Highlights
- Flash chromatography removes all impurities from commercial phenol red (PR) powder
- PR pH-measurement parameters are obtained for 275 ≤ T ≤ 308 K and 0 ≤ SP ≤ 40
- Simplified derivation for parameter e1 is applicable to any sulfonephthalein dye
- Optimal PR spectrophotometric (spec) pH-indicating range is roughly 5.9 to 7.7
- With PR and other dyes, spec pH measurements are now possible over pH range of 4 to 9
Abstract
Phenol red (PR) is one of several sulfonephthalein indicators used to provide rapid and precise spectrophotometric pH measurements of seawater and similar solutions. With an approximate pH-indicating range of 5.9 to 7.7, this dye is well suited to fill a critical gap in spectrophotometric pH-measurement capabilities – e.g., the slightly acidic waters of environments low in oxygen or high in carbon dioxide. For highest-quality measurements, the salinity and temperature dependence of indicator behavior must be established, but previous characterizations of PR were for impure indicator powder or for low-salinity solutions only. This work is the first to comprehensively characterize purified phenol red over wide ranges of temperature (T; absolute temperature in K) and salinity (SP; practical scale). Measurements of spectrophotometric pHT (total hydrogen ion concentration scale) are given by: pHT = –log(K2Te2) + log((R – e1)/(1 – Re4))
where K2T is the second dissociation constant of fully protonated PR, and e1, e2, and e4 are PR molar absorption coefficient ratios. The term R is the ratio of absorbances measured in the sample of interest at 558 and 433 nm. In this work, we derived a simplified method for determining the parameter e1 of any sulfonephthalein indicator and also fully characterized PR physical–chemical characteristics for 275.15 ≤ T ≤ 308.15 K and 0 ≤ SP ≤ 40, yielding:
e1 = –2.12261 × 10–3 + 1.37448 × 10–5T + 3.061 × 10–10SP0.5T2
e2 = 3.6429426 – 2.8139 × 10–3T
e4 = 8.0884775 × 10–2 + 6.2187 × 10–5SP – 14.093126T–1 – 5.005 × 10–12SP2T2
– log(K2Te2) = 6.0900807 – 2.6700911SP0.5 + 0.116252996SP – 2.5437592 × 10–2SP1.5 + 3.0176155 × 10–3SP2 – 1.396307 × 10–4SP2.5 + 7802.66T–1.5 + 0.7402604SP0.5ln(T) – 0.110614654SP0.5T0.5
To test the performance of this characterization, we measured pH at sea using both PR and meta-cresol purple (the standard indicator for measuring surface-to-deep open-ocean profiles) and found substantial agreement over the entire water column. The PR-based equation for measuring pHT can be combined with the parameterizations of other indicators to provide high-quality measurements over pH 4 to 9 for a wide range of solutions. This seamless continuity can be especially important in monitoring long-term change (e.g., ocean acidification) that may drive the pH of some waters of interest from the indicating range of one dye to another.
Fleger K. L., Byrne R. H. & Liu X., 2024. Physical–chemical characterization of purified phenol red for spectrophotometric pH measurements in riverine, estuarine, and oceanic waters. Talanta Open: 100380. doi: 10.1016/j.talo.2024.100380. Article.
