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In our unpublished studies concerning the effect of the medium on the electrophoretic velocity of the organism grown thereon, it became necessary to measure the pH of solid bacterial media. In the light of the errors exhibited by indicators, the question arose as to whether the colorimetric method (which is most commonly used in the bacteriological laboratory because it has the advantage of simplicity) is sufficiently accurate for this purpose. Whenever highly accurate values are necessary, the colorimetric values should be verified by potentiometric determinations. Previously, this has been done with liquid but not with solid media. Since we were dealing with a solid medium the question arose as to how to make pH determinations on such a medium potentiometrically. It was thought that if the medium was sufficiently buffered, it would be possible to do this by diluting the melted agar until it remained a liquid at room temperature, an idea suggested by buffer principles. 1 This is a resume of a thesis submitted by 0. G. Jensen in partial fulfilment of the requirements for the degree of Master of Science in Agricultural and Biological Chemistry in the Graduate School of The Pennsylvania State College, 1929. 2 Publication authorized by the Director of the Pennsylvania Agricultural Experiment Station as Technical Paper No. 513.

journal of bacteriology

A Comparison of Colorimetric and Potentiometric Methods for Hydrogen Ion Determination of Solid Bacteria Media, Using a Dilution Method Based on the Buffer Equation