LABORATORY WORK 333 
dissolve by heat. It takes considerable heat to dissolve the agar. Boil till 
the agar is completely dissolved and then replace the water that has evaporated by 
adding cold water till the original weight has been restored. 
(b) Adjust the acidity of the medium by one of the following methods: 
rt. Toasmall portion of the medium add 
a few drops of a solution of brom thymol 
blue.* If too alkaline, the medium turns 
this indicator blue, if too acid, it becomes 
yellow, while if of the correct acidity it 
becomes green. The medium is not likely 
to be too alkaline, but may be too acid. 
In that case proceed as follows: : 
Measure out ro c.c. of the medium, Normal |: 
place in an evaporating dish and dilute with Mtb": 
considerable water. Add 1 c.c. of the : 
brom thymol blue solution. Place the 
evaporating dish under a burette contain- 
ing 349 normal NaOH (one part normal 
NaOH diluted with nine parts of distilled 
water) (Fig. 56). Take the reading on 
the burette. Allow the solution to fall 
drop by drop into the evaporating dish, 
stirring between drops. As long as the 
material remains acid the color is yellow. 
Continue adding the NaOH until it is a 
distinct green. 
Take the reading upon the burette and 
determine the difference between the first 
and second reading. ‘The difference gives 
the number of cubic centimeters of 1/9 
normal NaOH needed for ro c.c. of the | 
cullure medium. Multiply by 50 to deter- 
mine the amount necessary for the half xe 
liter of the medium. Instead of adding 14/9 ZEE 
normal, add zormal NaOH to the medium, 
using of course only 149 as many cubic 
centimeters as indicated by the above BiG. 50. Two burettes arranged 
calculation. This will bring the medium or neutralizing culture media, 
to the correct reaction. 
2. Instead of using brom thymol blue, bacteriologists often use phenolphthalein. tf 
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* Di-bromo-thymol-sulphon-phthalein. A 0.04 per cent. solution in 95 percent. 
alcohol. 
{ Eight per cent. phenolphthalein in 50 per cent. alcohol. 
