Abstract
In most bacteriological laboratories of this country adjustment in the reaction of culture media by titration has largely replaced all other methods. The indicator most commonly employed is phenolphthalein, and the results are expressed in terms of the amount of normal alkali necessary to bring one liter of the medium to the desired reaction (Fuller Scale).
Recent studies have shown that the titrimetric method in its present form is inaccurate. The results of titrations done in this laboratory support the observations of Clark 1 that media titrated to the end point of phenolphthalein and corrected to definite degrees on the Fuller scale have different hydrogen ion concentrations.
An exact knowledge of the reaction of a medium can be gained only from a determination of its hydrogen ion concentration. It is our purpose to present a simple colorimetric method which makes possible the accurate and rapid determination of the hydrogen ion concentration of culture media and their adjustment to any optimum concentration of ionized hydrogen.
For our work we have made use of a set of standard solutions recommended by Levy, Rowntree, and Marriott 2 for determining the hydrogen ion concentration of the blood. These consist of standard phosphate mixtures containing phenolsulphonephthalein. The advantages of this indicator have been set forth by these workers.
The medium is tested first to ascertain what its ionization is before adjustment. This preliminary test can be carried out quickly: to 3 C.C. of fluid is added 0.3 C.C. of a 0.01 per cent. solution of phenolsulphonephthalein, the fluid being read directly in the comparator. 3 In most instances the culture fluid has been roughly adjusted by the usual methods so that its reaction falls within the limits of the scale (pH+ = 6.4 to pH+ = 8.4).
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