EXACT COLORIMETRIC pH DETERMINATION D. F. STEDMAN National Research Council, Ottawa, Canada
COLORIMETRIC
pH determination is not usually very exact since either the quality or depth of a color must be measured, and neither of these properties is very easy t o determine exactly. However, standard methods of improving the delicacy of measurements may be applied to this problem. The standard approach to such a measurement is to ,,measure zero," and make deviations as abrupt and as opposite as possible, which, translated into color language states that to give maximum accuracy the dye should have zero chromaticity, i.e., be neutral gray, at the pH measured, and acid or alkaline variation should produce complementary color changes. To investigate the possibilities of this method dyes were examined giving a trichromatic color balance, i.e., gray, at each pH value from 4.3 t o 5.8; and providing red and green respectively for acid or alkaline deviations. One group of dyes having excellent properties for this purpose are brom cresol green for the green alkaline component and methyl red for the red acid component; using either brom thymol blue or brom phenol blue to correct "off-shades" and improve the purity of the gray at each pH value as needed. The proportions of the dyes giving the best gray color at each pH value were determined using a carefully cali-
brated glass electrode for pH measurement. The table gives a summary of the dye concentrat,ions. For interpolation if needed, these concentrations are plotted as a locus on the triangle plots of the figure. I t should be noted, however, that to produce a clear diagram the hrom cresol green is plotted on the basis of O.lyosolution. Each of these dye proportions is of course adjusted for each pH value listed, and each gives a aimilar serie.5 of color changes:
++ 01.O2 ppHH unit, unit, grtlenish blue green + 0.05 pH unit, cold greenish gray
Tabulated pH, gray -0.05 pH unit, warm rosy gray -0.2 pH unit, pink -1.0 pH unit, red
The color change at the tabulated value is so sharp that a visual t,itration to about *0.01 pH unit can be made, and it is prohable that measurements made with the glass electrode are no more precise. .I further valuable point mith indicators adjusted in this manuer is that the titration is almost independent of the color of the room light used. If a white background is used for the titration, any normal room lighting is satisfactory, since a neutral gray on a white background is not affected appreciably even by considerable
PHof Complementw-Neutrd Indirstorr
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JOURNAL OF CHEMICAL EDUCATION
variations in the tint of the illumination. However, since these mixed indicators lack all definite color at their proper neutralization point, sharper titrations can usually be made if more indicator is used than is commonly necessary. Colorimetric indicators still have a wide application despite instrumentation, and a few commercial indicators do appear to be adjusted more or less along these lines. However, no systematic study appears to have been made using this method to improve the accuracy of colorimetric pH measurement, but there seems every probability that with the wide range of dyes now available the method could be extended widely to cover most pH values, and the examination of carbonate and bicarhonate, or mono-, di-, and tri-phosphate ions and similar differential analyses could be considerably simplified. If the method is extended, using other dyes for other pH ranges, it is not strictly necessary that the acid or alkaline colors of the dye mixture must be exactly complementary. If a gray color is given at the selected pH and brightly contrasting colors obtained for acid or alkaline deviations, results should be excellent, but it is important to select the indicators so that their contrasting colors do not overlap in the pH scale too widely; otherwise the gray shade extends over a wide pH range and dulls the brightness of the contrast obtained. This method of indicator adjustment appears to offer
VOLUME 35, NO. 9. SEPTEMBER, 1958
not only a means of exact pH determination but also quite a wide range of subjects suitable for student work, since it combines pH study with triangle composition plots, locus production, trichromatic color theory, and, if available, trichromatic or spectrophotometric instrumentation Amounts of Indicators to be Mixed to Make Complementary-Neutral Indicators for pH Range 4.3-5.8
pH
0 04% Standavd Indicators (.m l . ) Brom Brom Methyl phenol cresol blue pa red
Bmm fhymol blue
