Purification and characterization of thymol blue for spectrophotometric pH measurements in rivers, estuaries, and oceans

Highlights: • Thymol blue (TB) was purified using flash chromatography. • TB is useful for pH measurements above the range of meta-cresol purple (mCP). • Paired pH measurements directly link the equilibrium characteristics of TB and mCP. -- Abstract: Thymol blue (TB) is one of a suite of indicator dyes appropriate for spectrophotometric determinations of the pH of aqueous solutions. For measurements of seawater pH, meta-cresol purple (mCP) is most often used, but TB is especially well suited for measurements in surface or shallow waters where the pH may exceed the optimal indicating range of mCP (e.g., due to photosynthesis). This work presents flash chromatography procedures for purifying commercially available TB and describes physical–chemical characteristics of the purified dye, thus enabling the acquisition of spectrophotometric pH measurements over a wide range of practical salinities (S_P) and temperatures (T). The essential TB characteristics for 0 ≤ S_P ≤ 40 and 278.15 ≤ T ≤ 308.15 K are described by: ${\text{pH}}_{\text{T}}={-}_{\text{TB}}\left(\text{log}(K_2^{\text{T}}{e}_2)\right)+\log ((R-e_1)/(1-R{e}_4))$ ${}_{\text{TB}}(\text{log}\left(K_2^{\text{T}}{e}_2\right))=6.6942-0.001129S_{\text{p}}^{0.5}T-0.5926T^{-0.5}{S}_{\text{p}}+\text{619.40/}T+0.1441S_{\text{P}}-0.02591S_{\text{p}}^{1.5}+0.0034S_{\text{p}}^2-0.0001754S_{\text{p}}^{2.5}$ e₁ = −0.00132 + 0.00001600T e₄ = −0.005042 + 0.0002094T + 0.01916S_P^0.5/T where pH_T is pH determined on the total hydrogen ion concentration scale; R is the ratio of TB absorbances (A) at 435 and 596 nm (₅₉₆A/₄₃₅A); $K_2^{\text{T}}$ is the equilibrium constant for the second TB dissociation step on the total scale; and e₁, e_2, and e₄ are TB molar absorptivity ratios. This characterization was developed in a manner that ensures consistency with the primary TRIS buffer standards used in previously published characterizations of mCP. With this characterization, TB joins mCP as a sulfonephthalein indicator that has been characterized over the ranges of salinity and temperature required to make high-quality pH measurements in rivers, estuaries, and the open ocean. The full characterization of purified TB reported here extends the upper range of pH_T that can be accessed with precise spectrophotometric measurements by approximately 0.50 pH units.