stereoisomers. Problem 10. Absorption Spectroscopy Although is almost always determined by glass electrode in modern laboratories, situations pH exist in which optical measurements employing indicators can be used advantageously. One instance is the determination of in seawater. Because of the high concentration of dissolved pH salts, electrode-based determinations in seawater suffer from systematic errors that are too pH large for some applications. An example is determination of PCO2-driven changes in the pH ocean. Anthropogenic CO2 releases cause an annual shift in North Pacific surface waters of pH only about –0.0017. Thymol blue (molar mass 466.59 g·mol–1) is a dye that is a diprotic acid. The concentration of the non-ionized form, H2In0, is negligible near seawater pH and can be neglected. At 298K, the second ionization constant of thymol blue, corrected for the salinity of seawater, is = Ka2 of HIn– and In2– at two wavelengths are as Molar absorption coefficients (ελ) (λ) follows: Species nm ε436 nm ε596 1) HIn– 13900 44.2 In2– 1930 33800 Measurements were made on a sample of seawater contained in a 10.00 cm optical cell: Absorbance 436 nm Absorbance 596 nm Sample alone 0.052 0.023 Sample plus thymol blue indicator solution 0.651 0.882 Calculate the and the molar concentration of thymol blue in the sample. Because the value of pH has been salinity corrected, activity coefficients should be neglected (i.e., considered to Ka2 equal 1.000). Problem 11. Solution Equilibria
stereoisomers. Problem 10. Absorption Spectroscopy Although pH is almost always determined by glass electrode in modern laboratories, situations exist in which optical measurements employing indicators can be used advantageously. One instance is the determination of pH in seawater. Because of the high concentration of dissolved salts, electrode-based pH determinations in seawater suffer from systematic errors that are too large for some applications. An example is determination of PCO2-driven pH changes in the ocean. Anthropogenic CO2 releases cause an annual pH shift in North Pacific surface waters of only about –0.0017. Thymol blue (molar mass 466.59 g·mol–1) is a dye that is a diprotic acid. The concentration of the non-ionized form, H2In0, is negligible near seawater pH and can be neglected. At 298K, the second ionization constant of thymol blue, corrected for the salinity of seawater, is Ka2 = Molar absorption coefficients (ελ) of HIn– and In2– at two wavelengths (λ) are as follows: Species ε436 nm ε596 nm HIn– 13900 44.2 In2– 1930 33800 Measurements were made on a sample of seawater contained in a 10.00 cm optical cell: Absorbance 436 nm Absorbance 596 nm Sample alone 0.052 0.023 Sample plus thymol blue indicator solution 0.651 0.882 Calculate the pH and the molar concentration of thymol blue in the sample. Because the value of Ka2 has been salinity corrected, activity coefficients should be neglected (i.e., considered to equal 1.000). Problem 11. Solution Equilibria 1)