Visible spectra of methyl red, methyl orange, p-nitrophenol, p-nitroaniline, and bromophenol blue in calcium nitrate tetrahydrate melt have been recorded at 25 deg C. All the indicators react with the acidic calcium nitrate tetrahydrate melt. It is found that indicators having high pK_a value in the range <=4 can be used as probes if the spectra of acidic and basic forms of such indicators in the hydrate melt are well separated. Acidity function of calcium nitrate tetrahydrate melt has been determined from the spectral data of p-nitroaniline and bromophenol blue only and its values, 1.8 and 2.6 respectively are estimated from their respective spectra.
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