Phenolphthalein as a dilution indicator in gastric analysis

Conclusion

We have demonstrated the existence of two possible sources of error in the use of phenolphthalein as a dilution indicator in gastric analyses. (1) The solubility of phenolphthalein in the acid form, both in water and in dilute alcohol, is considerably lower than the concentration which it is necessary to employ in a test meal. Even if the latter be prepared immediately before it is administered to the subject, in the course of a two hour Rehfuss series determination a considerable amount will be lost by precipitation in the stomach and by filtration of the specimens to remove mucin and other solids preparatory to colorimetry. (2) Phenolphthalein, at a concentration of 10 mgm. per 100 ml. and a pH of about 12, the lowest pH value compatible with maximum conversion of the indicator to its red salt form, loses color at a significant rate. The second of these errors can be obviated by not alkalinizing the phenolphthalein solutions until just before they are introduced into the colorimeter cups for reading, but the first error constitutes an insurmountable barrier to the use of the substance as a dilution indicator. It follows, therefore, that all previous work based on the use of phenolphthalein in this way involves one or both of these errors unless otherwise demonstrated and should be repeated with the aid of a more reliable substance. To this end we have investigated phenol red. Our results indicate that this indicator possesses neither of the disadvantages of phenolphthalein, since it is soluble in the acid form and the rate of decolorization of the red alkali salt at pH 11–12 is so slow as to be insignificant. A preliminary report of this work has already been published (Hollander, Penner and Saltzman (11)).