Although pH is almost always determined by glass electrode in modern laboratories, situations exist — Analytical Chemistry Chemistry Question
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, H2In 0, is negligible near seawater pH and can be neglected. At 298 K, the second ionization constant of thymol blue, corrected for the salinity of seawater, is Ka2 = 10–8.090. Molar absorption coefficients (ελ) of HIn– and In2– at two wavelengths (λ) are as follows:
Measurements were made on a sample of seawater contained in a 10.00 cm optical cell:
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).
Model Answer
10.1 There are two equations for absorbance (background corrected):
At 436 nm, 0.651 – 0.052 = 10.00 [13900 c(HIn–) + 1930 c(In2–)]
At 596 nm, 0.882 – 0.023 = 10.00 [44.2 c(HIn–) + 33800 c(In2–)]
These can be solved to obtain c(HIn) = 3.96 · 10–6 mol dm–3 and c(In–) = 2.54 · 10–6 mol dm–3. Thus the concentration of the indicator is 6.47 · 10–6 mol dm–3.
Putting these values into the ionization constant equation:
log(Ka2) = –8.090 = log c(In2–) / c(HIn–) – pH, and pH = 7.897
The concentration of the indicator should be given to three significant figures and the pH to three decimal places.