Direct solubility determination in optically dense solutions of highly soluble chromophores by the optoacoustic technique: Acidity dependence for Phenol Red

•Optoacoustic spectroscopy is applied for direct solubility determination in highly light-absorbing saturated solutions.•No dilution or sample preparation is required.•The dependence of solubility of Phenol Red on solution acidity in the wide pH range is obtained.•Optoacoustic and conventional spectrophotometric measurements provide complementary data.

The optoacoustic (OA) technique capable for light-absorption measurements in highly concentrated solutions is applied for direct solubility assessment in highly absorbing saturated solutions without any dilution or sample preparation. Maximum solubility of Phenol Red in aqueous solutions at pH>11 is found to be 1.3±0.1 g/L. The experimental dependence of solubility of Phenol Red on solution acidity was obtained with optoacoustic and conventional spectrophotometric measurements. The data from both methods show complementary results. The model of the solubility of Phenol Red on solution acidity describing the experimental dependence is proposed, which cannot be attained at pHs above 7 without the OA-technique.

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