Spectrophotometric measurement of freshwater pH with purified meta‐cresol purple and phenol red

Abstract

Impurities in indicator salts can significantly bias spectrophotometric pH determinations. In this work, two purified sulfonephthalein indicators, meta-cresol purple (mCP) and phenol red (PR), were tested for analysis of freshwater pH on the free hydrogen ion concentration scale. These two purified indicators were characterized for the first time under low ionic strength conditions, providing their molar absorption coefficients and dissociation constants along with their temperature dependence from 8 8C to 30 8C. At 25 8C, the infinite dilution constants (pK I o ) were determined to be 8.6606 and 8.0642 for mCP and PR, respectively. The accuracy and precision of the method, evaluated with a variety of buffers with known pH, were found to be 10.0014 pH units and 60.0022 pH units, respectively (n 5 30). The pH values of different freshwater samples were also determined using both indicators. The mCP and PR results were all within 6 0.01 pH units of each other with three out of seven pH differences within 6 0.001 pH units, indicating the high consistency between these two indicator methods. The work presented here is the first parallel comparison with two purified indicators used to determine pH of the same freshwater samples.

Physical and Biogeochemical Controls on pH Dynamics in the Northern Gulf of Mexico During Summer Hypoxia

“…The accuracy of spectrophotometric pH measurement relies heavily on the accurate determination of the dissociation constants of the indicator dye (Clayton & Byrne, ; Liu et al, ). To derive the dissociation constants of the purified mCP in the salinity range of 0–40, Douglas and Byrne () and Müller and Rehder () used the same measurement results in freshwater (Lai et al, , ) and in the salinity range of 20–40 (Liu et al, ; Table ). It is thus not surprising that Liu et al (), Douglas and Byrne (), and Müller and Rehder () generated very similar pH spec results in the salinity range of 20–40 (Table and Figures a and b).…”

“…When the recommended constants of Millero et al () and Uppström () were applied in the CO2SYS calculation (Oschlies & Kähler, ), the differences between pH DICTA and the pH spec following Douglas and Byrne () and Müller and Rehder () were 0.001 ± 0.016 and 0.003 ± 0.016, respectively (Table ). The dissociation behavior of mCP may be sensitive to the ionic composition of the solution at very low ionic strength, which may affect the accuracy of the spectrophotometric pH measurements (Lai et al, ; Müller & Rehder, ). The changing ionic composition would also increase uncertainty of the carbonate calculation at low salinities.…”

“…The cause of the different behavior of the discrepancy between pH spec and pH DICTA in open ocean and coastal environment remains as an unresolved question. Further research is needed to better improve the accuracy of spectrophotometric pH analysis (Lai et al, ; Müller & Rehder, ) and to identify potential thermodynamic inconsistency in the CO 2 system calculation (Fong & Dickson, ;Williams et al, ). Data from coastal oceans such as the northern Gulf of Mexico provide a new angle to examine the long‐standing question our community has faced over decades.…”

“…Reliable data on the molar absorption coefficients of the purified mCP have been generated by Liu et al () and Patsavas et al (). As shown in Table , the salinity and temperature dependence of the dissociation constants of the purified mCP have been characterized in freshwater (Lai et al, , ), in salinity ranges of 5–20 (Müller & Rehder, ), and 20–40 (Liu et al, ). This allowed the development of p(K 2 e 2 ) model (equation ) for the purified mCP‐based spectrophotometric pH measurement for the conditions of 0 ≤ S ≤ 40 and 5 °C ≤ temperature ≤ 25 °C (Douglas & Byrne, ; Müller & Rehder, ).…”

Spectrophotometric pH measurements in the presence of dissolved organic matter and hydrogen sulfide

“…In regard to H 2 S and DOM, spectral disturbances need to be expected from chromophoric dissolved organic matter (CDOM), the fraction of DOM that absorbs light in the UV and visible spectrum (Bricaud et al ; Green and Blough ; Stedmon et al ). Concerning the characterization of mCP for freshwater conditions, it was already pointed out by Lai et al () that the effect of “colored dissolved organic matter on measurements needs to be more systematically studied in the future.” Furthermore, it was shown that DOM enhances the water solubility of nonionic organic solutes (Averett et al ). Although mCP has different molecular properties than the organic solutes studied by Averett et al (), comparable molecular interactions between DOM and mCP could alter the dissociation constants and/or extinction coefficients of the dye, both of which would cause erroneous pH readings.…”