Abstract
The sulfonphthalein indicators of Clark and Lubs have shown themselves quite stable and reliable in biological fluids. Methyl red, which is not a sulfonphthalein, is not altogether reliable, but was included in the Clark and Lubs series because it was indispensable in covering a certain range of H-ion concentration. Methyl red is easily reduced irreversibly to a colorless compound—frequently by microbic action—thereby impairing its utility as an indicator under all conditions.
A sulfonphthalein indicator has been synthesized which has an apparent dissociation constant almost identical with that of methyl red and which seems as stable and reliable as the rest of the sulfonphthaleins. This compound is tetra-brom m-cresol sulfonphthalein. It is made by the bromination in glacial acetic acid of m-cresol sulfonphthalein. The common name suggested for this compound is Brom Cresol Green. Its effective range as an acid-base indicator is between PH 4.0 and 6.0, with a color change from yellow to green to blue-green. Its apparent dissociation constant in terms of PH is 5.00 (that of methyl red is 4.95).
Color standards of Brom Cresol Green were unaffected after several months' exposure in test tubes to usual laboratory conditions, while similar standards of methyl red had faded irregularly and become totally useless. Brom Cresol Green may be used directly in a bacteriological culture medium, for instance, while this would be out of the question for methyl red.
Get full access to this article
View all access options for this article.