1. In presence of Igepon T, the pH of solutions, as measured colorimetrically with thymol blue and bromophenol blue as indicators, is found to be lower in value than that obtained with the glass electrode. The value of — Δ pH (pH by the glass electrode — pH by the colorimetric method) is found to be influenced by the presence of salts like sodium chloride, sodium sulphate and calcium chloride.
2. With thymol blue, an increase in salt concentration diminishes the value of — Δ pH, while with bromophenol blue the opposite effect is observed.
3. With calcium chloride, the effect of salt concentration on — Δ pH is far more pronounced than with sodium chloride.
4. The results are explained on the basis of the effect of the salts on zeta potential, in relation to the formation of complexes by the acidic and the basic forms of the indicator, with the wetting agent.