90 MANUAL OF MICROBIOLOGICAL METHODS 



concentration. Assume, for example, that the adjustment of a colorless 

 medium is to be made to pH 7.0 before sterilization. Test the pH of the 

 medium to establish whether acid or alkali will be required for adjustment 

 to pH 7. To determine the amount required, titrate 5 ml of the medium 

 plus 5 drops of the appropriate indicator (e.g., bromthymol blue) with 

 the diluted acid or alkali until the color almost matches that of 10 ml of 

 standard buffer pH 7.0 plus 5 drops of the same indicator. Next, add 

 water to the tube with the medium to bring the volume to 10 ml, mix well, 

 and make a proper comparison with the standard. If the color difference 

 is small, then small additions of either acid or alkali may be made to 

 bring about a correct match without changing significantly the necessary 

 volume relations. If the color difference is large, the titration should be 

 tried again. (In the case of a medium with inherent color, this should be 

 compensated as previously described.) 



From the titration value, a calculation can be made of the amount of 

 the stronger acid or alkali to be added to bring the bulk of the medium 

 to the desired pH. The pH of the medium is checked after the addition, 

 and when the pH is correctly adjusted, the medium is diluted with dis- 

 tilled water to the final volume. 



In making a colorimetric pH determination of a well-buffered medium 

 that is already colored, it is permissible to dilute the test sample of the 

 medium 1:5 or 1:10 with distilled water to thin out the inherent color 

 before proceeding with the test. The change in pH due to such dilution 

 of a well-buffered solution is usually negligible. On the other hand, cau- 

 tion must be observed in employing the dilution procedure on poorly 

 buffered solutions, because the results may be misleading should the 

 distilled water, or even the indicator solution, be too far from the desired 

 pH. 



THE MEASUREMENT OF OXIDATION-REDUCTION POTENTIALS 



Introduction. The oxidation-reduction reaction 



Clo + 21- -^ 2C1- + I2 



represents an exchange of electrons between the chlorine : chloride system 

 and the iodine : iodide system. These systems may be represented by the 

 hypothetical ' ' half -reactions ' ' 



CI2 + 2, ;=i 2C1- 

 I2 + 2, ;:± 21- 



to show the participation of electrons. In the interaction, chlorine is the 

 electron-acceptor, and iodide the electron-donor. 



