THE MEASUREMENT OF pH ix«-17 



cant results as to final pH and titratable acidity in cultures depend to 

 a large extent on comparisons made in media having buffer action 

 that is uniform and adjusted in amount to the purpose of the test. 

 A method for estimating such buffer action is as follows: 



Assume, for example, that the initial pH of a culture medium is 

 6.8 and that it is desired to measure the buffering capacity of the 

 medium between the pH limits 5.0 and 8.0. This can be done by 

 titrating an aliquot e.g., 5 ml., of the medium with 0.05 N HCl to 

 pH 5.0, and another aliquot with 0.05 N NaOH to pH 8.0. The 

 sum of these titers gives a simple and useful measure of the buffering 

 capacity of the medium within the pH zone 5.0 to 8.0. Brown (1921) 

 has described the procedure and some of its practical uses. 



The 'pH -adjustment of a culture medium. This is done with the 

 medium at about 80 to 90% of its final volume. Prepare approxi- 

 mately normal NaOH and HCl stock solutions, and also, about 100 

 ml. of each of these solutions diluted with distilled water exactly to 

 one-tenth 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 re- 

 quired, 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 medium to bring the volume to 10 ml., mix w^ell, 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 ne- 

 cessary volume relations. If the color difference is large, the titra- 

 tion 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 correctly adjusted, the medium is diluted 

 with distilled 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 to 5 or 1 to 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 negli- 

 gible. On the other hand, caution 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 



CIo+21- — ?-2Cl-+l2 



represents an exchange of electrons between the chlorine: chloride 

 system and the iodine: iodide system. These systems may be rep- 



