98 MANUAL OF MICROBIOLOGICAL METHODS 



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 determine 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-reduction 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 impossi- 

 ble 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 I. L. Baldwin. 1932. Oxidation-reduction potentials in relation 

 to the growth of an aerobic form of bacteria. /. BacterioL, 23, 369-398. 



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

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



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

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



Clark, W. M. 1928. "The Determination of Hydrogen-Ions," 3rd ed. The Wil- 

 liams & Wilkins Company, Baltimore. 



. 1948. "Topics in Physical Chemistry." The Williams & Wilkins Com- 

 pany, Baltimore. 



Clark, W. M., Barnett Cohen, et al. 1928. Studies on Oxidation-Reduction, I-X. 

 U. S. Public Health Service, Hyg. Lab. Bull. 151. 



Clark, W. M., and H. A. Lubs. 1917. The colorimetric determination of hydrogen- 

 ion concentration. /. BacterioL, 2, 1-34, 109-136, 191-236. 



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

 Health Rpts., 41, 3051-3074. 



. 1933. Reversible oxidation-reduction potentials in dye systems; (also) 



Reactions of oxidation-reduction indicators in biological material, and their 

 interpretation. Cold Spring Harbor Symposia Quant. BioL, 1, 195-204, 214-223. 

 1935. Oxidations and Reductions, chap. 19 in "A Textbook of Bio- 



chemistry," by B. Harrow and C. P. Sherwin. W. B. Saunders Company, 



Philadelphia. 

 Dole, M. 1941. "The Glass Electrode." John Wiley & Sons, Inc., 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." D. Van 



Nostrand Company, Inc., Princeton, N.J. (See Chap. 8.) 

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

 chemistry," 6th ed. The Williams & Wilkins Company, Baltimore. 

 Kolthoff, I. M., and Charles Rosenblum. 1937. "Acid-base Indicators." The 



Macmillan Company, New York. 

 Small, James. 1946. "pH and Plants." D. Van Nostrand Company, Inc., 



Princeton, N.J. 



