Abstract

The color change of phenol red when screened (optically filtered) by mixture with methylene blue is the same as the change of color of phenol red when screened with copper oxalato complex. An attempt has been made to differentiate the color changes of these two indicators with change of pH, by use of psychophysical data. The chromaticity coordinates of the colors of these indicators at different pH’s were determined. From these coordinates, the number of standard deviations of color matching, and consequently the color discrimination steps, were calculated on the basis of MacAdam ellipses. A new concept of Specific Color Discrimination, that is, the number of color discrimination steps per unit pH, has been introduced as a measure of visual sensitivity to indicator color change. It has been found that of the two screened indicators, phenol red plus methylene blue is the more sensitive; it shows a maximum ΔcpH value of 11.8 in the range of pH 7.9–8.0. The second indicator, phenol red with copper oxalato complex, is almost as good; it has a maximum ΔcpH. value of 8.6 at pH 7.2. The use of phenol red, alone, may give erroneous results, because it has two nearly equally sensitive ΔcpH. peaks, at pH 7.4 and 8.4. The method is recommended for the quantitative evaluation of the pH of most rapid color change of other indicators, which until now has been only arbitrarily determined, visually.

© 1964 Optical Society of America

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