Abstract
Although more accurate methods of measuring redox potentials in connection with bacterial oxidations are now available, 1 , 2 the Thunberg technique, 3 which has been extended by Quastel 4 , 5 and others, will no doubt continue to find wide application. It is especially useful where a large number of determinations are to be made, because of its greater simplicity and less time-consuming nature than the electrometric methods. At the same time, however, there are important sources of error. The purpose of this paper is to describe an improved method which either totally eliminates or greatly reduces the errors associated with the Thunberg technique as it has generally been used.
In a recent critical study, Yudkin 6 , 7 points out 6 factors which may interact to alter the reduction time of methylene blue by bacteria, when the suspension is evacuated and placed in a water bath. These are: (1) the temperature lag effect, (2) presence of residual oxygen, (3) gradual poisoning of the enzyme by methylene blue, (4) affinity of the enzyme for methylene blue, (5) presence of hydrogen donators in the suspension, and (6) the products of the action of the bacteria on the substrate which can act as hydrogen donators. The evacuation has nearly always been carried out for less than 2 minutes, with a water aspirator. Since Harvey 8 has shown that the rate of reduction of methylene blue by dehydrogenases in milk is proportional to the reduction in oxygen tension, the question naturally arises concerning the efficacy of one or 2 minutes'evacuation. This point was tested by luminous bacteria, which constitute a very good indicator for the presence of oxygen, 9 for their light dims at a partial pressure of about 2 mm. of mercury 10 and remains visible at 10−5 atmospheres, 11 emptied from the side arms to the suspension in the evacuated tubes, a bright flash of light occurred, showing the presence of dissolved oxygen.
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