ix,8-24 MANUAL OF METHODS FOR PURE CULTURE STUDY 



cator. This drawback cannot be overcome except by allowing suffi- 

 cient time for the biological system to overcome the poising* effect 

 of the added indicator. However, the time required may be very 

 long (especially in relation to the most active period of a growing 

 bacterial culture) so that it may be difficult or impossible to deter- 

 mine successive Eh values colorimetrically at brief intervals. 



Furthermore, the indicator may not merely come into simple 

 oxidation-reduction equilibrium with the components of the system 

 under test. It may act catalytically to displace the oxidation-reduc- 

 tion equilibrium that it is supposed to measure; or it may be toxic 

 toward living cells, or combine chemically with components of the 

 system under test. 



In summary, the indicator method, often applicable where it is 

 impossible to employ an electrode, may give results that require 

 considerable caution in interpretation, especially the results obtained 

 on unstable oxidation-reduction systems or on biological material 

 containing them. 



REFERENCES 



Allyn, W. p., and Baldwin, I. L. 1932. Oxidation-reduction potentials in re- 

 lation to the growth of an aerobic form of bacteria. J. Bact., 23, 369-398. 



BoRSOOK, H., and Schott, H. F. 1931. The role of the coenzyme in the succinate- 

 enzyme-fumarate equilibrium. J. Biol. Chem., 92, 535-557. 



Brown, J. H. 1921. Hydrogen ions, titration and the buffer index of bacteriological 

 media. J. Bact., 6, 555-568. 



Clark, W. M. 1928. The Determination of Hydrogen-Ions. 3rd Ed. Williams 

 and Wilkins, Baltimore. 



Clark, W. M. 1948. Topics in Physical Chemistry. Williams and Wilkins, Balti- 

 more. 



Cl.\rk, W. M., Cohen, Barnett, et al. 1928. Studies on Oxidation-Reduction, 

 I-X. Hygienic Laboratory Bulletin No. 151, U. S. Public Health Service, 

 Washington. 



Cl.\rk, W. M. and Lubs, H. A. 1917. The colorimetric determination of hydro- 

 gen-ion concentration. J. Bact., 2, 1-34, 109-136, 191-236. 



Cohen, Barnett. 1926. Indicator properties of some new sulfonphthaleins. 

 Public Health Rpts., 41, 3051-3074. 



Cohen, Barnett. 1933. Reversible oxidation-reduction potentials in dye systems; 

 (also) Reactions of oxidation-reduction indicators in biological material, and 

 their interpretation. Cold Spring Harbor Symposia on Quantitative Biology, 

 1, 195-204; 214-223. 



Cohen, Barnett. 1935. Oxidations and Reductions. Chapt. XIX in: A Text- 

 book of Biochemistry, by B. Harrow and C. P. Sherwin. W. B. Saunders 

 Co., Phila. 



Dole, M. 1941. The Glass Electrode. John Wiley and Sons, New York. 



Gillespie, L. J. 1920. Colorimetric determination of hydrogen-ion concentration 

 without buffer mixtures, with especial reference to soils. Soil Sci., 9, 115—136. 



Glasstone, Samuel. 1942. An Introduction to Electrochemistry. Van Nostrand, 

 N. Y. See Chapt. VIII. 



Hewitt, L. F. 1936. Oxidation-Reduction Potentials in Bacteriology and Bio- 

 chemistry. Ii-th Ed. London County Council. 



Kolthofp, I. M., and Rosenblum, Charles. 1937. Acid-base Indicators. Mac- 

 millan, New York. 



Small, James. 1946. pH and Plants. Van Nostrand, New York. 



^Poising action of an oxidation-reduction system is analogous to buffer action of an 

 acid-base system. (Compare paragraph on buffer action, p. 16.) 



