pH effects on absorption spectra: pKa determination by spectrophotometric method
We know that acid-base indicators change color depending on the pH of the solution. This property of indicators is used to locate equivalent point during acid-base titrations. We are familiar with many acid-base indicators, such as phenolphthalein, methyl orange, bromocresol green, etc. For example, phenolphthalein solution is colorless below pH 8.2 and pink in color above pH 10. Similarly, methyl orange indicator changes from red to (orange-) yellow color in pH range 3.1 – 4.4. Methyl orange is used to test whether a sample is highly acidic (< pH 3) or not. Potassium dichromate-chromate conversion is another well-known example showing pH-dependent color change. The equilibrium shifts towards dichromate ions in acid solution whereas mainly chromate ions form in basic solution.
Cr2O72- (aq. , orange) + 3H2O (l) ↔ 2CrO42- (aq. , yellow) + 2H3O+ (aq. )
These examples demonstrate how the colors of many substances depend on the pH of the solution. We have chosen here methyl orange as an example to demonstrate the effect of pH on the absorption spectra. Methyl orange and other acid-base indicators exhibit one or more pH-dependent ionic equilibriums. The dissociation equilibrium constants for such compounds can be determined by monitoring the pH-dependent color changes. Such spectroscopic determinations are especially useful in cases of small sample quantity or poor sample solubility. The knowledge of dissociation equilibrium constants of weak acids and weak bases is very useful in chemical, biochemical and pharmaceutical sciences.