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

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 (SP) and temperatures (T). The essential TB characteristics for 0 ≤ SP ≤ 40 and 278.15 ≤ T ≤ 308.15 K are described by: