TRANSIENTS IN ACID PRODUCTION 



451 



queiitly, after very long dark periods, a secondary acid evolution, 

 "S," may follow the acid giish. 



The general pattern of pH changes sho^^^l in Fig. 1 has been ob- 

 served hundreds of times, but it must be emphasized that the rates 

 and magnitudes of the effects shown vary widely in an as yet un- 

 predictable fashion from culture to culture and even in the same 

 suspension while it is under observation. 



That the anomalous pH changes are not artifacts of instrumenta- 

 tion but are related to photoreduction is shown by the fact that heat 

 treatments and certain inhibitors which inhibit photoreduction also 

 abolish all pH changes caused by light. 



Fig. 1. 



Since stirring of the suspension was found not to influence the 

 results, it was assumed until recently that the suspension remained 

 homogeneous with respect to pH. It has been found, however, that 

 the tendency of the bacteria to form a film on the surface of the glass 

 electrode causes the pH changes in this surface layer to exceed those 

 in the rest of the suspension. For rapid pH changes this inhomogeneity 

 may be quite marked even though the film maj^ be almost invisible, 

 and despite stirring of the solution. 



The build-up of the film may be prevented by frequently cleaning 

 the electrode. It is possible to correct for it by a separate measure- 

 ment of the pH changes due to the film of bacteria alone in plain 

 buffer after draining the bacterial suspension. A number of new 

 measurements have recently been made of the acid gush and initial 

 uptake where the film effect was kept negligibly small. These new 

 values have l)een substituted in this manuscript for the erroneously 

 large values reported at the Gatlinbin-g meeting. 



