The visible spectra of five sulfonephthalein indicators (thymol blue, bromophenol blue, bromocresol green, bromocresol purple, and phenol red) were deconvoluted into four Gaussian components. The spectra of the basic form of the indicator were described by three components and the acidic form by one component. The combination of the spectral model, pH, and pK data allowed quantative prediction of indicator spectra as a function of pH. A general equation was derived for the calculation of pH from two absorbance measurements of a solution which contains one or more indicators. The application of multiple indicators significantly expands the pH range over which pH measurements can be made. With the use of the modeled spectra for several indicators, optimal indicator combinations and measurement wavelengths for specific pH ranges were determined. The combination of phenol red and bromocresol green allowed determination of seawater pH over the range 3.0 to 8.2, which is suitable for oceanic pH and alkalinity determinations.
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