ELEMENTARY BACTERIOLOGY LABORATORY EXERCISES 
19 
SECTION 7: PREPARATION OF CULTURE MEDIA 
EXERCISE 30 
COLORIMETRIC DETERMINATION OF HYDROGEN ION 
CONCENTRATION 
Determine the hydrogen ion concentration of the samples of media furnished, 
using standard buffer solutions. Also use color charts in the place of the stan- 
dard buffers. 
First, by use of the spot plate 
to the medium being tested. 
Procedure 
Place 5 c.c. of the medium in 
tilled water. Add 5 drops of the 
the color obtained with the proper 
The accuracy of this method 
block which compensates for the 
determine the correct indicator solution to add 
a clean test tube and dilute with 5c.c. of dis- 
indicator solution and mix thoroughly. Match 
tube in the standard buffer series. 
may be increased by the use of a comparator 
turbidity of the solution under test. Observe 
carefully the method of using the comparator block. 
COLOR CHANGES OF INDICATORS USED IN DETERMINING pH 
’ 
Indicators 
Full Full Sensitive range 
acid alkaline pH values 
color color 
Thymol blue (T.B.) (acid range) 
Brom phenol blue (B.P.B.) 
Brom cresol green (B.C.G.) 
Methyl red (M.R.) 
Chlor phenol red (C.P.R.) 
Brom cresol purple (B.C.P.) 
Brom thymol blue (B.T.B.) 
Phenol red (P.R.) 
Cresol red (C.R.) 
Meta cresol purple (M.C.P.) 
Thymol blue (T.B.) 
Phenolphthalein 
Cresolphthalein 
red—yellow 
yellow—blue . 
yellow—blue 
red—yellow 
yellow—red 
yellow—purple 
yellow—blue 
yellow—red 
yellow—red 
yellow—red ... 
yellow—blue - 8. 
Colorless—red .... 8.0—9.6 
Colorless—red u.....eseseeeeceeeeneeee 8.2—9.8 
EXERCISE 31 
ADJUSTMENT OF CULTURE MEDIA REACTION 
After determining the hydrogen ion concentration, adjust 5 c.c. portions of the 
media furnished to a reaction of 
pH 7.0 by the addition of N/20 NaOH (or 
