary potato tubes of NONSOL glass are satisfactory. 

 Distilled water. 



Method 



1. Into each of 3 clean tubes place 9 cc. of distilled water and 1 cc. 

 (very accurately measured) of the medium to be titrated. 



2. Make the hydrogen ion determination in the usual manner, using 

 one of the tubes (tube I) as a color screen and adding phenol red to 

 another (tube II). (If the hydrogen ion concentration of the medium 

 lies outside the color range of phenol red but inside the range of brom 

 cresol purple (pH 5.8 to 6.8) the pH determination had better be made 

 in a separate sample which is then discarded.) 



Record the hydrogen ion concentration on the record sheet. 



3. Place the pH 8.0 standard in the comparator block behind the 

 color screen (tube I). From the burette cautiously add N/20 NaOH 

 to tube II (containing phenol red) until its hydrogen ion concentration 

 becomes pH 8.0 as determined by viewing it in the comparator block 

 beside tube I. 



Record figures in spaces a, b, and c of the record sheet. 



4. Place the pH 5.0 standard in the comparator block behind tube I. 

 Discard tube II and to a third tube of the diluted medium (tube III) add 

 methyl red. From a burette cautiously add N/20 HCl to tube III 

 until its hydrogen ion concentration becomes pH 5.0. 



Record figures in spaces e, f, and g of the record sheet. 



